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15. The Special Senses

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Historically, it was thought thatwe

had just ﬁve senses:smell, taste,

sight, hearing, and touch. Todaywe

recognize manymore. Some specialists

suggestthat there are at least 20, or per-

hapsas many as 40, different senses. Most

ofthese senses are part of what wasoriginally

classiﬁed as “touch.” These “general senses”

were discussed in chapter 14. The sense ofbalance is

now recognized asa “specialsense,” making a total of ﬁve special senses: smell,

taste, sight, hearing, and balance. Specialsenses are deﬁned as those senses

with highlylocalized receptors that provide speciﬁc information about the envi-

ronment. Thischapter describes olfaction (502), taste (504), the visual system

(508), and hearing and balance(527). We conclude the chapter with a lookat the

effectsof aging on the special senses (540).

The Special

Senses

Photograph of an isolated cochlea from the

innerear.

CHAPTER

Part 3 Integration and ControlSystems

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III. Integration and Control

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15. The Special Senses

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Part3 Integration and ControlSystems502

15.1a).Most of the nasal cavity is involved in respiration, with only a

small superior part devoted to olfaction.During normal respiration,

air passes through the nasal cavity without much ofit entering the ol-

factory recess.The major anatomic features of the nasal cavity are de-

scribed in chapter 23 in relation to respiration.The specialized nasal

epithelium ofthe olfactory recess is called the olfactory epithelium.

PREDICT

Explain whyit sometimes helps to inhale slowly and deeply through

the nose when trying to identifyan odor.

Olfaction

Objectives

■ Describe the histologic structure and function of the

olfactoryepithelium and the olfactory bulb.

■ Describe the CNS connections for smell.

Olfaction(ol-fak⬘shu˘n),the sense of smell, occurs in response

to odors that stimulate sensory receptors located in the extreme supe-

rior region of the nasal cavity, called the olfactory recess (ﬁgure

Cribriform plate of

ethmoid bone

Olfactory tract

Olfactory recess

Nasopharynx

Frontal bone

Nasal cavity

Palate

Olfactory bulb

Fibers of olfactory nerve

Olfactory

tract

Cribriform

plate

Connective

tissue

Olfactory

epithelium

Mucous layer

on epithelial

surface

Mitral cell

Association

neuron

Tufted cell

Olfactory bulb

Basal cell

Supporting cell

Olfactory neuron

Olfactory vesicle

Foramen

Axon

Dendrite

Cilia (olfactory hairs)

Figure 15.1

OlfactoryRecess, Epithelium, and Bulb

(a) The lateralwall of the nasalcavity (cut in sagittal section), showing the olfactory recess and olfactory bulb. (b) The olfactory cellswithin the olfactory epithelium

are shown. The olfactorynerve processes passing through the cribriform plate and the ﬁne structure of the olfactory bulb are also shown.

(a)

(b)

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III. Integration and Control

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15. The Special Senses

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Chapter 15 The Special Senses 503

sory cells, is lost about every 2 months as the olfactory epithe-

lium degenerates and is lost from the surface.Lost olfactory cells

are replaced by a proliferation ofbasal cells in the olfactory ep-

ithelium. This replacement of olfactory neurons is unique

among neurons,most of which are permanent cells that have a

very limited ability to replicate (see chapter 4).

NeuronalPathways for Olfaction

Axons from the olfactory neurons (cranial nerve I) enter the

olfactory bulb (see ﬁgure 15.1b),where they synapse w ith mitral

(mı¯⬘tra˘l;triangular cells; shaped like a bishop’s miter or hat) cells

ortufted cells. The mitral and tufted cells relay olfactory informa-

tion to the brain through the olfactory tracts and synapse with as-

sociation neuronsin the olfactory bulb. Association neurons also

receive input from nerve cell processes entering the olfactory bulb

from the brain.As a result of input from both mitral cells and the

brain, association neurons can modify olfactory information be-

fore it leaves the olfactory bulb.

Olfaction is the only major sensation that is relayed di-

rectly to the cerebral cortex without first passing through the

thalamus.Each olfactory tract terminates in an area of the brain

called the olfactory cortex (ﬁgure 15.2). The olfactory cortex is

in the frontal lobe,within the lateral ﬁssure of the cerebrum, and

can be divided structurally and functionally into three areas:lat-

eral,intermediate, and medial. The lateral olfactory area is in-

volved in the conscious perception of smell. The medial

olfactory area is responsible for visceral and emotional reac-

tions to odors and has connections to the limbic system,through

which it connects to the hypothalamus.Axons extend from the

intermediate olfactory area along the olfactory tract to the

bulb,synapse with the association neurons, and thus constitute a

major mechanism by which sensory information is modulated

within the olfactory bulb.

1. Describe the initiation of an action potential in an olfactory

neuron. Name all the structuresand cells thatthe action

potential would encounteron the way to the olfactory

cortex.

2. What is a primary odor? Name seven possible examples.

Howdo the primary odors relate to our ability to smell

manydifferent odors?

3. What type of neurons are olfactory neurons? What is

unique aboutolfactory neurons with respect to

replacement?

4. How is the sense of smell modiﬁed in the olfactory bulb?

5. Name the three areas of the olfactory cortex, and give their

functions.

6. Explain how the CNS connectionselicit various visceral and

consciousresponses to smell.

PREDICT

The olfactorysystem quickly adapts to continued stimulation, and a

particular odor becomesunnoticed before very long, even though

the odor moleculesare still present in the air. Describe as many

sitesas you can in the olfactory pathways where such adaptation

can occur.

Olfactory Epithelium and Bulb

Ten millionolfactory neurons are present within the olfactory ep-

ithelium (ﬁgure 15.1b).The axons of these bipolar neurons project

through numerous small foramina of the bony cribriform plate

(see chapter 7) to the olfactory bulbs. Olfactory tracts project

from the bulbs to the cerebral cortex.

The dendrites of olfactory neurons extend to the epithelial

surface of the nasal cavity,and their ends are modiﬁed into bul-

bous enlargements called olfactory vesicles (see ﬁgure 15.1b).

These vesicles possess cilia called olfactory hairs, which lie in a

thin mucous ﬁlm on the epithelial surface.

Airborne molecules enter the nasal cavity and are dissolved

in the ﬂuid covering the olfactory epithelium.Some of these mole-

cules, referred to as odorants (o¯⬘do˘r-ants; a molecule with an

odor), bind to chemoreceptor molecules of the olfactory hair

membranes.Although the exact nature of this interaction is not yet

fully understood, it appears that the chemoreceptors are mem-

brane receptor molecules that bind to odorants.Once an odorant

has become bound to a receptor,the cilia of the olfactory neurons

react by depolarizing and initiating action potentials in the olfac-

tory neurons.

The mechanism of olfactory discrimination is not com-

pletely known.Most physiologists believe that the wide variety of

detectable smells,which is about 4000 for the average person, are

actually combinations of a smaller number of primary odors.

Seven primary classes ofodors have been proposed: (1) camphora-

ceous,(2) musky,(3) ﬂoral, (4) pepperminty,(5) ethereal, (6) pun-

gent,and (7) putrid. It’s very unlikely, however,that this list is an

accurate representation of all primary odors, and some studies

point to the possibility ofas many as 50 primar y odors.

The threshold for the detection ofodors is ver y low,so very

few odorant molecules are required to trigger the response.Appar-

ently there is rather low speciﬁcity in the olfactory epithelium. A

given receptor may react to more than one type ofodorant.

The “Odor” ofNatural Gas

Methylmercaptan, which hasa nauseating odor similar to thatof rotten

cabbage, isadded to natural gas ata concentration of about 1 part per

million. A person can detectthe odor of about 1/25 billionth of a

milligram ofthe substance and therefore is aware of the presence ofthe

more dangerousbut odorless natural gas.

Odor SurveyResults

The NationalGeographic Society conducted a smellsur veyin 1986,

which wasthe largest sampling of its kind ever conducted. One and a

halfmillion people participated. Of six odors studied, 98%–99% of

those responding could smellisoamyl acetate (banana), eugenol

(cloves), mercaptans, and rose; but29% could not smell galaxolide

(musk), and 35% could notsmell androstenone (contained in sweat). Of

those responding to the survey, 1.2% could notsmell at all, a disorder

calledanosmia (an-oz⬘me¯-a˘).

The primary olfactory neurons have the most exposed

nerve endings of any neurons,and they are constantly being re-

placed.The entire olfactory epithelium, including the neurosen-

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Histology ofTaste Buds

Taste buds are oval structures embedded in the epithelium ofthe

tongue and mouth (ﬁgure 15.3f). Each of the 10,000 taste buds on

a person’s tongue consists oftwo ty pes of specialized epithelial

cells.One ty pe forms the exterior supporting capsule of the taste

bud,whereas the interior of each bud consists of about 50 taste or

gustatory cells. Like olfactory cells, cells of the taste buds are re-

placed continuously,each having a normal life span of about 10

days.Each taste cell has several microvilli, called gustatory hairs,

extending from its apex into a tiny opening in the epithelium called

thetaste or gustator y pore.

Function ofTaste

Substances called tastants(ta¯s⬘tants),dissolved in saliva,enter the

taste pore and,by various mechanisms, cause the taste cells to de-

polarize. These cells have no axons and don’t generate their own

action potentials. Neurotransmitters are released from the taste

cells and stimulate action potentials in the axons ofsensor y neu-

rons associated with them.

The taste of salt results when Na

⫹

diffuse through Na

⫹

channels (ﬁgure 15.4a) ofthe gustatory hairs or other cell surfaces

oftaste cells, resulting in depolarization of the cells. Hydrogen ions

⫹

) ofacids can cause depolarization of taste cells by one of three

mechanisms (ﬁgure 15.4b): (1) they can enter the cell directly

through H

⫹

channels,(2) they can bind to ligand-gated K

⫹

chan-

nels and block the exit of K

⫹

from the cell,or (3) they can open

ligand-gated channels for other positive ions and allow them to

diffuse into the cell. Sweet and bitter tastants bind to receptors

(ﬁgure 15.4cand d) on the gustatory hairs of taste cells and cause

depolarization through a G protein mechanism (see chapter 17).A

new taste,called umami (u¯-ma⬘me¯; loosely translated as savory) by

the Japanese,results when amino acids, such as glutamate, bind to

Tast e

Objectives

■ Describe the types and locations of papillae on the tongue,

and indicate which typeshave taste buds associated with

them.

■ Describe the histology and function of a typical taste bud.

■ List the ﬁve primary tastes, and indicate foreach taste how

depolarization of the taste cell occurs.

■ Describe the CNS pathways and cortical locationsfor taste.

The sensory structures that detect gustatory,or taste, stim-

uli are the taste buds.Most taste buds are associated with special-

ized portions of the tongue called papillae (pa˘-pil⬘e¯).Taste buds,

however,are also located on other areas of the tongue, the palate,

and even the lips and throat,especially in children. The four major

types of papillae are named according to their shape (ﬁgure 15.3):

vallate (val⬘a¯t;surrounded by a wall), fungiform (fu˘n⬘ji-fo¯rm;

mushroom-shaped), foliate (fo¯⬘le¯-a¯t; leaf-shaped), and filiform

(fil⬘i-fo¯rm; filament-shaped). Taste buds (ﬁgure 15.3c–e) are asso-

ciated with vallate,fungiform, and foliate papillae. Filiform papil-

lae are the most numerous papillae on the surface ofthe tongue but

have no taste buds.

Vallate papillae are the largest but least numerous ofthe

papillae.Eight to 12 of these papillae form a V-shaped row along

the border between the anterior and posterior parts of the tongue

(ﬁgure 15.3a).Fungiform papillae are scattered irregularly over the

entire superior surface of the tongue and appear as small red dots

interspersed among the far more numerous ﬁliform papillae.Foli-

ate papillae are distributed in folds on the sides of the tongue and

contain the most sensitive ofthe taste buds. They are most numer-

ous in young children and decrease with age. They are located

mostly posteriorly in adults.

Lateral olfactory area

Intermediate olfactory

area

Medial olfactory area

Olfactory tract

Frontal bone

Olfactory bulb

Fibers of olfactory nerve

Nasal bone

Nasal cavity

Axons of the olfactory neurons in the olfactory

epithelium project through foramina in the cribriform

plate to the olfactory bulb.

Axon of neurons in the olfactory bulb project

through the olfactory tract to the olfactory cortex.

The lateral olfactory area is involved in the

conscious perception of smell.

The medial olfactory area is involved in the visceral

and emotional reaction to odors.

The intermediate olfactory area receives input from

the medial and lateral olfactory areas.

Axons from the intermediate olfactory area project

along the olfactory tract to the olfactory bulb. Action

potentials carried by those axons modulate the

activity of the neurons in the olfactory bulb.

Figure 15.2

OlfactoryNeuronal Pathways and Cortex

Seeley−Stephens−Tate:

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Chapter 15 The Special Senses 505

receptors (ﬁgure 15.4e) on gustatory hairs of taste cells and cause

depolarization through a Gprotein mechanism.

The texture offood in the oral cavity also affects the percep-

tion oftaste. Hot or cold food temperatures may interfere with the

ability ofthe taste buds to function in tasting food. If a cold ﬂuid is

held in the mouth,the ﬂuid becomes warmed by the body, and the

taste becomes enhanced. On the other hand,adaptation is ver y

rapid for taste.This adaptation apparently occurs both at the level

ofthe taste bud and within the CNS. Adaptation may begin within

1 or 2 seconds after a taste sensation is perceived,and complete

adaptation may occur within 5 minutes.

Even though only ﬁve primary tastes have been identiﬁed,hu-

mans can perceive a fairly large number ofdifferent tastes, presum-

ably by combining the ﬁve basic taste sensations.As with olfaction,

the speciﬁcity ofthe receptor molecules is not perfect. For example,

artiﬁcial sweeteners have different chemical structures than the sug-

ars they are designed to replace and are often many times more

powerful than natural sugars in stimulating taste sensations.

Many ofthe sensations thought of as being taste are strongly in-

ﬂuenced by olfactory sensations.This phenomenon can be demon-

strated by pinching one’s nose to close the nasal passages,while trying

to taste something.With olfaction blocked, it’s difﬁcult to distinguish

Palatine

tonsil

Epiglottis

Root of

tongue

Dorsum of

tongue

Foramen

caecum

Terminal

sulcus

Epithelium

Foliate papilla

Fungiform papilla

Taste

bud

Taste bud

Taste

bud

Vallate papilla

Nerve fiber

of sensory

neuron

Supporting

cell

Epithelium

Filiform papilla

Surface of

the tongue

Epithelium

Taste pore

Gustatory

hair

Taste

cell

Filiform

papilla

Fungiform

papilla

Figure 15.3

Papillae and Taste Buds

(a) Surface ofthe tongue. (b) Filiform papillae. (c) Vallate papillae. (d) Foliate papillae. (e) Fungiform papillae. (f ) A taste bud. ( g) Scanning electron micrograph of

taste buds(fungiform and ﬁliform papillae) on the surface of the tongue.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

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Salt: Na

diffuse through Na

channels, resulting in depolarization.

Acid: Hydrogen ions (H

) from acids

can cause depolarization by one of

three mechanisms: (1) they can enter

the cell directly through H

channels,

(2) they can bind to gated K

channels,

closing the gate, and preventing K

from

entering the cell, or (3) they can open

ligand-gated channels for other

positive ions.

Sweet: Sugars, such as glucose, or

artificial sweeteners bind to receptors

and cause the cell to depolarize by

means of a G protein mechanism.

(GDP = guanosine diphosphate)

Bitter: Bitter tastants, such as quinine,

bind to receptors and cause

depolarization of the cell through a G

protein mechanism.

Glutamate (umami): Amino acids, such

as glutamate, bind to receptors and

cause depolarization through a G

protein mechanism.

Channel protein

Positive

ion

Sugar (or sweetener)

Receptor

Bitter tastant

Receptor

Glutamate

Receptor

βαγ

GDP

G protein with GDP

bound to the α subunit

βαγ

GDP

G protein with GDP

bound to the α subunit

G protein with GDP

bound to the α subunit

βαγ

Figure 15.4

Actionsof the Major Tastants

(a)

(b)

(c)

(d)

(e)

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Chapter 15 The Special Senses 507

between the taste ofa piece of apple and a piece of potato. Much of the

“taste”is lost by this action. Although all taste buds are able to detect

all ﬁve ofthe basic tastes, each taste cell is usually most sensitive to one.

Thresholds vary for the ﬁve primary tastes. Sensitivity for

bitter substances is the highest; sensitivities for sweet and salty

tastes are the lowest.Sugars, some other carbohydrates, and some

proteins produce sweet tastes;many proteins and amino acids pro-

duce umami tastes; acids produce sour tastes;metal ions tend to

produce salty tastes; and alkaloids (bases) produce bitter tastes.

Many alkaloids are poisonous;thus the high sensitivit y for bitter

tastes may be protective.On the other hand, humans tend to crave

sweet, salty,and umami tastes, perhaps in response to the body’s

need for sugars,carbohydrates, proteins, and minerals.

NeuronalPathways for Taste

Taste from the anterior two-thirds ofthe tongue, except from the

circumvallate papillae,is carried by means of a branch of the facial

nerve (VII) called the chorda tympani (ko¯r⬘da˘ tim⬘pa˘-ne¯;so

named because it crosses over the surface of the tympanic mem-

1. Axons of sensory neurons, which synapse

with taste receptors, pass through cranial

nerves VII, IX, and X and through the

ganglion of each nerve (enlarged portion

of each nerve).

2. The axons enter the brainstem and

synapse in the nucleus of the tractus

solitarius.

3. Axons from the nucleus solitarius synapse

in the thalamus.

4. Axons from the thalamus terminate in the

taste area of the cortex.

Taste area of cortex

Chorda tympani

Vagus nerve (X)

Foramen magnum

Facial nerve (VII)

Trigeminal nerve (V)

(lingual branch)

Glossopharyngeal nerve (IX)

Thalamus

Nucleus of

tractus

solitarius

VII

ProcessFigure 15.5

Pathwaysfor the Sense of Taste

The facialnerve (anterior two-thirds of the tongue), glossopharyngeal nerve (posterior one-third of the tongue), and vagus nerve (root of the tongue) allcarry taste

sensations. The trigeminalnerve is also shown. It carries tactile sensationsfrom the anterior two-thirds of the tongue. The chorda tympani from the facial nerve

(carrying taste input) joinsthe trigeminal nerve.

brane ofthe middle ear). Taste from the posterior one-third of the

tongue,the circumvallate papillae,and the super ior pharynx is car-

ried by means of the glossopharyngeal nerve (IX). In addition to

these two major nerves,the vagus nerve (X) carries a few ﬁbers for

taste sensation from the epiglottis.

These nerves extend from the taste buds to the tractus soli-

tarius of the medulla oblongata (ﬁgure 15.5).Fibers from this nu-

cleus decussate and extend to the thalamus. Neurons from the

thalamus project to the taste area ofthe cortex, which is at the ex-

treme inferior end ofthe postcentral g yrus.

7. Name and describe the four kinds of papillae found on the

tongue. Which oneshave taste buds associated with them?

8. Starting with the gustatory hair, name the structures and

cellsthat an action potential would encounter on the way to

the taste area of the cerebral cortex.

9. What is the life span of a normal gustatory cell?

10. What are the ﬁve primary tastes? Describe how each type of

tastantcauses depolarization of a taste cell.

11. How is the sense of taste related to the sense of smell?

Seeley−Stephens−Tate:

Anatomy and Physiology,

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III. Integration and Control

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15. The Special Senses

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Visual System

Objective

■ List the accessorystructures of the eye, and explain their

functions.

The visual system includes the eyes,the accessory structures,

and the optic nerves (II),tracts, and pathways. The eyes respond to

light and initiate afferent action potentials,which are transmitted

from the eyes to the brain by the optic nerves and tracts.The acces-

sory structures,such as eyebrows, eyelids,eyelashes, and tear glands,

help protect the eyes from direct sunlight and damaging particles.

Much ofthe information about the world around us is detected by

the visual system.Our education is largely based on visual input and

depends on our ability to read words and numbers.Visual input in-

cludes information about light and dark,color and hue.

Part3 Integration and ControlSystems508

AccessoryStructures

Accessory structures protect,lubricate, move,and in other ways aid

in the function of the eye. These structures include the eyebrows,

eyelids,conjunctiva, lacrimal apparatus, and extrinsic eye muscles.

Eyebrows

Theeyebrows (ﬁgure 15.6) protect the eyes by preventing perspira-

tion,which can irritate the eyes, from running down the forehead

and into them,and they help shade the eyes from direct sunlight.

Eyelids

Theeyelids, also called palpebrae(pal-pe¯⬘bre¯), with their associ-

ated lashes, protect the eyes from foreign objects.The space be-

tween the two eyelids is called the palpebral ﬁssure,and the angles

where the eyelids join at the medial and lateral margins of the eye

are called canthi(kan⬘thı¯;corners of the eye) (see ﬁgure 15.6).The

medial canthus contains a small reddish-pink mound called the

caruncle (kar⬘u˘ng-kl; a mound of tissue). The caruncle contains

some modiﬁed sebaceous and sweat glands.

The eyelids consist ofﬁve layers of tissue (ﬁgure 15.7). From

the outer to the inner surface,they are (1) a thin layer of integument

on the external surface;(2) a thin layer of areolar connective tissue;

(3)a layer of skeletal muscle consisting of the orbicularis oculi and

levator palpebrae superioris muscles;(4) a crescent-shaped layer of

dense connective tissue called the tarsal(tar⬘sa˘l)plate,which helps

maintain the shape ofthe e yelid;and (5) the palpebral conjunctiva

(described in the next section),which lines the inner surface of the

eyelid and the anterior surface ofthe eyeball.

Medial

canthus

(corner)

Caruncle

Pupil

Superior

palpebra

(eyelid)

Iris

Eyebrow

Lateral

canthus

(corner)

Inferior

palpebra

(eyelid)

Figure 15.6

The RightEye and Its Accessory Structures

Lower eyelid

(inferior palpebra)

Inferior oblique

muscle

Inferior rectus

muscle

Levator palpebrae

superioris muscle

Superior rectus

muscle

Smooth muscle to

tarsal plate

Orbicularis oculi muscle

Inferior conjunctival fornix

Palpebral fissure

Areolar connective tissue

Eyebrow

Orbicularis oculi muscle

Superior conjunctival

fornix

Palpebral conjunctiva

Bulbar conjunctiva

Tarsal (meibomian)

gland

Tarsal plate

Eyelash

Skin

Cornea

Figure 15.7

SagittalSection Through the Eye Showing Its AccessoryStructures

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Lacrimal Apparatus

The lacrimal (lak⬘ri-ma˘l) apparatus (ﬁgure 15.8) consists of a

lacrimal gland situated in the superolateral corner ofthe orbit and

a nasolacrimal duct beginning in the inferomedial corner ofthe or-

bit. The lacrimal gland is innervated by parasympathetic ﬁbers

from the facial nerve (VII).The gland produces tears, which leave

the gland through several ducts and pass over the anterior surface

of the eyeball. Tears are produced constantly by the gland at the

rate ofabout 1 mL/day to moisten the surface of the e ye,lubricate

the eyelids,and wash away foreign objects. Tears are mostly water,

with some salts,mucus, and lysozyme, an enzyme that kills certain

bacteria.Most of the ﬂuid produced by the lacrimal glands evapo-

rates from the surface ofthe eye, but excess tears are collected in the

medial corner of the eye by the lacrimal canaliculi. The opening

ofeach lacrimal canaliculus is called a punctum (pu˘ngk⬘tu˘m).The

upper and lower eyelids each have a punctum near the medial can-

thus.Each punctum is located on a small lump called the lacrimal

papilla.The lacrimal canaliculi open into a lacrimal sac, which in

turn continues into the nasolacrimal duct (see ﬁgure 15.8). The

nasolacrimal duct opens into the inferior meatus of the nasal cav-

ity beneath the inferior nasal concha (see chapter 23).

FacialNerve Damage

Facialnerve damage results in the inability to close the eyelid on the

affected side. With the abilityto blink being lost, tears cannotbe washed

acrossthe eye, and the conjunctiva and cornea become dry. A dry cornea

maybecome ulcerated, and, if not treated, eyesight maybe lost.

PREDICT

Explain whyit’s often possible to “taste” medications, such as

eyedrops, thathave been placed into the eyes. Why doesa person’s

nose “run” when he or she cries?

1. Tears are produced in the

lacrimal gland.

2. The tears pass over the surface

of the eye.

3. Tears enter the lacrimal

canaliculi.

4. Tears are carried through the

nasolacrimal duct.

5. Tears enter the nasal cavity from

the nasolacrimal duct.

Lacrimal

gland

Lacrimal

ducts

Lacrimal

canaliculi

Lacrimal

sac

Nasolacrimal

duct

Puncta

ProcessFigure 15.8

The LacrimalApparatus

Ifan object suddenly approaches the eye, the eyelids protect

the eye by rapidly closing and then opening (blink reﬂex).Blink-

ing,which normally occurs about 25 times per minute, also helps

keep the eye lubricated by spreading tears over the surface ofthe

eye. Movements ofthe eyelids are a function of skeletal muscles.

The orbicularis oculi muscle closes the lids,and the levator palpe-

brae superioris elevates the upper lid (see chapter 10).The eyelids

also help regulate the amount oflight entering the eye.

Eyelashes(see ﬁgures 15.6 and 15.7) are attached as a dou-

ble or triple row of hairs to the free edges of the eyelids. Ciliary

glands are modiﬁed sweat glands that open into the follicles of

the eyelashes to keep them lubricated.When one of these glands

becomes inﬂamed, it’s called a sty.Meibomian (mı¯-bo¯⬘me¯-an;

also called tarsal) glands are sebaceous glands near the inner

margins ofthe eyelids and produce sebum (se¯⬘bu˘m;an oily semi-

ﬂuid substance),which lubricates the lids and restrains tears from

ﬂowing over the margin of the eyelids.An infection or blockage

ofa meibomian gland is called a chalazion (ka-la¯⬘ze¯-on),or mei-

bomian cyst.

Conjunctiva

Theconjunctiva (kon-ju˘nk-tı¯⬘va˘) (see ﬁgure 15.7) is a thin,trans-

parent mucous membrane.The palpebral conjunctiva covers th e

inner surface ofthe eyelids, and the bulbar conjunctiva covers the

anterior surface of the eye.The points at which the palpebral and

bulbar conjunctivae meet are the superior and inferior conjuncti-

val fornices.

Conjunctivitis

Conjunctivitisis an inﬂammation of the conjunctiva caused byinfection

or some other irritation. An example ofconjunctivitis caused bya

bacterium isacute contagious conjunctivitis,also called pinkeye.

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ExtrinsicEye Muscles

Six extrinsic muscles of the eye (ﬁgures 15.9 and 15.10; also see

chapter 10) cause the eyeball to move.Four of these muscles run

more or less straight anteroposteriorly.They are the superior, in-

ferior,medial, and lateral rectus muscles. Two muscles,the supe-

rior and inferior oblique muscles, are placed at an angle to the

globe of the eye.

The movements ofthe eye can be described graphically by a

ﬁgure resembling the letter H. The clinical test for normal eye

movement is therefore called the H test. A person’s inability to

move his eye toward one part ofthe H may indicate dysfunction of

an extrinsic eye muscle or the cranial nerve to the muscle (the ac-

tions ofthe eye muscles are listed in table 10.7).

The superior oblique muscle is innervated by the trochlear

nerve (IV). The nerve is so named because the superior oblique

muscle goes around a little pulley,or trochlea, in the superomedial

Part3 Integration and ControlSystems510

Trochlea

Superior oblique

Posterior

Superior

Inferior

Medial rectus

Optic nerve

Levator palpebrae

superioris (cut)

Lateral rectus

Superior rectus

Trochlea

Superior oblique

Superior rectus

Lateral rectus

Inferior oblique

Inferior rectus

Optic nerve

Levator palpebrae

superioris (cut)

View

Figure 15.9

ExtrinsicMuscles of the Eye

(a) Superior view. (b) Lateralview.

(b)

(a)

Optic chiasm

Optic nerve

Lateral rectus

muscle

Medial rectus

muscle

Eyeball

Superior rectus

muscle

Figure 15.10

Photograph ofthe Eye and Its Associated

Structuresfrom a Superior View

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Chapter 15 The Special Senses 511

The sclera is continuous anteriorly with the cornea. The

cornea (ko¯r⬘ne¯-a˘) is an avascular, transparent structure that per-

mits light to enter the eye and bends,or refracts, that light as part

ofthe focusing system of the eye.The cornea consists of a connec-

tive tissue matrix containing collagen,elastic ﬁbers, and proteogly-

cans, with a layer of stratiﬁed squamous epithelium covering the

outer surface and a layer ofsimple squamous epithelium on the in-

ner surface.Large collagen ﬁbers are white, whereas smaller colla-

gen ﬁbers and proteoglycans are transparent. The cornea is

transparent,rather than white like the sclera, in part because fewer

large collagen ﬁbers and more proteoglycans are present in the

cornea than in the sclera.The transparency of the cornea also re-

sults from its low water content.In the presence of water, proteo-

glycans trap water and expand,which scatters light. In the absence

of water, the proteoglycans decrease in size and do not interfere

with the passage oflig ht through the matrix.

PREDICT

Predictthe effect of inﬂammation of the cornea on vision.

The Cornea

The centralpart of the cornea receives oxygen from the outside air. Soft

plasticcontact lenses worn for long periods must therefore be permeable

to air so thatair can reach the cornea.

The mostcommon eye injuries are cuts or tears of the cornea caused

byforeign objects like stones or sticks hitting the cornea. Extensive

injuryto the cornea may cause connective tissue deposition, thereby

making the cornea opaque.

The cornea wasone of the ﬁrst organs transplanted. Several

characteristicsmake it relativelyeasy to transplant: It’s easily accessible

and relativelyeasily removed; it’s avascular and therefore does not

require asextensive circulation as do other tissues; and it’s less

immunologicallyactive and therefore less likely to be rejected than

other tissues.

corner of the orbit. The lateral rectus muscle is innervated by the

abducens nerve (VI), so named because the lateral rectus muscle

abducts the eye. The other four extrinsic eye muscles are inner-

vated by the oculomotor nerve (III).

12. Describe and state the functions of the eyebrows, eyelids,

conjunctiva, lacrimal apparatus, and extrinsiceye muscles.

Anatomyof the Eye

Objectives

■ Describe the tunics of the eye, and give the function of each

of theirparts.

■ What are light refraction and reﬂection, and howare

imagesfocused on the retina?

■ Describe the structure and function of the cells in the layers

of the retina.

The eye is composed of three coats, or tunics (figure

15.11). The outer,or ﬁbrous, tunic consists of the sclera and

cornea; the middle, or vascular,tunic consists of the choroid,

ciliary body,and iris; and the inner,or ner vous,tunic consists of

the retina.

FibrousTunic

Thesclera (skle¯r⬘a˘) is the ﬁrm,opaque, white outer layer of the

posterior ﬁve-sixths of the eye. It consists of dense collagenous

connective tissue with elastic ﬁbers. The sclera helps maintain

the shape ofthe eye, protects its internal structures, and provides

an attachment point for the muscles that move it. Usually,a

small portion of the sclera can be seen as the “white of the eye”

when the eye and its surrounding structures are intact (see

ﬁgure15.6).

Optic nerve

Vitreous

humor

Retina

Choroid

Sclera

Conjunctiva

Cornea

Anterior chamber

Posterior chamber

Iris

Pupil

Lens

Suspensory

ligaments

Ciliary body

Figure 15.11

SagittalSection of the Eye Demonstrating Its Layers

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VascularTunic

The middle tunic ofthe eyeball is called the vascular tunic because

it contains most of the blood vessels of the eyeball (see ﬁgure

15.11).The arteries of the vascular tunic are derived from a num-

ber ofarteries called shor t ciliary arteries, which pierce the sclera

in a circle around the optic nerve.These ar teries are branches of

theophthalmic (of-thal⬘mik)artery, which is a branch of the in-

ternal carotid artery.The vascular tunic contains a large number of

melanin-containing pigment cells and appears black in color.The

portion ofthe vascular tunic associated with the sclera of the eye is

the choroid (ko⬘royd).The term choroid means membrane and

Part3 Integration and ControlSystems512

suggests that this layer is relatively thin (0.1–0.2 mm thick).Ante-

riorly,the vascular tunic consists of the ciliary body and iris.

Theciliary (sil⬘e¯-ar-e¯) bodyis continuous with the choroid,

and the irisis attached at its lateral margins to the ciliary body (ﬁg-

ure 15.12aand b). The ciliary body consists of an outer ciliary ring

and an inner group ofciliar y processes, which are attached to the

lens by suspensory ligaments. The ciliary body contains smooth

muscles called the ciliary muscles, which are arranged with the

outer muscle ﬁbers oriented radially and the central ﬁbers oriented

circularly.The ciliary muscles function as a sphincter,and contrac-

tion of these muscles can change the shape of the lens. (This

Sclera

Choroid

Retina

Ciliary muscle

Ciliary ring

Ciliary processes

Lens

Capsule of

the lens

Suspensory

ligaments

Ciliary body

Canal of Schlemm

Iris

Posterior

chamber

Cornea

compartment

Posterior

compartment

Ciliary

ring

Ciliary

processes

Suspensory

ligaments

Ciliary

body

Lens

Sphincter

pupillae

Dilator

pupillae

Figure 15.12

Lens, Cornea, Iris, and CiliaryBody

(a) The orientation isthe same as in ﬁgure 15.11. (b) The lensand ciliary body. (c) The sphincter pupillae muscles of the iris constrict the pupil. (d ) The dilator

pupillae musclesof the iris dilate the pupil.

(a)

(b)

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Chapter 15 The Special Senses 513

function is described in more detail on p. 515.) The ciliary

processes are a complex ofcapillaries and cuboidal epithelium that

produces aqueous humor.

Their is is the “colored part”of the eye, and its color differs

from person to person.Brown eyes have brown melanin pigment

in the iris. Blue eyes are not caused by a blue pigment but result

from the scattering of light by the tissue of the iris, overlying a

deeper layer ofblack pigment. The blue color is produced in a fash-

ion similar to the scattering oflight as it passes through the at mo-

sphere to form the blue skies from the black background ofspace.

The iris is a contractile structure,consisting mainly of smooth

muscle, surrounding an opening called the pupil.Lig ht enters the

eye through the pupil,and the iris regulates the amount of light by

controlling the size of the pupil. The iris contains two groups of

smooth muscles:a circular group called the sphincter pupillae (pu¯-

pil⬘e¯), and a radial group called the dilator pupillae (ﬁgure 15.12c

and d). The sphincter pupillae are innervated by parasympathetic

ﬁbers from the oculomotor nerve (III).When they contract, the iris

decreases or constricts the size ofthe pupil. The dilator pupillae are

innervated by sympathetic ﬁbers.When they contract, the pupil is

dilated.The ciliary muscles, sphincter pupillae, and dilator pupillae

are sometimes referred to as the intrinsic eye muscles.

Retina

The retina is the innermost, nervous tunic of the eye (see ﬁgure

15.11). It consists of the outer pigmented retina, which is pig-

mented simple cuboidal epithelium,and the inner sensory retina,

which responds to light.The sensor y retina contains 120 million

photoreceptor cells called rodsand another 6 or 7 million cones,

as well as numerous relay neurons.The retina covers the inner sur-

face of the eye posterior to the ciliary body.A more detailed de-

scription ofthe histology and function of the retina is presented on

page 516 and following.

Eye Pigment

The pupilappears black when you lookinto a person’s eye because of

the pigmentin the choroid and the pigmented portion of the retina. The

eye isa closed chamber, which allowslight to enter only through the

pupil. Lightis absorbed by the pigmented inner lining of the eye; thus

looking into itis like looking into a dark room. Ifa bright light is directed

into the pupil, however, the reﬂected lightis red because of the blood

vesselson the surface of the retina. This is whythe pupils of a person

looking directlyat a ﬂash camera often appear red in a photograph.

People with albinism lackthe pigment melanin, and the pupilalways

appearsred because no melanin is present to absorb light and prevent it

from being reﬂected from the backof the eye. The diffusely lighted blood

vesselsin the interior of the eye contribute to the red color of the pupil.

When the posterior region ofthe retina is examined with an

ophthalmoscope (of-thal⬘mo¯-sko¯p) (ﬁgure 15.13),several impor-

tant features can be observed.Near the center of the posterior retina

is a small yellow spot approximately 4 mm in diameter,the macula

lutea(mak⬘u¯-la˘ lu¯⬘te¯-a˘).In the center of the macula lutea is a small

pit,the fovea (fo¯⬘ve¯-a˘) centralis.The fovea and macula make up the

region ofthe retina where light is focused. The fovea is the portion

ofthe retina with the greatest visual acuity, the ability to see ﬁne im-

ages,because the photoreceptor cells are more tightly packed in that

portion of the retina than anywhere else.Just medial to the macula

lutea is a white spot,the optic disc, through which blood vessels en-

ter the eye and spread over the surface ofthe retina. This is also the

spot where nerve processes from the sensory retina meet, pass

through the outer two tunics,and exit the eye as the optic ner ve.

The optic disc contains no photoreceptor cells and does not re-

spond to light;therefore it’s called the blind spot of the eye.

OphthalmoscopicExamination of the Retina

Ophthalmoscopicexamination of the posterior retina can reveal some

generaldisorders of the body. Hypertension,or high blood pressure,

resultsin “nicking” (compression) of the retinalveins where the

abnormallypressurized arteries cross them. Increased cerebrospinalﬂuid

(CSF) pressureassociated with hydrocephalusmay cause swelling of the

opticdisc. This swelling is referred to aspapilledema (pa˘-pil-e-de¯⬘ma˘).

Compartmentsof the Eye

Two major compartments exist within the eye,a larger compart-

ment posterior to the lens and a much smaller compartment ante-

rior to the lens (see ﬁgure 15.11).The anter ior compartment is

divided into two chambers:the anterior chamber lies between the

cornea and iris,and a smaller posterior chamber lies between the

iris and lens (see ﬁgure 15.12).These two chambers are ﬁlled with

aqueous humor,which helps maintain intraocular pressure. The

Macula

lutea

Fovea centralis

Optic

disc

Retinal

vessels

Figure 15.13

OphthalmoscopicView of the Left Retina

(a) The posterior wallof the retina as seen when looking through the pupil.

Notice the vesselsentering the eye through the optic disc (the optic nerve)

and the macula lutea with the fovea (the partof the retina with the greatest

visualacuity). (b) Demonstration of the blind spot. Close your right eye. Hold

the ﬁgure in frontof your left eye and stare at the ⫹. Move the ﬁgure toward

your eye. Ata certain point, when the image of the spot is over the opticdisc,

the red spotseems to disappear.

(a)

(b)

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pressure within the eye keeps the eye inﬂated and is largely respon-

sible for maintaining the shape ofthe eye. The aqueous humor also

refracts light and provides nutrition for the structures of the ante-

rior chamber,such as the cornea,which has no blood vessels. Aque-

ous humor is produced by the ciliary processes as a blood ﬁltrate

and is returned to the circulation through a venous ring at the base

of the cornea called the canal ofSchlemm (shlem),or the scleral

venous sinus (see ﬁgure 15.12). The production and removal of

aqueous humor results in “circulation”of aqueous humor and

maintenance of a constant intraocular pressure. If circulation of

the aqueous humor is inhibited,a defect called glaucoma (glaw-

ko¯⬘ma˘),which is an abnormal increase in intraocular pressure,can

result (see the Clinical Focus on “Eye Disorders”).

The posterior compartment ofthe eye is much larger than the

anterior compartment. It’s surrounded almost completely by the

retina and is ﬁlled with a transparent jellylike substance,the vitre-

ous(vit⬘re¯-u˘s) humor.The vitreous humor is not produced as rap-

idly as is the aqueous humor,and its turnover is extremely slow.The

vitreous humor helps maintain intraocular pressure and therefore

the shape ofthe eyeball, and it holds the lens and the retina in place.

It also functions in the refraction oflight in the eye.

Lens

Thelens is an unusual biologic structure.Transparent and bicon-

vex,with the greatest convexity on its posterior side, the lens con-

sists ofa layer of cuboidal epithelial cells on its anterior surface and

a posterior region ofvery long columnar epithelial cells called lens

ﬁbers.Cells from the anterior epithelium proliferate and give rise

to the lens ﬁbers at the equator ofthe lens. The lens ﬁbers lose their

nuclei and other cellular organelles and accumulate a special set of

Part3 Integration and ControlSystems514

proteins called crystallines. This crystalline lens is covered by a

highly elastic transparent capsule.

The lens is suspended between the two eye compartments by

the suspensory ligaments ofthe lens, which are connected from the

ciliary body to the lens capsule.

13. Name the three layers (tunics) of the eye, describe the parts

orstructures each forms, and explain their functions.

14. How does the pupil constrict? How does it dilate? What is

the blind spot?

15. Name the two compartments of the eye and the substances

thatﬁll each compartment.

16. What is the function of the canal of Schlemm and the ciliary

processes?

17. Describe the lens of the eye, and explain how the lens is

held in place.

Functionsof the Complete Eye

The eye functions much like a camera.The iris allows light into the

eye, and the lens, cornea,and humors focus the lig ht onto the

retina.The light striking the retina is converted into action poten-

tials that are relayed to the brain.

Light

The electromagnetic spectrum is the entire range of wavelengths

or frequencies of electromagnetic radiation from very short

gamma waves at one end of the spectrum to the longest radio

waves at the other end (ﬁgure 15.14).Visible light is the portion

of the electromagnetic spectrum that can be detected by the hu-

man eye.Light has characteristics of both particles (photons) and

380 nm 430 nm 500 nm 560 nm

0.001 nm 1 nm 10 nm 1000 nm 0.01 cm 1 cm 1 m 100 m

600 nm 650 nm 750 nm

Increasing energy

Increasing wavelength

Visible light

Gamma rays X-rays Infrared Microwaves Radio waves

light

Figure 15.14

The ElectromagneticSpectrum

The spectrum ofvisible light is pulled out and expanded. The wavelengthsof the various colors are also depicted.

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Chapter 15 The Special Senses 515

waves, with a wavelength between 400 and 700 nm.This range

sometimes is called the range ofvisible light or, more correctly,the

visible spectrum. Within the visible spectrum, each color has a

different wavelength.

LightRefraction and Reﬂection

An important characteristic oflight is that it can be refracted (bent).

As light passes from air to a denser substance like glass or water,its

speed is reduced.If the surface of that substance is at an angle other

than 90 degrees to the direction the light rays are traveling,the rays

are bent as a result ofvariation in the speed of light as it encounters

the new medium.This bending of light is called refraction.

If the surface of a lens is concave,with the lens thinnest in

the center,the light rays diverge as a result of refraction.If the sur-

face is convex,with the lens thickest in the center, the light rays

tend to converge.As light rays converge,they ﬁnally reach a point

at which they cross.This point is called the focal point, and caus-

ing light to converge is called focusing.No image is formed ex-

actly at the focal point,but an inverted, focused image can form

on a surface located some distance past the focal point.How far

past the focal point the focused image forms depends on a num-

ber of factors.A biconvex lens causes light to focus closer to the

lens than does a lens with a single convex surface.Furthermore,

the more nearly spherical the lens,the closer to the lens the light is

focused;the more ﬂattened the biconcave lens, the more distant is

the point where the light is focused.

If light rays strike an object that is not transparent, they

bounce off the surface. This phenomenon is called reﬂection. If

the surface is very smooth,such as the surface of a mirror, the light

rays bounce off in a speciﬁc direction.If the surface is rough, the

light rays are reﬂected in several directions and produce a more dif-

fuse reﬂection.We can see most solid objects because of the light

reﬂected from their surfaces.

Focusing of Imageson the Retina

The focusing system of the eye projects a clear image on the

retina.Light rays converge as they pass from the air through the

convex cornea.Additional convergence occurs as light encounters

the aqueous humor,lens, and vitreous humor. The greatest con-

trast in media density is between the air and the cornea; there-

fore,the greatest amount of convergence occurs at that point.The

shape of the cornea and its distance from the retina are ﬁxed,

however,so that no adjustment in the location of the focal point

can be made by the cornea.Fine adjustment in focal point loca-

tion is accomplished by changing the shape of the lens. In gen-

eral, focusing can be accomplished in two ways.One is to keep

the shape ofthe lens constant and move it nearer or farther from

the point at which the image will be focused,such as occurs in a

camera,microscope, or telescope. The second way is to keep the

distance constant and to change the shape of the lens, which is

the technique used in the eye.

As light rays enter the eye and are focused,the image formed

just past the focal point is inverted (ﬁgure 15.15).Action potentials

that represent the inverted image are passed to the visual cortex of

the cerebrum, where they are interpreted by the brain as being

right side up.

VisualImage Inversion

Because the visualimage is inverted when it reachesthe retina, the

image ofthe world focused on the retina is upside down. The brain

processesinformation from the retina so that the world is perceived the

way“it really is.” If, as an experiment, a person wears glasses thatinver t

the image entering the eye, he or she willsee the world upside down for a

few days, after which time the brain adjuststo the new input to set the

world rightside up again. If the glasses are then removed, another

adjustmentperiod is required before the world is made right by the brain.

When the ciliary muscles are relaxed, the suspensory liga-

ments of the ciliary body maintain elastic pressure on the lens,

thereby keeping it relatively ﬂat and allowing for distant vision (ﬁg-

ure 15.15a). The condition in which the lens is ﬂattened so that

nearly parallel rays from a distant object are focused on the retina

Ciliary muscles in the

ciliary body relaxed

Near vision

Lens flattened

Suspensory ligaments

(tension high)

Ciliary muscles in the

ciliary body contract, moving

ciliary body toward lens

Lens thickened

Suspensory ligaments

(tension low)

Distant vision

Figure 15.15

Focusand Accommodation by the Eye

The focalpoint (FP) is where light rays cross. (a) Distantimage. The lens is

ﬂattened, and the image isfocused on the retina. (b) Accommodation for near

vision. The lensis more rounded, and the image isfocused on the retina.

(a)

(b)

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the retina.The main factor affecting the depth of focus is

the size ofthe pupil. If the pupillary diameter is small, the

depth offocus is greater than if the pupillary diameter is

large.With a smaller pupillary opening, an object may

therefore be moved slightly nearer or farther from the eye

without disturbing its focus.This is particularly important

when viewing an object at close range because the interest

in detail is much greater,and therefore the acceptable

margin for error is smaller.When the pupil is constricted,

the light entering the eye tends to pass more nearly through

the center ofthe lens and is more accurately focused than

light passing through the edges ofthe lens. Pupillary

diameter also regulates the amount oflight entering the eye.

The dimmer the light,the greater the pupil diameter must

be.As the pupil constricts during close vision, therefore,

more light is required on the object being observed.

3. Convergence.Because the light rays entering the eyes from a

distant object are nearly parallel,both pupils can pick up

the light rays when the eyes are directed more or less

straight ahead.As an object moves closer, however,the eyes

must be rotated medially so that the object is kept focused

on corresponding areas ofeach retina. Otherwise the object

appears blurry.This medial rotation of the eyes is

accomplished by a reﬂex which stimulates the medial rectus

muscle ofeach eye. This movement of the eyes is called

convergence.Convergence can easily be observed. Have

someone stand facing you.Have the person reach out one

hand and extend an index ﬁnger as far in front ofhis face as

possible.While the person keeps his gaze ﬁxed on the ﬁnger,

have him slowly bring the ﬁnger in toward his nose until he

ﬁnally touches it.Notice the movement of his pupils during

this movement.What happens?

PREDICT

Explain how severalhours of reading can cause eyestrain, or eye

fatigue. Describe whatstructures are involved.

18. What causes light to refract? What is a focal point? What is

emmetropia?

19. Describe the changes that occur in the lens, pupil, and

extrinsiceye muscles as an object moves from 25 feet away

to 6 inchesaway. What is meant by the termsnear point

and farpoint of vision?

Structure and Function ofthe Retina

Leonardo da Vinci,in speaking of the eye, said, “Who would be-

lieve that so small a space could contain the images ofall the uni-

verse?”The retina of each eye, which gives us the potential to see

the whole world,is about the size and thickness of a postage stamp.

The retina consists of a pigmented retina and a sensory

retina.The sensory retina contains three layers of neurons: pho-

toreceptor, bipolar, and ganglionic. The cell bodies of these

neurons form nuclear layers separated by plexiform layers,where

the neurons of adjacent layers synapse with each other (ﬁgure

15.16).The outer plexiform (plexuslike) layer is between the pho-

toreceptor and bipolar cell layers.The inner plexiform layer is be-

tween the bipolar and ganglionic cell layers.

Part3 Integration and ControlSystems516

is referred to as emmetropia(em-e˘-tro¯⬘pe¯-a˘; measure) and is the

normal resting condition of the lens.The point at w hich the lens

does not have to thicken for focusing to occur is called the far

point ofvision and normally is 20 feet or more from the eye.

When an object is brought closer than 20 feet to the eye,

three events occur to bring the image into focus on the retina:ac-

commodation by the lens,constriction of the pupil, and conver-

gence ofthe eyes.

1. Accommodation.When focusing on a nearby object, the

ciliary muscles contract as a result ofparasy mpathetic

stimulation from the oculomotor nerve (III).This

sphincterlike contraction pulls the choroid toward the lens

to reduce the tension on the suspensory ligaments.This

allows the lens to assume a more spherical form because of

its own elastic nature (ﬁgure 15.15b).The more spherical

lens then has a more convex surface,causing greater

refraction oflight. This process is called accommodation.

As light strikes a solid object,the rays are reﬂected in

every direction from the surface ofthe object. Only a small

portion ofthe light r ays reﬂected from a solid object,however,

pass through the pupil and enter the eye ofany given person.

An object far away from the eye appears small compared to a

nearby object because only nearly parallel light rays enter the

eye from a distant object (see ﬁgure 15.15a).Converging rays

leaving an object closer to the eye can also enter the eye (see

ﬁgure 15.15b),and the object appears larger.

When rays from a distant object reach the lens,they

don’t have to be refracted to any great extent to be focused on

the retina,and the lens can remain fairly ﬂat. When an object

is closer to the eye,the more obliquely directed rays must be

refracted to a greater extent to be focused on the retina.

As an object is brought closer and closer to the eye,

accommodation becomes more and more difﬁcult because

the lens cannot become any more convex.At some point,the

eye no longer can focus the object,and it’s seen as a blur.The

point at which this blurring occurs is called the near point

ofvision, which is usually about 2–3 inches from the eye for

children,4–6 inches for a young adult, 20 inches for a 45-

year-old adult,and 60 inches for an 80-year-old adult. This

increase in the near point ofvision, called presbyopia,

occurs because the lens becomes more rigid with increasing

age,which is primarily why some older people say they

could read with no problem ifthey only had longer arms.

Vision Charts

When a person’svision is tested, a chart is placed 20 feetfrom the eye,

and the person isasked to read a line of letters that is standardized for

normalvision. If the person can read the line, the vision isconsidered to

be 20/20, which meansthat the person can see at 20 feet whatpeople

with normalvision can see at 20 feet. If, on the other hand, the person

can see wordsonly at 20 feet that people with normalvision can see at

40 feet, the vision isconsidered 20/40.

2. Pupil constriction.Another factor involved in focusing is the

depth offocus, which is the greatest distance through

which an object can be moved and still remain in focus on

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Chapter 15 The Special Senses 517

Thepigmented retina, or pigmented epithelium, consists of

a single layer of cells.This layer of cells is ﬁlled with melanin pig-

ment and, together with the pigment in the choroid, provides a

black matrix,which enhances visual acuity by isolating individual

photoreceptors and reducing light scattering.Pigmentation is not

strictly necessary for vision,however. People with albinism (lack of

pigment) can see,although their visual acuity is reduced because of

some light scattering.

The layer ofthe sensor y retina nearest the pigmented retina

is the layer ofrods and cones. The rods and cones are photorecep-

tor cells,which are sensitive to stimulation from “visible”light. The

light-sensitive portion ofeach photoreceptor cell is adjacent to the

pigmented layer.

Rods

Rods are bipolar photoreceptor cells involved in noncolor vision

and are responsible for vision under conditions of reduced light

(table 15.1). The modiﬁed,dendritic, light-sensitive part of rod

cells is cylindrical, with no taper from base to apex (figure

15.17a).This rod-shaped photoreceptive part of the rod cell con-

tains about 700 double-layered membranous discs. The discs

contain rhodopsin (ro¯-dop⬘sin), which consists of the protein

opsincovalently bound to a pigment called retinal (derived from

vitamin A).

Function of Rhodopsin

Figure 15.18 depicts the changes that rhodopsin undergoes in re-

sponse to light. In the resting (dark) state, the shape of opsin

keeps 11-cis-retinal tightly bound to the internal surface of

opsin.As light is absorbed by rod cells, opsin changes shape from

11-cis-retinal to all-trans-retinal.These changes activate the at-

tached G protein, called transducin (trans-doo⬘sin), which

closes Na

channels, resulting in hyperpolarization of the cell

(ﬁgure 15.19).

Opsin Mutants

Opsin isa protein composed of 338 amino acids. Mutation at amino acid

23 or 28, in the extracellular plug covering the externalopening of the

molecule, which keepsretinal associated with opsin, causesretinitis

pigmentosa.This is a genetic disorder consisting ofprogressive retinal

degeneration. During thisdegeneration, pigment inﬁltrates the sensory

retina, decreasing itsfunction and constricting the visualﬁelds. Night

blindness,or nyctalopia, (the decreased ability to see in reduced light)

mayalso occur in retinitis pigmentosa. Night blindnessalso may occur

asa result of vitamin A deﬁciency or as the result ofanother mutation at

amino acid 90 ofthe opsin molecule. This mutation occursin the second

ofthe seven helical regions of the protein opsin and may affectthe

attachmentof retinal to opsin.

Choroid

Pigment cell

layer

Photoreceptor

layer

Outer plexiform

layer

Bipolar layer

Inner plexiform

layer

Ganglionic

layer

Fibers to

optic nerve

Optic nerve

Light

source

Nerve

fibers

Interplexiform

cell

Amacrine

cell

Ganglion

cell

Bipolar

cell

Horizontal

cell

Rod cell

Cone cell

Pigment

cell

Direction

of action

potential

propagation

Pigmented

retina

Sensory

retina

Figure 15.16

Retina

Section through the retina with itsmajor layers labeled.

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This hyperpolarization in the photoreceptor cells is some-

what remarkable,because most neurons respond to stimuli by de-

polarizing.When photoreceptor cells are not exposed to light and

are in a resting,nonactivated state, some of the Na

⫹

channels in

their membranes are open,and Na

⫹

ﬂow into the cell.This inﬂux

ofNa

⫹

causes the photoreceptor cells to release the neurotransmit-

ter glutamate from their presynaptic terminals (see ﬁgure 15.19).

Glutamate binds to receptors on the postsynaptic membranes of

Part3 Integration and ControlSystems518

the bipolar cells ofthe retina, causing them to hyperpolarize. Thus,

glutamate causes an inhibitory postsynaptic potential (IPSP) in the

bipolar cells.

When photoreceptor cells are exposed to light, the Na

⫹

channels close,fewer Na

⫹

enter the cell,and the amount of gluta-

mate released from the presynaptic terminals decreases.As a result,

the hyperpolarization in the bipolar cells decreases, and the

cells depolarize sufﬁciently to release neurotransmitters, which

Table 15.1

Photoreceptive

Photoreceptive End Molecule Function Location

Rod

Cylindrical Rhodospin Noncolor vision; vision under conditions Over most of retina; none

of low light in fovea

Cone

Conical Iodopsin Color vision; visual acuity Numerous in fovea and macula

lutea; sparse over rest of retina

Rods and Cones

Opsin

Retinal

Rhodopsin

Gated Na

channel

Disc

Outer membrane

Disc

membrane

Extracellular

plug

Inside

of disc

membrane

Outside

of disc

membrane

Folding of outer

membrane to

form discs

Disc

Nuclei

Axons

Rod

Cone

Synaptic

ending

Outer

segment

Inner

segment

αβγ

G protein

(transducin)

Figure 15.17

SensoryReceptor Cells of the Retina

(a) Rod cell. (b) Cone cell. (c) An enlargementofthe discs in the outer segment. (d ) An enlargement of one of the discs, showing the relation of rhodopsin and a

gated Na

⫹

channelto the membrane.

(a)

(b)

(c)

(d)

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stimulate ganglionic cells to generate action potentials.The num-

ber of Na

⫹

channels that close and the degree to which they close

is proportional to the amount oflight exposure.

At the ﬁnal stage of this light-initiated reaction, retinal is

completely released from the opsin.This free retinal may then be

converted back to vitamin A,from which it was originally derived.

The total vitamin A/retinal pool is in equilibrium so that under

normal conditions the amount offree retinal is relatively constant.

To create more rhodopsin,the altered retinal must be converted

back to its original shape,a reaction that requires energy. Once the

retinal resumes its original shape,its recombination with opsin is

spontaneous,and the newly formed rhodopsin can again respond

to light.

Light and dark adaptation is the adjustment of the eyes to

changes in light.Adaptation to light or dark conditions, which oc-

curs when a person comes out ofa darkened building into the sun-

light or vice versa, is accomplished by changes in the amount of

available rhodopsin.In bright light excess rhodopsin is broken down

so that not as much is available to initiate action potentials,and the

eyes become “adapted”to bright light. Conversely, in a dark room

more rhodopsin is produced,making the retina more light-sensitive.

Chapter 15 The Special Senses 519

PREDICT

Ifbreakdown of rhodopsin occurs rapidlyand production is slow, do

eyesadapt more rapidly to light or dark conditions?

Light and dark adaptation also involves pupil reﬂexes.The

pupil enlarges in dim light to allow more light into the eye and con-

tracts in bright light to allow less light into the eye.In addition, rod

function decreases and cone function increases in light conditions,

and vice versa during dark conditions. This occurs because rod

cells are more sensitive to light than cone cells and because

rhodopsin is depleted more rapidly in rods than in cones.

Cones

Color vision and visual acuity are functions ofcone cells. Color is a

function of the wavelength of light, and each color results from a

certain wavelength within the visible spectrum.Even though rods

are very sensitive to light,they cannot detect color, and sensory in-

put that ultimately reaches the brain from these cells is interpreted

by the brain as shades ofg ray.Cones require relatively bright light

to function.As a result, as the light decreases, so does the color of

objects that can be seen until, under conditions of very low

1. Retinal (in an inactive

configuration called II-

cis

is attached inside opsin to

make rhodopsin.

2. Light causes opsin to change

shape, and retinal changes

shape from II-

cis

-retinal to

all-

trans

-retinal. This activated

rhodopsin also activates the

attached G protein (called

transducin), which closes

channels, resulting in

hyperpolarization of the cell.

3. All-

trans

-retinal detaches from

opsin.

4. All-

trans

-retinal is converted

to II-

cis

-retinal, a process that

requires energy.

5. II-c

-retinal attaches to opsin,

which returns to its original

(dark) configuration

II-

cis

-retinal

II-

cis

-retinal

Opsin

Retinal

Rhodopsin

Cross section

Light

Transducin

(G protein)

inactive

Transducin

(G protein)

active

channels close

Cell hyperpolarization

Opsin

(dark configuration)

Opsin (light

configuration)

All-

trans

-retinal

All-

trans

-retinal

Energy (ATP)

αβγ

βγ

ProcessFigure 15.18

Rhodopsin Cycle

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illumination, the objects appear gray.This occurs because as the

light decreases,fewer cone cells respond to the dim light.

Cones are bipolar photoreceptor cells with a conical light-

sensitive part that tapers slightly from base to apex (see ﬁgure

15.17b).The outer segments of the cone cells, like those of the rods,

consist ofdouble-layered discs. The discs are slightly more numer-

Part3 Integration and ControlSystems520

ous and more closely stacked in the cones than in the rods.Cone

cells contain a visual pigment,iodopsin (ı¯-o¯-dop⬘sin), which con-

sists ofretinal combined with a photopigment opsin protein. Three

major types ofcolor-sensitive opsin exist: blue, red, and green; each

closely resembles the opsin proteins ofrod cells but with somewhat

different amino acid sequences. These color photopigments

Rod cell

(unstimulated)

Rhodopsin

(dark configuration)

Rhodopsin

(light configuration)

Gated Na

channel open

(dark configuration)

Gated Na

channel closed

(light configuration)

Transducin

(G protein)

inactive

Transducin

(G protein)

active

1. In the dark, the rod cell is unstimulated.

Rhodopsin is inactive and the attached

G protein, transducin, is also inactive.

Gated Na

channels are open and Na

diffuse into the rod cell.

2. Glutamate is constantly released from

the unstimulated rod cell.

3. The glutamate released from rod cells

inhibits bipolar cells from releasing

neurotransmitters so that ganglionic

cells, with which the bipolar cells

synapse, do not generate action

potentials.

1. In the light, the rod cell is stimulated.

Rhodopsin is activated and the

attached G protein, transducin, is

also activated. The activated

G protein causes gated Na

channels

to close and Na

is blocked from

entering the cell resulting in

hyperpolarization.

2. Glutamate release from the

stimulated rod cell decreases.

3. The bipolar cells, no longer inhibited,

release neurotransmitters, which

stimulate ganglionic cells to generate

action potentials.

Glutamate is

continuously

released

Bipolar cell

inhibited

Rod cell

(hyperpolarized)

Glutamate

release

decreases

Bipolar cell

no longer

inhibited

αβγ

βγ

– 35

Time (s)

Light pulse

Hyperpolarization

– 30

– 25

(mV)

ProcessFigure 15.19

Rod CellHyperpolarization

(a) Changesin the rod cell membrane potential following the opsin and retinalcell shape changes is a hyperpolarization. (b) Unstimulated rod cell (dark).

(c)Stimulated rod cell (light).

(a)

(b)

(c)

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Chapter 15 The Special Senses 521

function in much the same manner as rhodopsin, but whereas

rhodopsin responds to the entire spectrum of visible light, each

iodopsin is sensitive to a much narrower spectrum.

Most people have one red pigment gene and one or more

green pigment genes located in a tandem array on each X chromo-

some.An enhancer gene on the X chromosome apparently deter-

mines that only one color opsin gene is expressed in each cone cell.

Only the ﬁrst or second gene in the tandem array is expressed in

each cone cell,so that some cone cells express only the red pigment

gene and others express only one ofthe green pigment genes.

As can be seen in ﬁgure 15.20,although considerable overlap

occurs in the wavelength oflight to which these pigments are sen-

sitive,each pigment absorbs light of a certain range of wavelengths.

As light of a given wavelength,representing a certain color, strikes

the retina,all cone cells containing photopigments capable of re-

sponding to that wavelength generate action potentials.Because of

the overlap among the three types ofcones, especially between the

green and red pigments, different proportions of cone cells re-

spond to each wavelength,thus allowing color perception over a

wide range. Color is interpreted in the visual cortex as combina-

tions of sensory input originating from cone cells. For example,

when orange light strikes the retina,99% of the red-sensitive cones

respond, 42% of the green-sensitive cones respond,and no blue

cones respond.When yellow light strikes the retina,the response is

shifted so that a greater number ofgreen-sensitive cones respond.

The variety of combinations created allows humans to distinguish

several million gradations oflig ht and shades of color.

Seeing Red

Noteveryone sees the same red. Two forms of the red photopigment are

common in humans. Approximately60% of people have the amino acid

serine in position 180 ofthe red opsin protein, whereas 40% have

alanine in thatposition. That subtle difference in the protein results in

slightlydifferent absorption characteristics(see ﬁgure 15.20). Even

though we were each taughtto recognize red when we see a certain

color, we apparentlydon’t see that color in quite the same way. This

difference maycontribute to people having different favorite colors.

Distribution of Rodsand Cones in the Retina

Cones are involved in visual acuity,in addition to their role in color

vision. The fovea centralis is used when visual acuity is required,

such as for focusing on the words ofthis page. The fovea centralis

has about 35,000 cones and no rods.The 120 million rods are 20

times more plentiful than cones over most ofthe remaining retina,

however.They are more highly concentrated away from the fovea

and are more important in low-light conditions.

PREDICT

Explain whyat night a person may notice a movement “outof the

corner ofher eye,” but, when she tries to focus on the area where she

noticed the movement, itappears as though nothing is there.

InnerLayers of the Retina

The middle and inner nuclear layers of the retina consist of two

major types of neurons: bipolar and ganglion cells. The rod and

cone photoreceptor cells synapse with bipolar cells,which in turn

synapse with ganglion cells. Axons from the ganglion cells pass

over the inner surface ofthe retina (see ﬁgure 15.16), except in the

area of the fovea centralis,converge at the optic disc, and exit the

eye as the optic nerve(II). The fovea centralis is devoid of ganglion

cell processes,resulting in a small depression in this area; thus the

name fovea, meaning small pit. As a result of the absence of gan-

glion cell processes in addition to the concentration of cone cells

mentioned previously, visual acuity is further enhanced in the

fovea centralis because light rays don’t have to pass through as

many tissue layers before reaching the photoreceptor cells.

Rod and cone cells differ in the way they interact with bipo-

lar and ganglion cells.One bipolar cell receives input from numer-

ous rods,and one ganglion cell receives input from several bipolar

cells so that spatial summation of the signal occurs and the signal

is enhanced,thereby allowing awareness of stimulus from very dim

light sources but decreasing visual acuity in these cells.Cones, on

the other hand,exhibit little or no convergence on bipolar cells so

that one cone cell may synapse with only one bipolar cell.This sys-

tem reduces light sensitivity but enhances visual acuity.

Within the inner layers ofthe retina, association neurons

are present also,which modify the signals from the photorecep-

tor cells before the signal ever leaves the retina (see ﬁgure 15.16).

Horizontal cells form the outer plexiform layer and synapse

with photoreceptor cells and bipolar cells. Amacrine (am⬘a˘-

krin) cells form the inner plexiform layer and synapse with

bipolar and ganglion cells.Interplexiform cells form the bipo-

lar layer and synapse with amacrine,bipolar, and horizontal cells

Figure 15.20

Wavelengthsto Which Each of the Three

VisualPigments are Sensitive: Blue,

Green,Red

There are actuallytwo forms of the red pigment. One, found in 60% of the

population, hasa serine at position 180; and the other, found in 40% of the

population, hasan alanine at position 180. Each red pigment hasa slightly

differentwavelength sensitivity.

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to form a feedback loop.Association neurons are either excita-

tory or inhibitory on the cells with which they synapse.These as-

sociation cells enhance borders and contours,thereby increasing

the intensity at boundaries, such as the edge of a dark object

against a light background.

20. What is the function of the pigmented retina and of the

choroid?

21. Describe the changes that occur in a rod cell after light

strikesrhodopsin. How does rhodopsin re-form? Why is the

response of a rod cell to a stimulusunusual?

22. How do dark and light adaptation occur?

23. What are the three types of cone cells? How do they function

to produce the colorswe see?

24. Describe the arrangement of rods and cones in the fovea,

the macula lutea, and the peripheryof the eye.

25. Starting with a rod or cone cell, name the cells or structures

thatan action potential encounters while traveling to the

visual cortex.

Motion Pictures

Action potentialspass from the retina through the optic nerve at the rate

of20–25/s. We “see” a given image for a fraction of a second longer

than itactually appears. Motion pictures take advantage ofthese two

facts. When stillphotographs are ﬂashed on a screen at the rate of 24

framesper second, they appear to ﬂow into each other, and a motion

picture results.

NeuronalPathways for Vision

Objective

■ Outline the CNS pathway forvisual input, and describe

whathappens to images from each half of the visual ﬁelds.

The optic nerve (II) (ﬁgure 15.21) leaves the eye and exits the

orbit through the optic foramen to enter the cranial cavity.Just in-

side the vault and just anterior to the pituitary,the optic nerves are

connected to each other at the optic chiasm(kı¯⬘azm).Gang lion

cell axons from the nasal retina (the medial portion ofthe retina)

cross through the optic chiasm and project to the opposite side of

the brain.Ganglion cell axons from the temporal retina (the lateral

portion of the retina) pass through the optic nerves and project to

the brain on the same side ofthe body w ithout crossing.

Beyond the optic chiasm,the route of the ganglionic axons is

called the optic tract(see ﬁgure 15.21). Most of the optic tract ax-

ons terminate in the lateral geniculate nucleus of the thalamus.

Some axons do not terminate in the thalamus but separate from

the optic tract to terminate in the superior colliculi,the center for

visual reﬂexes (see chapter 13).Neurons of the lateral geniculate

ganglion form the ﬁbers of the optic radiations, which project to

thev isual cortex in the occipital lobe. Neurons of the visual cor-

tex integrate the messages coming from the retina into a single

message, translate that message into a mental image, and then

transfer the image to other parts of the brain,where it is evaluated

and either ignored or acted on.

Part3 Integration and ControlSystems522

The projections ofganglion cells from the retina can be related

to thevisual ﬁelds (see ﬁgure 15.21).The visual ﬁeld of one eye can

be evaluated by closing the other eye.Everything that can be seen with

the one open eye is the visual ﬁeld ofthat eye. The visual ﬁeld of each

eye can be divided into a temporal part (lateral) and a nasal part (me-

dial).In each eye, the temporal part of the visual ﬁeld projects onto

the nasal retina,whereas the nasal part of the visual ﬁeld projects to

the temporal retina.The projections and nerve pathways are arranged

in such a way that images entering the eye from the right part ofeach

visual ﬁeld project to the left side ofthe brain. Conversely,the left part

ofeach visual ﬁeld projects to the right side of the brain.

TunnelVision

Because the opticchiasm lies just anterior to the pituitary, a pituitary

tumor can putpressure on the optic chiasm and may resultin visual

defects. Because the nerve ﬁberscrossing in the optic chiasm are

carrying information from the temporalhalves of the visual ﬁelds, a

person with opticchiasm damage cannot see objects in the temporal

halvesof the visual ﬁelds, a condition called tunnelvision. Tunnel vision

isoften an early sign of a pituitary tumor.

PREDICT

The linesat A and B in the ﬁgure depict two lesionsin the visual

pathways. The effectof a lesion at Ain the optic radiations on the visual

ﬁeldsis depicted (with the right and left ﬁelds separated) in the ovals.

The blackareasindicate what parts of the visual ﬁelds are defective.

Describe the effectthat the lesion at B hason the visual ﬁelds (see

ﬁgure 15.21 for help).

Left visual

field

Right visual

field

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Chapter 15 The Special Senses 523

The visual ﬁelds of the eyes partially overlap (see ﬁgure

15.21).The region of overlap is the area of binocular vision, seen

with two eyes at the same time,and it is responsible for depth per-

ception, the ability to distinguish between near and far objects and

to judge their distance.Because humans see the same object with

both eyes,the image of the object reaches the retina of one eye at a

slightly different angle from that ofthe other. With experience, the

brain can interpret these differences in angle so that distance can

be judged quite accurately.

26. What is a visual ﬁeld? How do the visual ﬁelds project to the

brain?

27. Explain how binocular vision allows for depth perception.

Temporal

part

of left

visual

field

Nasal

part

of left

visual

field

Left eye

Lens

1. Each visual field is divided into a

temporal and nasal half.

2.After passing through the lens, light from

each half of a visual field projects to the

opposite side of the retina.

3. An optic nerve consists of axons

extending from the retina to the optic

chiasm.

4.In the optic chiasm, axons from the

nasal part of the retina cross and project

to the opposite side of the brain. Axons

from the temporal part of the retina do

not cross.

5.An optic tract consists of axons that have

passed through the optic chiasm (with or

without crossing) to the thalamus.

Visual

cortex

Superior

colliculi

Optic nerve

Optic

tracts

Optic chiasm

Lateral

geniculate

nuclei of

thalamus

Optic

radiations

Temporal

retina (lateral

part)

Nasal retina

(medial part)

6.The axons synapse in the lateral

geniculate nuclei of the thalamus.

Collateral branches of the axons in the

optic tracts synapse in the superior

colliculi.

7.An optic radiation consists of axons

from thalamic neurons that project to

the visual cortex.

8.(

) The right part of each visual field

(

dark green

and

light blue

) projects to

the left side of the brain, and the left

part of each visual field projects to the

right side of the brain (

light green

and

dark blue

Left visual field

Binocular

Right monocular

Left monocular

Temporal

part of left

visual field

Optic

nerves

Superior

colliculi

Optic

tracts

Lateral geniculate

nuclei of thalamus

Temporal

part of right

visual field

Optic

radiations

Visual

cortex

Optic nerve

Optic chiasm

Optic tract

Thalamus

Optic radiations

Visual cortex

Optic

chiasm

Occipital lobe

Nasal parts of

visual fields

ProcessFigure 15.21

VisualPathways

(a) Pathwaysfor the left eye (superior view). (b) Pathways for both eyes(superior view). (c) Overlap of the ﬁelds of vision (superior view). (d ) Photograph of the

visualnerves, tracts, and pathways (inferior view).

(a)

(b)

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Clinical Focus Eye Disorders

Myopia

Myopia (mı¯-o¯⬘pe¯-a˘), or nearsightedness, is

the ability to see close objectsclearly, but

distant objects appear blurry. Myopia isa

defectof the eye in which the focusing sys-

tem, the cornea and lens, is opticallytoo

powerful, or the eyeball is too long (axial

myopia). As a result, the focalpoint is too

near the lens, and the image isfocused in

frontof the retina (ﬁgure Aa).

Myopia iscorrected by a concave lens

that counters the refractive power of the

eye. Concave lenses cause the light rays

coming to the eye to diverge and are there-

fore called “minus” lenses(ﬁgure Ab).

Another technique for correcting my-

opia isradial keratotomy (ker⬘a˘-tot⬘o¯-me¯),

which consistsof making a series of four

to eight radiating cuts in the cornea. The

cuts are intended to slightly weaken the

dome of the cornea so that it becomes

more ﬂattened and eliminatesthe myopia.

One problem with the technique isthat it

is difficult to predict exactly how much

ﬂattening will occur. In one studyof 400

patients 5 years after the surgery, 55%

had normal vision, 28% were stillsome-

what myopic, and 17% had become hy-

peropic. Another problem is that some

patients are bothered by glare following

radialkeratotomy because the slits appar-

entlydon’t heal evenly.

An alternative procedure being investi-

gated islaser corneal sculpturing, in which

a thin portion ofthe cornea is etched away

to make the cornea lessconvex. The advan-

tage ofthis procedure is that the results can

be more accurately predicted than those

from radialkeratotomy.

Hyperopia

Hyperopia (hı¯-per-o¯⬘pe¯-a˘), or farsighted-

ness, is the ability to see distant objects

clearly, butclose objects appear blurry. Hy-

peropia is a disorder in which the cornea

and lenssystem is optically too weak or the

eyeball is too short. The image isfocused

behind the retina (ﬁgure Ac).

Hyperopia can be corrected byconvex

lensesthat cause light rays to converge as

they approach the eye (ﬁgure Ad). Such

lensesare called “plus” lenses.

Presbyopia

Presbyopia (prez-be¯-o¯⬘pe¯-a˘) is the normal,

presentlyunavoidable, degeneration of the

accommodation power of the eye thatoc-

curs as a consequence ofaging. It occurs

because the lens becomes sclerotic and

less ﬂexible. The eye is presbyopic when

the near point of vision hasincreased be-

yond 9 inches. The average age for onsetof

presbyopia is the midforties. Avid readers

or people engaged in ﬁne, close workmay

develop the symptomsearlier.

Presbyopia can be corrected bythe use

of “reading glasses” that are worn only

for close work and are removed when the

person wants to see at a distance. It’s

sometimesannoying to keep removing and

replacing glassesbecause reading glasses

hamper vision ofonly a few feet away. This

Convex lens corrects hyperopia

Hyperopia (farsightedness)

Part3 Integration and ControlSystems524

Figure A

VisualDisorders and Their Correction byVarious Lenses

FPis the focal point. (a) Myopia (nearsightedness). (b) Correction ofmyopia with a concave lens. (c) Hyperopia (farsightedness). (d ) Correction ofhyperopia

with a convexlens.

(c)

(d)

Myopia (nearsightedness) Concave lens corrects myopia

(a) (b)

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Chapter 15 The Special Senses 525

problem maybe corrected by the use of half

glasses, or bybifocals, which have a differ-

entlens in the top and the bottom.

Astigmatism

Astigmatism(a˘-stig⬘ma˘-tizm) isa type of re-

fractive error in which the qualityof focus is

affected. If the cornea or lens is notuni-

formlycurved, the light rays don’t focus at a

single pointbut fall as a blurred circle. Reg-

ular astigmatism can be corrected by

glasses that are formed with the opposite

curvature gradation. Irregular astigmatism

isa situation in which the abnormal form of

the cornea ﬁts no speciﬁc pattern and is

verydifﬁcult to correct with glasses.

Strabismus

Strabismus(stra-biz⬘mu˘s) isa lackof paral-

lelism oflight paths through the eyes. Stra-

bismus can involve only one eye or both

eyes, and the eyesmay turn in (convergent)

or out (divergent). In concomitantstrabis-

mus,the most common congenital type, the

angle between visual axes remains con-

stant, regardless of the direction of the

gaze. In noncomitantstrabismus, the angle

varies, depending on the direction of the

gaze, and deviatesas the gaze changes.

In some cases, the image thatappears

on the retina of one eye maybe consider-

ably different from that appearing on the

other eye. This problem iscalled diplopia

(di-plo¯⬘pe¯-a˘, -double vision) and isoften the

resultof weak or abnormal eye muscles.

RetinalDetachment

Retinaldetachment is a relatively common

problem that can result in complete blind-

ness. The integrityof the retina depends on

the vitreous humor, which keepsthe retina

pushed againstthe other tunics of the eye. If

a hole or tear occursin the retina, ﬂuid may

accumulate between the sensory and pig-

mented retina, therebyseparating them. This

separation, or detachment, maycontinue un-

tilthe sensory retina has become totally de-

tached from the pigmented retina and folded

into a funnel-like form around the opticnerve.

When the sensoryretina becomes separated

from itsnutrient supply in the choroid, it de-

generates, and blindnessfollows. Causes of

retinaldetachment include a severe blow to

the eye or head; a shrinking of the vitreous

humor, which mayoccur with aging; or dia-

betes. The space between the sensoryand

pigmented retina, called the subretinal

space, isalso important in keeping the retina

from detaching, aswell as in maintaining the

health of the retina. The space containsa

gummy substance that glues the sensory

retina to the pigmented retina.

Color Blindness

Color blindnessresults from the dysfunc-

tion of one or more of the three photopig-

ments involved in color vision. If one

pigmentis dysfunctional and the other two

are functional, the condition is called

dichromatism.An example of dichromatism

isred-green color blindness (ﬁgure B).

The genes for the red and green pho-

topigmentsare arranged in tandem on the X

chromosome, which explains why color

blindnessis over eight times more common

in malesthan in females (see chapter 29).

Six exonsexist for each gene. The red

and green genes are 96%–98% identical

and, asa result, the exonsmay be shufﬂed to

form hybrid genesin some people. Some of

the hybrid genes produce proteins with

nearly normal function, butothers do not.

Exon 5 is the most criticalfor determining

normal red-green function. If the ﬁfth exon

from a green gene replaces a red pigment

gene that has the ﬁfth exon, the protein

made from the gene responds to wave-

lengths more toward the green pigment

range. The person hasa red perception deﬁ-

ciencyand is notable to distinguish between

red and green. If the ﬁfth exon from a red

gene replacesa green pigment gene that has

the ﬁfth exon, the protein made from the

gene respondsto wavelengths more toward

the red pigment range. The person has a

green perception deﬁciencyand is also not

able to distinguish between red and green.

Apparentlyonly about 3 of the over 360

amino acidsin the color opsin proteins(those

atpositions 180 in exon 3 and those at 277

and 285 in exon 5) are keyto determining

their wavelength absorption characteristics.

Ifthose amino acids are altered by hydroxyla-

tion, the absorption shiftstoward the red end

ofthe spectrum. If they are not hydroxylated,

the absorption shiftstoward the green end.

NightBlindness

Everyone seesless clearly in the darkthan in

the light. A person with night blindness,

however, maynot see wellenough in a dimly

litenvironment to function adequately. Pro-

gressive night blindnessresults from gen-

eral retinal degeneration. Stationary night

blindness results from nonprogressive ab-

normalrod function. Temporary night blind-

nesscan result from a vitamin A deﬁciency.

Patientswith night blindness can now

be helped with special electronic optical

devices. These include monocular pocket

scopesand binocular goggles that electron-

icallyamplify light.

Continued

Figure B

Color BlindnessCharts

(a) A person with normalcolor vision can see the number 74, whereasa person with red-green color

blindnesssees the number 21. (b) A person with normal color vision can see the number 42. A person

with red color blindnesssees the number 2, and a person with green color blindnesssees the number 4.

Reproduced from Ishihara’s Tests for Colour Blindnesspublished by Kanehara & Co., Ltd., Tokyo, Japan, but tests for

color blindness cannot be conducted with this material. For accurate testing, the original plates should be used.

(a)

(b)

Seeley−Stephens−Tate:

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Part3 Integration and ControlSystems526

Glaucoma

Glaucoma(ﬁgure Ca) is a disease of the eye

involving increased intraocular pressure

caused by a buildup of aqueous humor. It

usuallyresults from blockage of the aqueous

veins or the canal of Schlemm, restricting

drainage ofthe aqueous humor, or from over-

production of aqueous humor. Ifuntreated,

glaucoma can lead to retinal, opticdisc, and

opticnerve damage. The damage resultsfrom

the increased intraocular pressure, which is

sufﬁcientto close off the blood vessels, caus-

ing starvation and death ofthe retinal cells.

Glaucoma isone of the leading causes

ofblindness in the United States, affecting

2% ofpeople over age 35, and accounting

for 15% of all blindness. Fifty thousand

people in the United Statesare blind as the

result of glaucoma, and it occurs three

times more often in blackpeople than in

white people. The symptomsinclude a slow

closing in ofthe ﬁeld of vision. No pain or

rednessoccurs, nor do light ﬂashes occur.

Glaucoma hasa strong hereditary ten-

dencybut may develop after surgery or with

the use ofcertain eyedrops containing corti-

sone. Everyone older than 40 should be

checked every 2–3 years for glaucoma;

those older than 40 who have relativeswith

glaucoma should have an annualcheckup.

During a checkup, the ﬁeld ofvision and the

opticnerve are examined. Ocular pressures

can also be measured. Glaucoma isusually

treated with eyedrops, which do notcure

the problem butkeep it from advancing. In

some cases, laser or conventionalsurgery

maybe used.

Cataract

Cataract (ﬁgure Cb) is a clouding of the lens

resulting from a buildup ofproteins. The lens

relieson the aqueous humor for its nutrition.

Any lossof this nutrient source leads to de-

generation ofthe lensand, ultimately, opacity

ofthe lens (i.e., a cataract). A cataractmay oc-

cur with advancing age, infection, or trauma.

A certain amountof lensclouding occurs

in 65% ofpatients older than 50 and 95% of

patients older than 65. The decision of

whether to remove the cataractdepends on

the extentto which light passage is blocked.

Over 400,000 cataracts are removed in the

United Stateseach year. Surgery to remove a

cataract isactually the removal of the lens.

The posterior portion of the lens capsule is

left intact. Although the cornea can stillac-

complish light convergence, with the lens

gone, the rayscannotbe focused as well, and

an artiﬁciallens must be supplied to help ac-

complish focusing. In mostcases, an artiﬁcial

lensis implanted into the remaining portion

ofthe lens capsule at the time that the natu-

rallens is removed. The implanted lenshelps

to restore normalvision, but glasses may be

required for near vision.

Macular Degeneration

Macular degeneration (ﬁgure Cc) is very

common in older people. Itdoes not cause

total blindness but results in the loss of

acute vision. Thisdegeneration has a variety

of causes, including hereditary disorders,

infections, trauma, tumor, or most often,

poorlyunderstood degeneration associated

with aging. Because no satisfactorymedical

treatmenthas been developed, optical aids,

such asmagnifying glasses, are used to im-

prove visualfunction.

Figure C

Defectsin Vision

Visualimages as seen with variousdefects in

vision. (a) Glaucoma. (b)Cataract. (c) Macular

degeneration. (d) Diabeticretinopathy.

Continued

Diabetes

Loss of visual function isone of the most

common consequencesof diabetes because

a major complication ofthe disease is dys-

function ofthe peripheral circulation. Defec-

tive circulation to the eye mayresult in retinal

degeneration or detachment. Diabeticretinal

degeneration (ﬁgure Cd) isone of the leading

causesof blindness in the United States.

Infections

Trachoma(tr a˘-ko¯⬘ma˘) is the leading cause

of blindnessworldwide. It is caused by an

intracellular microbialinfection (Chlamydia

trachomatis)of the corneal epithelial cells,

resulting in scar tissue formation in the

cornea. The bacteria are spread from one

eye to another eye by towels, ﬁngers, and

other objects. Five hundred million casesof

trachoma existin the world, and 7 million

people are blind or visuallyimpaired as a

resultof it.

Neonatal gonorrheal ophthalmia (of-

thal⬘-me¯-a˘) isa bacterial infection (Neisse-

ria gonorrhoeae) of the eye that causes

blindness. If the mother has gonorrhea,

which is a sexuallytransmitted disease of

the reproductive tract, the bacteria can in-

fect the newborn during delivery. The dis-

ease can be prevented by treating the

infant’s eyes with silver nitrate, tetracy-

cline, or erythromycin drops.

(a) (b)

Seeley−Stephens−Tate:

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Hearing and Balance

Objectives

■ Describe the structuresthat are part of the external, middle,

and innerears.

■ Explain how the parts of the earare able to convert sound

wavesinto action potentials.

■ Describe the auditory pathways in the CNS.

■ Describe the static and kineticlabyrinths, and explain how

theyfunction in balance.

■ Outline the CNS pathways forbalance.

The organs of hearing and balance are divided into three

parts:external, middle, and inner ears (ﬁgure 15.22). The external

and middle ears are involved in hearing only,whereas the inner ear

functions in both hearing and balance.

Theexternal ear includes the aur icle (aw⬘ri-kl; ear) and the

external auditory meatus (me¯-a¯⬘tu˘s;the passageway from the

outside to the eardrum). The external ear terminates medially at

theeardrum, ortympanic (tim-pan⬘ik)membrane. The middle

earis an air-ﬁlled space within the petrous portion of the tempo-

ral bone,which contains the auditory ossicles. The inner ear con-

tains the sensory organs for hearing and balance. It consists of

interconnecting ﬂuid-ﬁlled tunnels and chambers within the

petrous portion ofthe temporal bone.

AuditoryStructures and Their Functions

External Ear

The auricle,or pinna (pin⬘a˘), is the ﬂeshy part of the external ear

on the outside ofthe head and consists primarily of elastic cartilage

covered with skin (ﬁgure 15.23).Its shape helps to collect sound

waves and direct them toward the external auditory meatus.The

external auditory meatus is lined with hairsand ceruminous(se˘-

roo⬘mi-nu˘s) glands, which produce cerumen, a modiﬁed sebum

commonly called earwax.The hairs and cerumen help prevent for-

eign objects from reaching the delicate eardrum.Overproduction

ofcerumen, however, may block the meatus.

The tympanic membrane, or eardrum,is a thin, semitrans-

parent,nearly oval, three-layered membrane that separates the ex-

ternal ear from the middle ear.It consists of a low,simple cuboidal

epithelium on the inner surface and a thin stratiﬁed squamous

epithelium on the outer surface,with a layer of connective tissue

between.Sound waves reaching the tympanic membrane through

the external auditory meatus cause it to vibrate.

TympanicMembrane Rupture

Rupture ofthe tympanic membrane results in deafness. Foreign objects

thrustinto the ear, pressure, or infections of the middle ear can rupture

the tympanicmembrane. Sufﬁcient differential pressure between the

middle ear and the outside air can also cause rupture ofthe tympanic

membrane. Thiscan occur in ﬂyers, divers, or individuals who are hiton

the side ofthe head by an open hand.

Middle Ear

Medial to the tympanic membrane is the air-filled cavity of the

middle ear (see figure 15.22).Two covered openings, the round

and oval windows,on the medial side of the middle ear separate

it from the inner ear.Two openings provide air passages from

the middle ear.One passage opens into the mastoid air cells in

the mastoid process of the temporal bone. The other passage-

way,the auditory, or eustachian (u¯-sta¯⬘shu˘n) tube, opens into

the pharynx and equalizes air pressure between the outside air

and the middle ear cavity.Unequal pressure between the middle

ear and the outside environment can distort the eardrum,

dampen its vibrations,and make hearing difficult. Distortion of

the eardrum,which occurs under these conditions, also stimu-

lates pain fibers associated with it. Because of this distortion,

when a person changes altitude,sounds seem muffled, and the

eardrum may become painful.These symptoms can be relieved

by opening the auditory tube to allow air to pass through the

auditory tube to equalize air pressure. Swallowing, yawning,

chewing,and holding the nose and mouth shut while gently try-

ing to force air out of the lungs are methods used to open the

auditory tube.

The middle ear contains three auditory ossicles:the malleus

(mal⬘e¯-u˘s;hammer), incus (ing⬘ku˘s;anvil), and stapes (sta¯⬘pe¯z;

stirrup),which transmit vibrations from the ty mpanic membrane

to the oval window.The handle of the malleus is attached to the

inner surface of the tympanic membrane, and vibration of the

membrane causes the malleus to vibrate as well. The head of the

malleus is attached by a very small synovial joint to the incus,

which in turn is attached by a small synovial joint to the stapes.The

foot plate ofthe stapes ﬁts into the oval window and is held in place

by a ﬂexible annular ligament.

Chapter 15 The Special Senses 527

Seeley−Stephens−Tate:

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III. Integration and Control

Systems

15. The Special Senses

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Chorda Tympani

A structure thatstudents might be somewhat surprised to ﬁnd in the

middle ear isthe chorda tympani. It’s a branch ofthe facial nerve carrying

taste impulsesfrom the anterior two-thirds of the tongue. It crosses over

the inner surface ofthe tympanic membrane (see ﬁgures 15.22 and

15.29). The chorda tympani hasnothing to do with hearing but isjust

passing through. Thisnerve can be damaged, however, during ear surgery

or bya middle ear infection, resulting in loss of taste sensation from the

anterior two-thirdsof the tongue on the side innervated by that nerve.

Part3 Integration and ControlSystems528

InnerEar

The tunnels and chambers inside the temporal bone are called the

bony labyrinth (lab⬘i-rinth; a maze; ﬁgure 15.24). Because the

bony labyrinth consists oftunnels within the bone, it cannot easily

be removed and examined separately.The bony labyrinth is lined

with periosteum,and when the inner ear is shown separately (ﬁg-

ure 15.25a), the periosteum is what is depicted.Inside the bony

labyrinth is a similarly shaped but smaller set ofmembranous tun-

nels and chambers called the membranous labyrinth. The mem-

branous labyrinth is ﬁlled with a clear ﬂuid called endolymph,and

the space between the membranous and bony labyrinth is ﬁlled

with a ﬂuid called perilymph. Perilymph is very similar to cere-

brospinal ﬂuid,but endolymph has a high concentration of potas-

sium and a low concentration of sodium,which is opposite from

perilymph and cerebrospinal ﬂuid.

The bony labyrinth is divided into three regions: cochlea,

vestibule,and semicircular canals. The vestibule (ves⬘ti-bool) and

semicircular canals are involved primarily in balance, and the

cochlea (kok⬘le¯-a˘) is involved in hearing. The membranous

labyrinth of the cochlea is divided into three parts: the scala

vestibuli,the scala tympani, and the cochlear duct.

The oval window communicates with the vestibule ofthe in-

ner ear,which in turn communicates with a cochlear chamber,the

scala vestibuli (ska¯⬘la˘ ves-tib⬘u¯-le¯; see ﬁgure 15.25a). The scala

Auricle

Vestibulocochlear

nerve

Cochlear nerve

Inner

ear

Vestibule

Cochlea

Round window

Facial nerve

Semicircular

canals

Oval

window

Tympanic

membrane

Chorda

tympani

Auditory tube

Auditory ossicles

in the middle ear

Malleus Incus Stapes

Temporal

bone

External

auditory

meatus

External ear

Figure 15.22

External, Middle, and Inner Ear

Helix

Antitragus

Lobule

Tragus

External

auditory

meatus

Figure 15.23

Structuresof the Auricle (the Right Ear)

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Chapter 15 The Special Senses 529

vestibuli extends from the oval window to the helicotrema(hel⬘i-

ko¯-tre¯⬘ma˘; a hole at the end of a helix or spiral) at the apex of the

cochlea; a second cochlear chamber,the scala tympani (tim⬘pa˘-

ne¯), extends from the helicotrema, back from the apex, parallel to

the scala vestibuli,to the membrane of the round window.

The scala vestibuli and the scala tympani are the

perilymph-ﬁlled spaces between the walls of the bony and mem-

branous labyrinths.A layer of simple squamous epithelium is at-

tached to the periosteum of the bone surrounding each of these

chambers.The wall of the membranous labyrinth that bounds the

scala vestibuli is called the vestibular membrane (Reissner’s

membrane);the wall of the membranous labyrinth bordering the

scala tympani is the basilar membrane (ﬁgure 15.25b and c). The

space between the vestibular membrane and the basilar membrane

is the interior of the membranous labyrinth and is called the

cochlear duct orscala media, which is ﬁlled with endolymph.

The vestibular membrane consists ofa double layer of squa-

mous epithelium and is the simplest region of the membranous

labyrinth.The vestibular membrane is so thin that it has little or no

mechanical effect on the transmission ofsound waves through the

inner ear; therefore, the perilymph and endolymph on the two

sides of the vestibular membrane can be thought of mechanically

as one ﬂuid.The role of the vestibular membrane is to separate the

two chemically different ﬂuids.The basilar membrane is somewhat

more complex and is ofmuch greater physiologic interest in rela-

tion to the mechanics ofhearing. It consists of an acellular portion

with collagen ﬁbers,ground substance, and sparsely dispersed elas-

tic ﬁbers and a cellular part with a thin layer ofvascular connective

tissue that is overlaid with simple squamous epithelium.

The basilar membrane is attached at one side to the bony

spiral lamina, which projects from the sides of the modiolus

(mo¯⬘dı¯⬘o¯-lus), the bony core of the cochlea, like the threads of a

screw,and at the other side to the lateral wall of the bony labyrinth

by thespir al ligament, a local thickening of the periosteum. The

distance between the spiral lamina and the spiral ligament (i.e.,the

width of the basilar membrane) increases from 0.04 mm near the

oval window to 0.5 mm near the helicotrema.The collagen ﬁbers of

the basilar membrane are oriented across the membrane between

the spiral lamina and the spiral ligament,somewhat like the strings

of a piano. The collagen ﬁbers near the oval window are both

shorter and thicker than those near the helicotrema.The diameter

of the collagen ﬁbers in the membrane decreases as the basilar

membrane widens.As a result, the basilar membrane near the oval

window is short and stiff, and responds to high-frequency vibra-

tions, whereas that part near the helicotrema is wide and limber

and responds to low-frequency vibrations.

The cells inside the cochlear duct are highly modiﬁed to

form a structure called the spiral organ, or the organ of Corti

(ﬁgure 15.25band c). The spiral organ contains supporting epithe-

lial cells and specialized sensory cells called hair cells,which have

hairlike projections at their apical ends.In children, these projec-

tions consist of one cilium (kinocilium) and about 80 very long

microvilli,often referred to as stereocilia; but in adults the cilium

is absent from most hair cells (ﬁgures 15.25dand 15.26). The hair

Semicircular

canals

Membranous labyrinth

Membranous

labyrinth

Oval

window

Round

window

Cochlea

Vestibule

Cross section

through semicircular

canal

Cross section

through the

cochlea

Endolymph

Fibrous bands

Perilymph

Bony

labyrinth

Bony labyrinth

Periosteum

(boundary of

bony labyrinth)

Periosteum

(boundary of

bony labyrinth)

Bone

Figure 15.24

The Inner Ear: Bonyand Membranous Labyrinths

The crosssections are taken through a semicircular canal and the cochlea to show the relationship between the bonyand membranous labyrinths.

Seeley−Stephens−Tate:

Anatomy and Physiology,

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15. The Special Senses

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cells are arranged in four long rows extending the length of the

cochlear duct.The tips of the hairs are embedded within an acellu-

lar gelatinous shelf called the tectorial (tek-to¯r⬘e¯-a˘l)membrane,

which is attached to the spiral lamina.

Hair cells have no axons,but the basilar regions of each hair

cell are covered by synaptic terminals ofsensory neurons, the cell

bodies of which are located within the cochlear modiolus and are

grouped into a cochlear, or spiral ganglion (see ﬁgures 15.25b

and 15.31). Afferent ﬁbers of these neurons join to form the

cochlear nerve. This nerve then joins the vestibular nerve to be-

Part3 Integration and ControlSystems530

come the vestibulocochlear nerve(VIII), which traverses the in-

ternal auditory meatus and enters the cranial vault.

28. Name the three regions of the ear, and list each region’s

parts.

29. Describe the relationship between the tympanic membrane,

the earossicles, and the oval window of the ear.

30. What is the function of the external auditory meatus and of

the auditorytube?

31. Explain how the cochlear duct is divided into three

compartments. Whatis found in each compartment?

Semicircular

canals

Vestibule

Oval window

Round window

Cochlea

Helicotrema

Cochlear ganglion

Cochlear

nerve

Scala vestibuli (filled

with perilymph)

Vestibular membrane

Tectorial membrane

Cochlear duct (filled

with endolymph)

Spiral ligament

Basilar membrane

Scala tympani (filled

with perilymph)

Spiral lamina

Membranous

labyrinth

Periosteum of

bone (inner lining

of bony labyrinth)

Vestibular membrane

Tectorial membrane

Microvilli

Spiral lamina

Hair cell

Nerve endings of

cochlear nerve

Spiral

ligament

Basilar

membrane

Cochlear duct

Cochlear nerve

Hair cell

Spiral

organ

Supporting

cells

Figure 15.25

Structure ofthe Cochlea

(a)The inner ear. The outer surface (gray) is the periosteum lining the inner surface of the bony labyrinth. (b) A crosssection of the cochlea. The outer layer is the

periosteum lining the inner surface ofthe bony labyrinth. The membranous labyrinth isvery small in the cochlea and consists of the vestibular and basilar

membranes. The space between the membranousand bony labyrinth consistsof two parallel tunnels: the scala vestibuli and scala tympani. (c) An enlarged section

ofthe cochlear duct (membranous labyrinth). (d) A greatly enlarged individual sensory hair cell.

(a)

(b)

(c)

(d)

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Chapter 15 The Special Senses 531

Auditory Function

Vibration of matter such as air,water, or a solid material creates

sound.No sound occurs in a vacuum. When a person speaks, the

vocal cords vibrate,causing the air passing out of the lungs to vi-

brate.The vibrations consist of bands of compressed air followed

by bands of less compressed air (ﬁgure 15.27a).These vibrations

are propagated through the air as sound waves,somewhat like rip-

ples are propagated over the surface ofwater.Volume, or loudness,

is a function of wave amplitude, or height, measured in decibels

(ﬁgure 15.27b).The greater the amplitude, the louder is the sound.

Pitchis a function of the wave frequency (i.e., the number of waves

or cycles per second) measured in hertz (Hz) (ﬁgure 15.27c).The

higher the frequency,the hig her the pitch.The normal range of

human hearing is 20–20,000 Hz and 0 or more decibels (db).

Sounds louder than 125 db are painful to the ear.

Human Speech and Hearing Impairment

The range ofnormal human speech is 250–8000 Hz. Thisis the range

thatis tested for the possibility of hearing impairment because it’sthe

mostimportant for communication.

Figure 15.26

Scanning Electron Micrograph ofCochlear

Hair CellMicrovilli

Lower frequency

(lower pitch)

Lower amplitude

(lower volume)

Sound wave

Compressed

air

Compressed

air

Compressed

air

Less

compressed

air

One cycle

Time

Tuning fork

Time

Amplitude

Time

Amplitude

Amplitude (volume)

Higher frequency

(higher pitch)

Less

compressed

air

Higher amplitude

(higher volume)

Figure 15.27

Sound Waves

(a) Each sound wave consistsof a region of compressed air between two regionsof less compressed air (blue bars). The sigmoid waves correspond to the regions of

more compressed air (peaks) and lesscompressed air (troughs). The green shadowed arearepresents the width of one cycle (distance between peaks). When

something like a tuning forkor vocal cordsvibrate, the movements of the object alternate between compressing the air and decompressing the air, or making the air

lesscompressed, thus producing sound. (b) Depicts low- and high-volume sound waves. Compare the relative lengthsof the arrows indicating the wave height

(amplitude). (c) Depictslower and higher pitch sound. Compare the relative number ofpeaks (frequency) within a given time interval (between arrows).

(a)

(b)

(c)

10,000x

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Timbre (tam⬘br,tim⬘br) is the resonance quality or overtones

of a sound.A smooth sigmoid curve is the image of a “pure”sound

wave, but such a wave almost never exists in nature.The sounds

made by musical instruments or the human voice are not smooth

sigmoid curves but rather are rough, jagged curves formed by nu-

Part3 Integration and ControlSystems532

merous, superimposed curves of various amplitudes and frequen-

cies.The roughness of the curve accounts for the timbre. Timbre al-

lows one to distinguish between,for example, an oboe and a French

horn playing a note at the same pitch and volume.The steps involved

in hearing are listed in table 15.2 and are illustrated in ﬁgure 15.28.

External

auditory

meatus

Tympanic

membrane

Malleus

Incus

Round

window

Auditory tube

Stapes

Oval window

Scala vestibuli

Scala tympani

Space between

bony labyrinth

and membranous

labyrinth (contains

perilymph)

Cochlear duct

(contains endolymph)

Vestibular

membrane

Basilar

membrane

Tectorial

membrane

Cochlear nerve

Membranous

labyrinth

Spiral organ

Helicotrema

1. Sound waves strike the tympanic membrane and cause it to vibrate.

2.Vibration of the tympanic membrane causes the three bones of the

middle ear to vibrate.

3.The foot plate of the stapes vibrates in the oval window.

4.Vibration of the foot plate causes the perilymph in the scala vestibuli

to vibrate.

5.Vibration of the perilymph causes displacement of the basilar mem-

brane. Short waves (high pitch) cause displacement of the basilar

membrane near the oval window, and longer waves (low pitch) cause

displacement of the basilar membrane some distance from the oval

window. Movement of the basilar membrane is detected in the hair cells

of the spiral organ, which are attached to the basilar membrane.

6.Vibrations of the perilymph in the scala vestibuli and of the endolymph in

the cochlear duct are transferred to the perilymph of the scala tympani.

7.Vibrations in the perilymph of the scala tympani are transferred to the

round window, where they are dampened.

ProcessFigure 15.28

Effectof Sound Waves on Cochlear Structures

Table 15.2

1. The auricle collects sound waves that are then conducted

through the external auditory meatus to the tympanic

membrane, causing it to vibrate.

2. The vibrating tympanic membrane causes the malleus, incus, and

stapes to vibrate.

3. Vibration of the stapes produces vibration in the perilymph of

the scala vestibuli.

4. The vibration of the perilymph produces simultaneous vibration

of the vestibular membrane and the endolymph in the cochlear

duct.

5. Vibration of the endolymph causes the basilar membrane to

vibrate.

6. As the basilar membrane vibrates, the hair cells attached to the

membrane move relative to the tectorial membrane, which remains

stationary.

7. The hair cell microvilli, embedded in the tectorial membrane,

become bent.

8. Bending of the microvilli causes depolarization of the hair cells.

9. The hair cells induce action potentials in the cochlear neurons.

10. The action potentials generated in the cochlear neurons are

conducted to the CNS.

11. The action potentials are translated in the cerebral cortex and are

perceived as sound.

Steps Involved in Hearing

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Chapter 15 The Special Senses 533

External Ear

The auricle collects sound waves that are then conducted through

the external auditory meatus toward the tympanic membrane.

Sound waves travel relatively slowly in air,332 m/s, and a signiﬁ-

cant time interval may elapse between the time a sound wave

reaches one ear and the time that it reaches the other.The brain can

interpret this interval to determine the direction from which a

sound is coming.

Middle Ear

Sound waves strike the tympanic membrane and cause it to vibrate.

This vibration causes vibration of the three ossicles of the middle

ear,and by this mechanical linkage vibration is transferred to the

oval window.More force is required to cause vibration in a liquid

like the perilymph of the inner ear than is required in air;thus, the

vibrations reaching the perilymph must be ampliﬁed as they cross

the middle ear.The footplate of the stapes and its annular ligament,

which occupy the oval window,are much smaller than the tympanic

membrane.Because of this size difference,the mechanical force of

vibration is ampliﬁed about 20-fold as it passes from the tympanic

membrane,through the ossicles, and to the oval window.

Two small skeletal muscles are attached to the ear ossicles

and reﬂexively dampen excessively loud sounds (ﬁgure 15.29).This

sound attenuation reﬂexprotects the delicate ear structures from

damage by loud noises. The tensor tympani (ten⬘so¯r tim⬘pa˘ne¯)

muscle is attached to the malleus and is innervated by the trigemi-

nal nerve (V). The stapedius (sta¯-pe¯⬘de¯-u˘s) muscle is attached to

the stapes and is supplied by the facial nerve (VII).The sound at-

tenuation reﬂex responds most effectively to low-frequency sounds

and can reduce by a factor of100 the energy reaching the oval win-

dow.The reﬂex is too slow to prevent damage from a sudden noise,

such as a gunshot,and it cannot function effectively for longer than

about 10 minutes,in response to prolonged noise.

PREDICT

Whateffect does facial nerve damage have on hearing?

InnerEar

As the stapes vibrates, it produces waves in the perilymph of the

scala vestibuli (see ﬁgure 15.28).Vibrations of the perilymph are

transmitted through the thin vestibular membrane and cause si-

multaneous vibrations of the endolymph.The mechanical effect is

as though the perilymph and endolymph were a single ﬂuid. Vi-

bration of the endolymph causes distortion of the basilar mem-

brane.Waves in the perilymph ofthe scala vestibuli are transmitted

also through the helicotrema and into the scala tympani.Because

the helicotrema is very small,however,this transmitted vibration is

probably of little consequence. Distortions of the basilar mem-

brane, together with weaker waves coming through the heli-

cotrema, cause waves in the scala tympani perilymph and

ultimately result in vibration of the membrane of the round win-

dow.Vibration of the round window membrane is important to

hearing because it acts as a mechanical release for waves from

within the cochlea.If this window were solid,it would reﬂect the

waves, which would interfere with and dampen later waves.The

round window also allows relief of pressure in the perilymph be-

cause ﬂuid is not compressible, thereby preventing compression

damage to the spiral organ.

The distortion ofthe basilar membrane is most important to

hearing. As this membrane distorts, the hair cells resting on the

basilar membrane move relative to the tectorial membrane,which

remains stationary.The hair cell microvilli,which are embedded in

the tectorial membrane, become bent, causing depolarization of

the hair cells. The hair cells then induce action potentials in the

cochlear neurons that synapse on the hair cells,apparently by di-

rect electrical excitation through electrical synapses rather than by

neurotransmitters.

Superior ligament

of malleus

Posterior ligament

of incus

Incus

Chorda tympani

nerve

Stapedius muscle

Posterior

Head of malleus

Anterior ligament

of malleus

Handle of malleus

Tensor tympani

muscle

Auditory tube

Tympanic membrane

Stapes

Figure 15.29

Musclesof the Middle Ear

Medialview of the middle ear (as though viewed from the inner ear), showing the three ear ossicleswith their ligaments and the two muscles of the middle ear: the

tensor tympani and the stapedius.

Seeley−Stephens−Tate:

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The hairs ofthe hair cells are bathed in endolymph. Because

of the difference in the potassium and sodium ion concentrations

between the perilymph and endolymph,an approximately 80 mV

potential exists across the vestibular membrane between the two

ﬂuids.This is called the endocochlear potential. Because the hair

cell hairs are surrounded by endolymph,the hairs have a greater

electric potential than if they were surrounded by perilymph. It’s

believed that this potential difference makes the hair cells much

more sensitive to slight movement than they would be if sur-

rounded by perilymph.

The part of the basilar membrane that distorts as a result of

endolymph vibration depends on the pitch of the sound that cre-

ated the vibration and, as a result, on the vibration frequency

within the endolymph.The width of the basilar membrane and the

length and diameter of the collagen ﬁbers stretching across the

membrane at each level along the cochlear duct determine the lo-

cation of the optimum amount of basilar membrane vibration

produced by a given pitch (ﬁgure 15.30).Higher-pitched tones

cause optimal vibration near the base, and lower-pitched tones

cause optimal vibration near the apex ofthe basilar membr ane.As

the basilar membrane vibrates, hair cells along a large part of the

basilar membrane are stimulated.In areas of minimum vibration,

the amount ofstimulation may not reach threshold. In other areas,

a low frequency of afferent action potentials may be transmitted,

whereas in the optimally vibrating regions of the basilar mem-

brane,a high frequency of action potentials is initiated.

Loud Noisesand Hearing Loss

Prolonged or frequentexposure to excessively loud noises can cause

degeneration ofthe spiral organ at the base of the cochlea, resulting in

high-frequencydeafness. The actual amount of damage can varygreatly

from person to person. High-frequencyloss can cause a person to miss

hearing consonantsin a noisy setting. Loud music, ampliﬁed to 120 db,

can impair hearing. The defectsmay not be detectable on routine

diagnosis, butthey include decreased sensitivity to sound in speciﬁc

narrow frequencyranges and a decreased ability to discriminate

between two pitches. Loud music, however, isnot asharmful as is the

sound ofa nearby gunshot, which is a sudden sound occurring at140

db. The sound istoo sudden for the attenuation reﬂex to protect the

inner ear structures, and the intensityis great enough to cause auditory

damage. In fact, gunshotnoise is the most common recreationalcause

ofserious hearing loss.

Afferent action potentials conducted by cochlear nerve ﬁbers

from all along the spiral organ terminate in the superior olivary

nucleus in the medulla oblongata (ﬁgure 15.31; see chapter 13).

These action potentials are compared to one another,and the

strongest action potential,corresponding to the area of maximum

basilar membrane vibration, is taken as standard.Efferent action

potentials then are sent from the superior olivary nucleus back to

the spiral organ to all regions where the maximum vibration did

not occur.These action potentials inhibit the hair cells from initi-

ating additional action potentials in the sensory neurons. Thus,

only action potentials from regions of maximum vibration are

received by the cortex,where they become consciously perceived.

Part3 Integration and ControlSystems534

By this process,tones are localized along the cochlea.As a re-

sult of this localization, neurons along a given portion of the

cochlea send action potentials only to the cerebral cortex in re-

sponse to speciﬁc pitches.Action potentials near the base of the

basilar membrane stimulate neurons in a certain part of the audi-

tory cortex,which interpret the stimulus as a high-pitched sound,

whereas action potentials from the apex stimulate a different part

ofthe cortex, which interprets the stimulus as a low-pitched sound.

PREDICT

Suggestsome possible sites and mechanismsto explain why certain

people have “perfectpitch” and other people are “tone deaf.”

Sound volume,or loudness, is a function of sound wave am-

plitude. As high-amplitude sound waves reach the ear,the peri-

lymph,endolymph, and basilar membrane vibrate more intensely,

and the hair cells are stimulated more intensely.As a result of the

increased stimulation, more hair cells send action potentials at a

higher frequency to the cerebral cortex,where this information is

perceived as a greater sound volume.

32. Starting with the auricle, trace a sound wave into the inner

earto the point at which action potentials are generated in

the cochlearnerve.

PREDICT

Explain whyit’s much easier to perceive subtle musicaltones when

musicis played somewhat softly as opposed to very loudly.

NeuronalPathways for Hearing

The special senses of hearing and balance are both transmitted by

the vestibulocochlear (VIII) nerve.The term vestibular refers to the

vestibule of the inner ear,which is involved in balance. The term

cochlear refers to the cochlea and is that portion of the inner ear

7000 Hz

5000 Hz

4000

3000

1500

200

1000

800

600

Apex

Base

20,000 Hz

Figure 15.30

Effectof Sound Waves on Points Along the

Basilar Membrane

Pointsof maximum vibration along the basilar membrane resulting from

stimulation bysounds of various frequencies (in hertz).

Seeley−Stephens−Tate:

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Chapter 15 The Special Senses 535

involved in hearing.The vestibulocochlear nerve functions as two

separate nerves carrying information from two separate but closely

related structures.

The auditory pathways within the CNS are very complex,

with both crossed and uncrossed tracts (see ﬁgure 15.31).Uni-

lateral CNS damage therefore usually has little effect on hearing.

The neurons from the cochlear ganglion synapse with CNS neu-

rons in the dorsal or ventral cochlear nucleus in the superior

medulla near the inferior cerebellar peduncle.These neurons in

turn either synapse in or pass through the superior olivary nu-

cleus.Neurons terminating in this nucleus may synapse with ef-

ferent neurons returning to the cochlea to modulate pitch

perception.Nerve ﬁbers from the superior olivary nucleus also

project to the trigeminal (V) nucleus,which controls the tensor

tympani, and the facial (VII) nucleus, which controls the

stapedius muscle.This reﬂex pathway dampens loud sounds by

initiating contractions ofthese muscles. This is the sound atten-

uation reﬂex described previously.Neurons synapsing in the su-

perior olivary nucleus may also join other ascending neurons to

the cerebral cortex.

Ascending neurons from the superior olivary nucleus travel

in the lateral lemniscus.All ascending ﬁbers synapse in the infe-

rior colliculi,and neurons from there project to the medial genic-

ulate nucleusof the thalamus, where they synapse with neurons

that project to the cortex.These neurons terminate in the auditory

cortex in the dorsal portion of the temporal lobe within the lateral

ﬁssure and, to a lesser extent,on the superolateral surface of the

temporal lobe (see chapter 13).Neurons from the inferior collicu-

lus also project to the superior colliculus,where reﬂexes that turn

the head and eyes in response to loud sounds are initiated.

33. Describe the neuronal pathways for hearing from the

cochlearnerve to the cerebral cortex.

Balance

The organs of balance are divided structurally and functionally

into two parts.The ﬁrst, the static labyrinth, consists of the ut ri-

cle(oo⬘tri-kl) and saccule (sak⬘u¯l) ofthe vestibule and is primarily

involved in evaluating the position ofthe head relative to gravity,

although the system also responds to linear acceleration or

Thalamus

Auditory

cortex

Auditory

cortex

Inferior colliculus

Superior olivary

nucleus

Medial

geniculate

nucleus

Cochlear

ganglion

Nerve to

tensor

tympani

Cochlear

nucleus

Nerve to

stapedius

1. Sensory axons from the cochlear

ganglion terminate in the cochlear

nucleus in the brainstem.

2. Axons from the neurons in the

cochlear nucleus project to the

superior olivary nucleus or to the

inferior colliculus.

3. Axons from the inferior colliculus

project to the medial geniculate

nucleus of the thalamus.

4. Thalamic neurons project to the

auditory cortex.

5. Neurons in the superior olivary

nucleus send axons to the inferior

colliculus, back to the inner ear, or to

motor nuclei in the brainstem that

send efferent fibers to the middle ear

muscles.

ProcessFigure 15.31

CentralNervous System Pathways for Hearing

Seeley−Stephens−Tate:

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Clinical Focus Deafnessand Functional Replacement of the Ear

Deafness can have many causes. In gen-

eral, two categoriesof deafness exist: con-

duction and sensorineural (or nerve)

deafness. Conduction deafness involves a

mechanical deﬁciency in transmission of

sound waves from the external ear to the

spiralorgan and may often be corrected sur-

gically. Hearing aidshelp people with such

hearing deﬁcienciesby boosting the sound

volume reaching the ear. Sensorineural

deafnessinvolves the spiral organ or nerve

pathwaysand is more difﬁcult to correct.

Research iscurrently being conducted

on ways to replace the hearing pathways

with electric circuits. One approach in-

volves the direct stimulation of nerves by

electricimpulses. There has been consider-

able successin the area of cochlear nerve

stimulation. Certain typesof sensorineural

deafnessin which the hair cellsof the spiral

organ are impaired can now be partiallycor-

rected. Prostheses are available that con-

sist of a microphone for picking up the

initial sound waves, a microelectronic

processor for converting the sound into

electric signals, a transmission system for

relaying the signalsto the inner ear, and a

long, slender electrode thatis threaded into

the cochlea. Thiselectrode delivers electric

signals directly to the endings of the

Antenna

Contacts

Transmitter

Receiver

Electrode

Cochlea rotated to

show bipolar contacts

touching spiral organ

1.A receiver, transmitter, and antenna

are implanted under the skin near

the auricle.

2.A small lead from the transmitter is

fed through the external auditory

meatus, tympanic membrane, and

middle ear into the cochlea.

3.In the cochlea, the cochlear nerve can

be directly stimulated by electric

impulses from the receiver.

deceleration,such as when a person is in a car that is increasing or

decreasing speed.The second, the kinetic labyrinth, is associated

with the semicircular canals and is involved in evaluating move-

ments ofthe head.

Most of the utricular and saccular walls consist of simple

cuboidal epithelium. The utricle and saccule,however, each con-

tain a specialized patch of epithelium about 2–3 mm in diameter

Part3 Integration and ControlSystems536

called the macula (mak⬘u¯-la˘;ﬁgure 15.32a and b). The macula of

the utricle is oriented parallel to the base ofthe skull, and the mac-

ula ofthe saccule is perpendicular to the base of the skull.

The maculae resemble the spiral organ and consist ofcolumnar

supporting cells and hair cells.The “hairs”of these cells, which consist

of numerous microvilli, called stereocilia,and one cilium, called a

kinocilium (kı¯-no¯-sil⬘e¯-u˘m),are embedded in a gelatinous mass

Figure D

Cochlear Implant

cochlear nerve (ﬁgure D). High-frequency

sounds are picked up bythe microphone

and transmitted through speciﬁccircuits to

terminate near the oval window, whereas

low-frequency sounds are transmitted far-

ther up the cochlea to cochlear nerve end-

ingsnear the helicotrema.

Research iscurrently underway to de-

velop implantsdirectly into the cochlear nu-

cleus of the brainstem for patients with

vestibulocochlear nerve damage. These im-

plantshave electrodes of various lengths to

stimulate parts of the cochlear nucleus, at

variousdepths from the surface, which re-

spond to soundsof different frequencies.

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Chapter 15 The Special Senses 537

weighted by the presence ofotoliths (o¯⬘to¯-liths) composed of protein

and calcium carbonate (ﬁgure 15.32b).The gelatinous mass moves in

response to gravity,bending the hair cells and initiating action poten-

tials in the associated neurons. Deﬂection of the hairs toward the

kinocilium results in depolarization of the hair cell, whereas deﬂec-

tion of the hairs away from the kinocilium results in hyperpolariza-

tion ofthe hair cell. If the head is tipped, otoliths move in response to

gravity and stimulate certain hair cells (ﬁgure 15.33).The hair cells are

constantly being stimulated at a low level by the presence of the

otolith-weighted covering ofthe macula; but as this covering moves

in response to gravity,the pattern of intensity of hair cell stimulation

changes. This pattern of stimulation and the subsequent pattern of

action potentials from the numerous hair cells ofthe maculae can be

translated by the brain into speciﬁc information about head position

or acceleration.Much of this information is not perceived consciously

but is dealt with subconsciously.The body responds by making subtle

tone adjustments in muscles ofthe back and neck, which are intended

to restore the head to its proper neutral,balanced position.

The kinetic labyrinth (ﬁgure 15.34) consists ofthree semicir-

cular canalsplaced at nearly rig ht angles to one another,one lying

nearly in the transverse plane,one in the coronal plane, and one in

the sagittal plane (see chapter 1).The arrangement of the semicircu-

lar canals enables a person to detect movement in all directions.The

base ofeach semicircular canal is expanded into an ampulla (ﬁgure

15.34a).Within each ampulla,the epithelium is specialized to form a

crista ampullaris (kris⬘ta˘ am-pu¯-lar⬘u˘s). This specialized sensory

epithelium is structurally and functionally very similar to that ofthe

maculae.Each crista consists of a ridge or crest of epithelium with a

curved gelatinous mass, the cupula (koo⬘poo-la˘),suspended over

the crest. The hairlike processes of the crista hair cells,similar to

those in the maculae, are embedded in the cupula (ﬁgure 15.34b).

The cupula contains no otoliths and therefore doesn’t respond to

gravitational pull. Instead,the cupula is a ﬂoat that is displaced by

ﬂuid movements within the semicircular canals.Endolymph move-

ment within each semicircular canal moves the cupula, bends the

hairs,and initiates action potentials (ﬁgure 15.35).

As the head begins to move in a given direction,the en-

dolymph does not move at the same rate as the semicircular canals

(see ﬁgure 15.35). This difference causes displacement of the

cupula in a direction opposite to that ofthe movement of the head,

resulting in relative movement between the cupula and the en-

dolymph. As movement continues,the ﬂuid of the semicircular

canals begins to move and “catches up”with the cupula,and stim-

ulation is stopped. As movement of the head ceases, the en-

dolymph continues to move because of its momentum, causing

displacement of the cupula in the same direction as the head had

been moving.Because displacement of the cupula is most intense

when the rate of head movement changes, this system detects

changes in the rate of movement rather than movement alone.As

with the static labyrinth, the information obtained by the brain

from the kinetic labyrinth is largely subconscious.

34. What are the functions of the saccule and utricle? Describe

the macula and itsfunctions.

35. What is the function of the semicircular canals? Describe

the crista ampullarisand its mode of operation.

Utricle

Saccule

Vestibule

Utricular

macula

Saccular

macula

Gelatinous matrix

(otolithic membrane)

Otoliths

Kinocilium

Stereocilia

(microvilli)

Nerve fibers

of vestibular

nerve

Hair cell

Support cells

Part of

macula

Figure 15.32

Structure ofthe Macula

(a) Vestibule showing the location ofthe utricular and saccular maculae. (b) Enlargementof the utricular macula, showing hair cells and otoliths in the macula.

(c)An enlarged hair cell, showing the kinocilium and stereocilia.

(a)

(b)

(c)

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Space Sickness

Space sicknessis a balance disorder occurring in zero gravityand

resulting from unfamiliar sensoryinput to the brain. The brain must

adjustto these unusual signals, or severe symptoms like headaches and

dizzinessmay result. Space sicknessis unlike motion sickness in that

motion sicknessresults from an excessive stimulation ofthe brain,

whereasspace sickness results from too little stimulation asa result of

weightlessness.

NeuronalPathways for Balance

Neurons synapsing on the hair cells ofthe maculae and cristae am-

pullares converge into the vestibular ganglion, where their cell

Part3 Integration and ControlSystems538

bodies are located (ﬁgure 15.36).Sensor y ﬁbers from these neu-

rons join sensory ﬁbers from the cochlear ganglion to form the

vestibulocochlear nerve (VIII) and terminate in the vestibular nu-

cleus within the medulla oblongata. Axons run from this nucleus

to numerous areas ofthe CNS, such as the spinal cord, cerebellum,

cerebral cortex,and the nuclei controlling extrinsic eye muscles.

Balance is a complex process not simply conﬁned to one

type of input.In addition to vestibular sensory input, the vestibu-

lar nucleus receives input from proprioceptive neurons through-

out the body,and from the visual system. People are asked to close

their eyes while balance is evaluated in a sobriety test because al-

cohol affects the proprioceptive and vestibular components of

balance (cerebellar function) to a greater extent than it does the

visual portion.

Endolymph in

utricle

Gelatinous matrix

Hair cell

Supporting cell

Macula

Force of gravity

Vestibular nerve fibers

Figure 15.33

Function ofthe Vestibule in Maintaining Balance

(a) Asthe position of the head changes, such aswhen a person bends over, the maculae respond to changes in position of the head relative to gravity by moving in

the direction ofgravity. (b) In an upright position, the maculae don’t move.

(a) (b)

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Chapter 15 The Special Senses 539

Reﬂex pathways exist between the kinetic part ofthe vestibular

system and the nuclei controlling the extrinsic eye muscles (oculo-

motor,trochlear, and abducens). A reﬂex pathway allows mainte-

nance of visual ﬁxation on an object while the head is in motion.

This function can be demonstrated by spinning a person around

about 10 times in 20 seconds,stopping him or her,and observing eye

movements. The reaction is most pronounced if the individual’s

head is tilted forward about 30 degrees while spinning,thus bringing

the lateral semicircular canals into the horizontal plane.A slight os-

cillatory movement ofthe eyes occurs. The eyes track in the direction

of motion and return with a rapid recovery movement before re-

peating the tracking motion. This oscillation of the eyes is called

Ampullae

Vestibular

nerve

Hair

cell

Semicircular

canals

Cupula

Crista

ampullaris

Cupula

Nerve

fibers

to vestibular

nerve

Figure 15.34

Semicircular Canals

(a) Semicircular canalsshowing location of the crista ampullarisin the

ampullae ofthe semicircular canals. (b) Enlargement of the crista ampullaris,

showing the cupula and hair cells. (c) Enlargementof a hair cell.

Endolymph causes

movement of cupula

Cupula

Crista ampullaris

Hair cell

Movement of semicircular

canal with body movement

Endolymph in

semicircular canal

(a)

(b) (c)

Figure 15.35

Function ofthe Semicircular Canals

The crista ampullarisresponds to ﬂuid movements within the semicircular

canals. (a) When a person isat rest, the crista ampullarisdoes not move.

(b)As a person begins to move in a given direction, the semicircular canals

begin to move with the body(blue arrow), but the endolymph tends to remain

stationaryrelative to the movement (momentum force: red arrow pointing in

the opposite direction ofbody and semicircular canal movement), and the

crista ampullarisis displaced bythe endolymph in a direction opposite to the

direction ofmovement.

(a)

(b)

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nystagmus(nis-tag⬘mu˘s).Ifasked to walk in a straight line,the indi-

vidual deviates in the direction of rotation,and if asked to point to

an object,his or her ﬁnger deviates in the direction of rotation.

36. Describe the neuronal pathways for balance.

Effects of Aging on the

Special Senses

Objective

■ Describe changes that occur in the special senses with

aging.

Elderly people experience only a slight loss in the ability to

detect odors. However,the ability to correctly identify speciﬁc

odors is decreased,especially in men over age 70.

In general, the sense of taste decreases as people age. The

number ofsensor y receptors decreases and the ability of the brain

to interpret taste sensations declines.

Part3 Integration and ControlSystems540

Responses to taste change in some elderly people who are

ﬁghting cancer. One side effect of radiation treatment and

chemotherapy is the gastrointestinal discomfort resulting from the

treatments. The patients experience a loss of appetite because of

conditioned taste aversions resulting from treatment.

The lenses of the eyes lose ﬂexibility as a person ages because

the connective tissue ofthe lenses becomes more rigid. Consequently

there is ﬁrst a reduction and then an eventual loss in the ability ofthe

lenses to change shape.This condition, called presbyopia, is the most

common age-related change in the eyes.It is discussed more fully in

the Clinical Focus on “Eye Disorders”earlier in the chapter.

The most common visual problem in older people requiring

medical treatment,such as surgery, is the development of cataracts.

Macular degeneration is the second most common defect, glau-

coma is third,and diabetic retinopathy is fourth. These defects are

also described more fully in the Clinical Focus on “Eye Disorders.”

The number of cones decreases,especially in the fovea cen-

tralis. These changes cause a gradual decline in visual acuity and

color preception.

Vestibular

area

Posterior

ventral

nucleus

Trochlear motor

nucleus

Oculomotor

nucleus

Abducens motor

nucleus

Cerebellum

Vestibular

nerve

Vestibular

ganglion

Vestibular

nucleus

Spinovestibular

tract

Vestibulospinal

tract

Thalamus

1. Sensory axons from the vestibular

ganglion pass through the vestibular

nerve to the vestibular nucleus, which

also receives input from several other

sources, such as proprioception from

the legs.

2. Vestibular neurons send axons to the

cerebellum, which influences postural

muscles, and to the motor nuclei

(oculomotor, trochlear, and

abducens), which control extrinsic

eye muscles.

3. Vestibular neurons also send axons

to the posterior ventral nucleus of the

thalamus.

4. Thalamic neurons project to the

vestibular area of the cortex.

ProcessFigure 15.36

CentralNervous System Pathways for Balance

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

III. Integration and Control

Systems

15. The Special Senses

Companies, 2004

Clinical Focus Ear Disorders

Otosclerosis

Otosclerosis (o¯⬘to¯-skle¯-ro¯⬘sis) isan ear dis-

order in which spongybone grows over the

oval window and immobilizes the stapes,

leading to progressive lossof hearing. This

disorder can be surgically corrected by

breaking awaythe bony growth and the im-

mobilized stapes. During surgery, the

stapesis replaced by a small rod connected

bya fat pad or a synthetic membrane to the

ovalwindow at one end and to the incus at

the other end.

Tinnitus

Tinnitus(ti-nı¯⬘tu˘s) consists of noises such

asringing, clicking, whistling, or booming

in the ears. These noisesmay occur as a

result of disorders in the middle or inner

ear or along the central neuronal

pathways.

Motion Sickness

Motion sicknessconsists of nausea, weak-

ness, and other dysfunctions caused by

stimulation of the semicircular canalsdur-

ing motion, such asin a boat, automobile,

airplane, swing, or amusementpark ride. It

may progress to vomiting and incapacita-

tion. Antiemeticssuch as anticholinergic or

antihistamine medicationscan be taken to

counter the nausea and vomiting associated

with motion sickness. Scopolamine isan

anticholinergicdrug that reduces the excit-

ability of vestibular receptors. Cyclizine

(Marezine), dimenhydrinate (Dramamine),

and diphenhydramine (Benadryl) are anti-

histaminesthat affect the neural pathways

from the vestibule. Scopolamine can be ad-

ministered transdermally in the form ofa

patch placed on the skin behind the ear

(Transdermal-Scop). A patch lasts about

3days.

OtitisMedia

Infections of the middle ear, called otitis

media,are quite common in young children.

These infections usually result from the

spread ofinfection from the mucous mem-

brane of the pharynx through the auditory

tube to the mucouslining of the middle ear.

The symptomsof otitis media, consisting of

low-grade fever, lethargy, and irritability, are

often noteasily recognized by the parent as

signsof middle ear infection. The infection

can also cause a temporarydecrease or loss

ofhearing because ﬂuid buildup has damp-

ened the tympanicmembrane or ossicles.

Earache

Earache can result from otitis media, otitis

externa (inﬂammation ofthe external audi-

tory meatus), dental abscesses, or tem-

poromandibular jointpain.

Chapter 15 The Special Senses 541

As people age,the number of hair cells in the cochlea de-

creases.This decline doesn’t occur equally in both ears. As a re-

sult, because direction is determined by comparing sounds

coming into each ear,elderly people may experience a decreased

ability to localize the origin of certain sounds. In some people,

this may lead to a general sense of disorientation. In addition,

CNS defects in the auditory pathways can result in difﬁculty un-

derstanding sounds with echoes or background noise. Such

deﬁcit makes it difﬁcult for elderly people to understand rapid or

broken speech.

With age,the number of hair cells in the saccule, utricle, and

ampullae decrease.The number of otoliths also declines.As a re-

sult,elderly people experience a decreased sensitivity to gravity, ac-

celeration,and rotation. Because of these decreases, elderly people

experience dizziness (instability) and vertigo (a feeling of spin-

ning). They often feel that they can’t maintain posture and are

prone to fall.

37. Explain the changes in taste, vision, hearing, and balance

thatoccur with aging.

Olfaction

(p. 502)

Olfaction is the sense ofsmell.

OlfactoryEpithelium and Bulb

1. Olfactory neurons in the olfactory epithelium are bipolar neurons.

Their distal ends are enlarged as olfactory vesicles,which have long

cilia.The cilia have receptors that respond to dissolved substances.

2. At least seven (perhaps 50) primary odors exist.The olfactory

neurons have a very low threshold and accommodate rapidly.

NeuronalPathways of Olfaction

1. Axons from the olfactory neurons extend as olfactory nerves to the

olfactory bulb,where they synapse with mitral and tufted cells.

Axons from these cells form the olfactory tracts.Association

neurons in the olfactory bulbs can modulate output to the olfactory

tracts.

2. The olfactory tracts terminate in the olfactory cortex.The lateral

olfactory area is involved in the conscious perception ofsmell, the

intermediate area with modulating smell,and the medial area with

visceral and emotional responses to smell.

Taste

(p. 504)

Taste buds usually are associated with circumvallate,fungiform, and foli-

ate papillae.Filiform papillae do not have taste buds.

Histologyof Taste Buds

1. Taste buds consist ofsupport and gustatory cells.

2. The gustatory cells have gustatory hairs that extend into taste pores.

Function ofTaste

1. Receptors on the hairs detect dissolved substances.

2. Five basic types oftaste exist: sour, salty, bitter,sweet, and umami.

SUMMARY

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

III. Integration and Control

Systems

15. The Special Senses

Companies, 2004

NeuronalPathways for Taste

1. The facial nerve carries taste sensations from the anterior two-thirds

ofthe tongue, the glossopharyngeal nerve from the posterior one-

third ofthe tongue, and the vagus nerve from the epiglottis.

2. The neural pathways for taste extend from the medulla oblongata to

the thalamus and to the cerebral cortex.

VisualSystem

(p. 508)

AccessoryStructures

1. The eyebrows prevent perspiration from entering the eyes and help

shade the eyes.

2. The eyelids consist ofﬁve tissue layers. They protect the eyes from

foreign objects and help lubricate the eyes by spreading tears over

their surface.

3. The conjunctiva covers the inner eyelid and the anterior part ofthe

eye.

4. Lacrimal glands produce tears that ﬂow across the surface ofthe eye.

Excess tears enter the lacrimal canaliculi and reach the nasal cavity

through the nasolacrimal canal.Tears lubricate and protect the eye.

5. The extrinsic eye muscles move the eyeball.

Anatomyof the Eye

1. The ﬁbrous tunic is the outer layer ofthe eye. It consists of the sclera

and cornea.

• The sclera is the posterior four-ﬁfths of the eye. It is white

connective tissue that maintains the shape ofthe eye and provides

a site for muscle attachment.

• The cornea is the anterior one-ﬁfth of the eye. It is transparent and

refracts light that enters the eye.

2. The vascular tunic is the middle layer ofthe eye.

• The iris is smooth muscle regulated by the autonomic nervous

system.It controls the amount of light entering the pupil.

• The ciliary muscles control the shape of the lens. They are smooth

muscles regulated by the autonomic nervous system.The ciliary

process produces aqueous humor.

3. The retina is the inner layer ofthe eye and contains neurons

sensitive to light.

• The macula lutea (fovea centralis) is the area of greatest visual acuity.

• The optic disc is the location through which nerves exit and blood

vessels enter the eye.It has no photosensory cells and is therefore

the blind spot ofthe eye.

4. The eye has two compartments.

• The anterior compartment is ﬁlled with aqueous humor,which

circulates and leaves by way ofthe canal of Schlemm.

• The posterior compartment is ﬁlled with vitreous humor.

5. The lens is held in place by the suspensory ligaments,which are

attached to the ciliary muscles.

Functionsof the Complete Eye

1. Light is that portion of the electromagnetic spectrum that humans

can see.

2. When light travels from one medium to another,it can bend or

refract.Light striking a concave surface refracts outward

(divergence).Light striking a convex surface refracts inward

(convergence).

3. Converging light rays meet at the focal point and are said to be

focused.

4. The cornea,aqueous humor, lens, and vitreous humor all refract

light.The cornea is responsible for most of the convergence, whereas

the lens can adjust the focal point by changing shape.

• Relaxation of the ciliary muscles causes the lens to ﬂatten,

producing the emmetropic eye.

• Contraction of the ciliary muscles causes the lens to become more

spherical.This change in lens shape enables the eye to focus on

objects that are less than 20 feet away,a process called

accommodation.

Part3 Integration and ControlSystems542

5. The far point ofv ision is the distance at which the eye no longer has

to change shape to focus on an object.The near point of vision is

the closest an object can come to the eye and still be focused.

6. The pupil becomes smaller during accommodation,increasing the

depth offocus.

Structure and Function ofthe Retina

1. The pigmented retina provides a black backdrop for increasing

visual acuity.

2. Rods are responsible for vision in low illumination (night vision).

• A pigment, rhodopsin, is split by light into retinal and opsin,

producing hyperpolarization in the rod.

• Light adaptation is caused by a reduction of rhodopsin; dark

adaptation is caused by rhodopsin production.

3. Cones are responsible for color vision and visual acuity.

• Cones are of three types, each with a different photopigment. The

pigments are most sensitive to blue,red, and green lights.

• Perception of many colors results from mixing the ratio of the

different types ofcones that are active at a given moment.

4. Most visual images are focused on the fovea centralis,which has a

very high concentration ofcones. Moving away from the fovea,

fewer cones (the macula lutea) are present;mostly rods are in the

periphery ofthe retina.

5. The rods and the cones synapse with bipolar cells that in turn

synapse with ganglion cells,which form the optic nerves.

6. Association neurons in the retina can modify information sent to

the brain.

NeuronalPathways for Vision

1. Ganglia cell axons extend to the lateral geniculate ganglion ofthe

thalamus,where they synapse. From there neurons form the optic

radiations that project to the visual cortex.

2. Neurons from the nasal visual ﬁeld (temporal retina) ofone eye and

the temporal visual ﬁeld (nasal retina) ofthe opposite eye project to

the same cerebral hemisphere.Axons from the nasal retina cross in

the optic chiasm,and axons from the temporal retina remain

uncrossed.

3. Depth perception is the ability to judge relative distances ofan

object from the eyes and is a property ofbinocular vision. Binocular

vision results because a slightly different image is seen by each eye.

Hearing and Balance

(p. 527)

The osseous labyrinth is a canal system within the temporal bone that con-

tains perilymph and the membranous labyrinth.Endolymph is inside the

membranous labyrinth.

AuditoryStructures and Their Functions

1. The external ear consists ofthe auricle and external auditory

meatus.

2. The middle ear connects the external and inner ears.

• The tympanic membrane is stretched across the external auditory

meatus.

• The malleus, incus, and stapes connect the tympanic membrane to

the oval window ofthe inner ear.

• The auditory tube connects the middle ear to the pharynx and

functions to equalize pressure.

• The middle ear is connected to the mastoid air cells.

3. The inner ear has three parts:the semicircular canals; the vestibule,

which contains the utricle and the saccule;and the cochlea.

4. The cochlea is a spiral-shaped canal within the temporal bone.

• The cochlea is divided into three compartments by the vestibular

and basilar membranes.The scala vestibuli and scala tympani

contain perilymph.The cochlear duct contains endolymph and

the spiral organ (organ ofCorti).

• The spiral organ consists of hair cells that attach to the tectorial

membrane.

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

III. Integration and Control

Systems

15. The Special Senses

Companies, 2004

Chapter 15 The Special Senses 543

AuditoryFunction

1. Sound waves are funneled by the auricle down the external auditory

meatus,causing the tympanic membrane to vibrate.

2. The tympanic membrane vibrations are passed along the auditory

ossicles to the oval window ofthe inner ear.

3. Movement ofthe stapes in the oval window causes the perilymph,

vestibular membrane,and endolymph to vibrate, producing

movement ofthe basilar membrane. Movement of the basilar

membrane causes displacement ofthe hair cells in the spiral organ

and the generation ofaction potentials, which travel along the

vestibulocochlear nerve.

4. Some vestibulocochlear nerve axons synapse in the superior olivary

nucleus.Efferent neurons from this nucleus project back to the

cochlea,where they regulate the perception of pitch.

5. The round window protects the inner ear from pressure buildup

and dissipates waves.

NeuronalPathways for Hearing

1. Axons from the vestibulocochlear nerve synapse in the medulla.

Neurons from the medulla project axons to the inferior colliculi,

where they synapse.Neurons from this point project to the thalamus

and synapse.Thalamic neurons extend to the auditory cortex.

2. Efferent neurons project to cranial nerve nuclei responsible for

controlling muscles that dampen sound in the middle ear.

Balance

1. Static balance evaluates the position ofthe head relative to gravity

and detects linear acceleration and deceleration.

• The utricle and saccule in the inner ear contain maculae. The

maculae consist ofhair cells with the hairs embedded in a

gelatinous mass that contains otoliths.

• The gelatinous mass moves in response to gravity.

2. Kinetic balance evaluates movements ofthe head.

• Three semicircular canals at right angles to one another are

present in the inner ear.The ampulla of each semicircular canal

contains the crista ampullaris,which has hair cells with hairs

embedded in a gelatinous mass,the cupula.

• When the head moves,endolymph w ithin the semicircular canal

moves the cupula.

NeuronalPathways for Balance

1. Axons from the maculae and the cristae ampullares extend to the

vestibular nucleus ofthe medulla. Fibers from the medulla run to

the spinal cord,cerebellum, cortex,and nuclei that control the

extrinsic eye muscles.

2. Balance also depends on proprioception and visual input.

Effectsof Aging on the Special Senses

(p. 540)

Elderly people experience a decline in function ofall special functions:

olfaction,taste, vision, hearing, and balance. These declines can result in

loss ofappetite, visual impairment, disorientation, and risk of falling.

1. Olfactory neurons

a. have projections called cilia.

b. have axons that combine to form the olfactory nerves.

c. connect to the olfactory bulb.

d. have receptors that react with odorants dissolved in ﬂuid.

e. all of the above.

2. Which ofthese statements is not true with respect to olfaction?

a. Olfactory sensation is relayed directly to the cerebral cortex

without passing through the thalamus.

b. Olfactory neurons are replaced about every two months.

c. The lateral olfactory area of the cortex is involved in the

conscious perception ofsmell.

d. The medial olfactory area ofthe cortex is responsible for visceral

and emotional reactions to odors.

e. The olfactory cortex is in the occipital lobe of the cerebrum.

3. Gustatory (taste) cells

a. are found only on the tongue.

b. extend through tiny openings called taste buds.

c. have no axons but release neurotransmitter when stimulated.

d. have axons that extend directly to the taste area ofthe cerebral

cortex.

4. Which ofthese is not one of the basic tastes?

a. spicy

b. salt

c. bitter

d. umami

e. sour

5. Which ofthese types of papillae have no taste buds associated with

them?

a. circumvallate

b. ﬁliform

c. foliate

d. fungiform

6. Tears

a. are released onto the surface ofthe eye near the medial corner of

the eye.

b. in excess are removed by the canal ofSchlemm.

c. in excess can cause a sty.

d. can pass through the nasolacrimal duct into the oral cavity.

e. contain water,salts, mucus, and lysozyme.

7. The ﬁbrous tunic ofthe eye includes the

a. conjunctiva.

b. sclera.

c. choroid.

d. iris.

e. retina.

8. The ciliary body

a. contains smooth muscles that attach to the lens by suspensory

ligaments.

b. produces the vitreous humor.

c. is part of the iris of the eye.

d. is part ofthe sclera.

e. all of the above.

9. The lens normally focuses light onto the

a. optic disc.

b. iris.

c. macula lutea.

d. cornea.

e. ciliary body.

10. Given these structures:

1. lens

2. aqueous humor

3. vitreous humor

4. cornea

REVIEW AND COMPREHENSION

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

III. Integration and Control

Systems

15. The Special Senses

Companies, 2004

Choose the arrangement that lists the structures in the order that

light entering the eye encounters them.

a. 1,2,3,4

b. 1,4,2,3

c. 4,1,2,3

d. 4,2,1,3

e. 4,3,2,1

11. Aqueous humor

a. is the pigment responsible for the black color ofthe choroid.

b. exits the eye through the canal ofSchlemm.

c. is produced by the iris.

d. can cause cataracts ifoverproduced.

e. is composed ofproteins called cr ystallines.

12. Contraction of the smooth muscle in the ciliary body causes the

a. lens to ﬂatten.

b. lens to become more spherical.

c. pupil to constrict.

d. pupil to dilate.

13. Given these events:

1. medial rectus contracts

2. lateral rectus contracts

3. pupils dilate

4. pupils constrict

5. lens ofthe eye ﬂattens

6. lens ofthe eye becomes more spherical

Assume you are looking at an object 30 feet away.If you suddenly

look at an object that is 1 foot away,which events occur?

a. 1,3,6

b. 1,4,5

c. 1,4,6

d. 2,3,6

e. 2,4,5

14. Given these events:

1. bipolar cells depolarize

2. decrease in glutamate released from presynaptic terminals of

photoreceptor cells

3. light strikes photoreceptor cells

4. photoreceptor cells depolarized

5. photoreceptor cells hyperpolarized

Choose the arrangement that lists the correct order ofevents, starting

with the photoreceptor cells in the resting,nonactivated state.

a. 1,2,3,4,5

b. 2,4,3,5,1

c. 3,4,2,5,1

d. 4,3,5,2,1

e. 5,3,4,1,2

15. Given these neurons in the retina:

1. bipolar cells

2. ganglionic cells

3. photoreceptor cells

Choose the arrangement that lists the correct order ofthe cells

encountered by light as it enters the eye and travels toward the

pigmented retina.

a. 1,2,3

b. 1,3,2

c. 2,1,3

d. 2,3,1

e. 3,1,2

16. Which ofthese photoreceptor cells is not correctly matched with its

function?

a. rods—vision in low light

b. rods—visual acuity

c. cones—color vision

Part3 Integration and ControlSystems544

17. Concerning dark adaptation,

a. the amount ofr hodopsin increases.

b. the pupils constrict.

c. it occurs more rapidly than light adaptation.

d. all ofthe above.

18. In the retina there are cones that are most sensitive to a particular

color.Given this list of colors:

1. red

2. yellow

3. green

4. blue

Indicate which colors correspond to speciﬁc types ofcones.

a. 2,3

b. 3,4

c. 1,2,3

d. 1,3,4

e. 1,2,3,4

19. Given these areas of the retina:

1. macula lutea

2. fovea centralis

3. optic disc

4. periphery ofthe retina

Choose the arrangement that lists the areas according to the density

ofcones, starting with the area that has the highest density of cones.

a. 1,2,3,4

b. 1,3,2,4

c. 2,1,4,3

d. 2,4,1,3

e. 3,4,1,2

20. Concerning axons in the optic nerve from the right eye,

a. they all go to the right occipital lobe.

b. they all go to the left occipital lobe.

c. they all go to the thalamus.

d. some go to the right occipital lobe,and some go to the left

occipital lobe.

21. A lesion that destroyed the left optic tract ofa boy eliminates vision

in his

a. left nasal visual ﬁeld.

b. left temporal visual ﬁeld.

c. right temporal visual ﬁeld

d. both a and b.

e. both a and c.

22. A person with an abnormally long eyeball (anterior to posterior)

is and uses a to correct his or her

vision.

a. nearsighted,concave lens

b. nearsighted,convex lens

c. farsighted,concave lens

d. farsighted,convex lens

23. Which ofthese structures is found within or is a part of the external

ear?

a. oval window

b. auditory tube

c. ossicles

d. auricle

e. cochlear duct

24. Given these ear bones:

1. incus

2. malleus

3. stapes

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

III. Integration and Control

Systems

15. The Special Senses

Companies, 2004

Chapter 15 The Special Senses 545

Choose the arrangement that lists the ear bones in order from the

tympanic membrane to the ear.

a. 1,2,3

b. 1,3,2

c. 2,1,3

d. 2,3,1

e. 3,2,1

25. Given these structures:

1. perilymph

2. endolymph

3. vestibular membrane

4. basilar membrane

Choose the arrangement that lists the structures in the order sound

waves coming from the outside encounter them in producing sound.

a. 1,3,2,4

b. 1,4,2,3

c. 2,3,1,4

d. 2,4,1,3

e. 3,4,2,1

26. The spiral organ is found within the

a. cochlear duct.

b. scala vestibuli.

c. scala tympani.

d. vestibule.

e. semicircular canals.

27. An increase in the loudness ofsound occurs as a result of an

increase in the ofthe sound wave.

a. frequency

b. amplitude

c. resonance

d. both a and b

28. Interpretation ofdifferent sounds is possible because of the abilit y

ofthe to vibrate at different frequencies and

stimulate the .

a. vestibular membrane,vestibular nerve

b. vestibular membrane,spiral organ

c. basilar membrane,vestibular nerve

d. basilar membrane,spiral organ

29. Which structure is a specialized receptor found within the utricle?

a. macula

b. crista ampullaris

c. spiral organ

d. cupula

30. Damage to the semicircular canals affects the ability to detect

a. linear acceleration.

b. the position ofthe head relative to the ground.

c. the movement ofthe head in all directions.

d. all ofthe above.

Answers in Appendix F

1. Describe all the special sensations involved when a person picks up

an apple and bites into it.What types of receptors are involved?

Which aspects ofthe taste of the apple are actually taste and which

are olfaction?

2. An elderly man with normal vision develops cataracts.He is

surgically treated by removing the lenses ofhis eyes. What kind of

glasses would you recommend he wear to compensate for the

removal ofhis lenses?

3. Some animals have a reﬂective area in the choroid called the

tapetum lucidum.Light entering the eye is reﬂected back instead of

being absorbed by the choroid.What would be the advantage of this

arrangement? The disadvantage?

4. Perhaps you have heard someone say that eating carrots is good for

the eyes.What is the basis for this claim?

5. On a camping trip Jean Tights rips her pants.That evening she is

going to repair the rip.As the sun goes down, the light becomes

more and more dim.When she tries to thread the needle, it is

obvious that she is not looking directly at the needle but is looking a

few inches to the side.Why does she do this?

6. A man stares at a black clock on a white wall for several minutes.

Then he shifts his view and looks at only the blank white wall.

Although he is no longer looking at the clock,he sees a light clock

against a dark background.Explain what happened.

7. Describe the results ofa lesion of the optic chiasm.

8. Persistent exposure to loud noise can cause loss ofhearing,

especially for high-frequency sounds.What part of the ear is

probably damaged? Be as speciﬁc as possible.

9. Professional divers are subject to increased pressure as they descend

to the bottom ofthe ocean. Sometimes this pressure can lead to

damage to the ear and loss ofhearing. Describe the normal

mechanisms that adjust for changes in pressure,suggest some

conditions that might interfere with pressure adjustment,and

explain how the increased pressure might cause loss ofhearing.

10. Ifa vibr ating tuning fork is placed against the mastoid process of

the temporal bone,the vibrations are perceived as sound, even if the

external auditory meatus is plugged.Explain how this could happen.

Answers in Appendix G

CRITICAL THINKING

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

III. Integration and Control

Systems

15. The Special Senses

Companies, 2004

Part3 Integration and ControlSystems546

1. Inhaling slowly and deeply allows a large amount ofair to be drawn

into the olfactory recess,whereas not as much air enters during

normal breaths.Snifﬁng (rapid, repeated air intake) is effective for

the same reason.

2. Adaptation can occur at several levels in the olfactory system.First,

adaptation can occur at the receptor cell membrane,where receptor

sites are ﬁlled or become less sensitive to a speciﬁc odor.Second,

association neurons within the olfactory bulb can modify sensitivity

to an odor by inhibiting mitral cells or tufted cells.Third, neurons

from the intermediate olfactory area ofthe cerebrum can send

action potentials to the association neurons in the olfactory bulb to

inhibit further sensory action potentials.

3. Eyedrops placed into the eye tend to drain through the nasolacrimal

duct into the nasal cavity.Recall that much of what is considered

“taste”is actually smell. The medication is detected by the olfactory

neurons and is interpreted by the brain as taste sensation.Crying

produces extra tears,which are conducted to the nasal cavity,

causing a “runny”nose.

4. Inﬂammation ofthe cornea involves edema, the accumulation of

ﬂuid.Fluid accumulation in the cornea increases its water content,

and because water causes the proteoglycans to expand,the

transparency ofthe lens decreases, interfering with normal vision.

5. Eye strain,or eye fatigue, occurs primarily in the ciliary muscles. It

occurs because close vision requires accommodation.

Accommodation occurs as the ciliary muscles contract,releasing the

tension ofthe suspensory ligaments, and allowing the lens to

become more rounded.Continued close vision requires

maintenance ofaccommodation, which requires that the ciliary

muscles remain contracted for a long time,resulting in their fatigue.

6. Rhodopsin breakdown is associated with adaptation to bright light

and occurs rapidly,whereas rhodopsin production occurs slowly

and is associated with adaptation to conditions oflittle light. Eyes

adapt rather quickly to bright light but quite slowly to very dim

light.

7. Rod cells distributed over most ofthe retina are involved in both

peripheral vision (out ofthe corner of the eye) and vision under

conditions ofvery dim light. When attempting to focus directly on

an object,however, a person relies on the cones within the macula

lutea;although the cones are involved in visual acuity,they don’t

function well in dim light;thus the object may not be seen at all.

8. A lesion in the right optic nerve at Bresults in loss of vision in the

right visual ﬁeld (see following illustration).

9. The stapedius muscle,attached to the stapes, is innervated by the

facial nerve (VII).Loss of facial nerve function eliminates part of

the sound attenuation reﬂex,although not all of it, because the

tensor tympani muscle,innervated by the trigeminal nerve, is still

functional.A reduction in the sound attenuation reﬂex results in

sounds being excessively loud in the affected ear.A reduced reﬂex

can also leave the ear more susceptible to damage by prolonged loud

sounds.

10. “Perfect pitch”is the ability to precisely reproduce a pitch just by

being told its name or reading it on a sheet ofmusic, with no other

musical support,such as from piano accompaniment. This

remarkable talent as well as conditions such as tone deafness (the

complete inability to recognize or reproduce musical pitches) or a

decreased ability to perceive tone differences could occur at a

number oflocations. The structure of the basilar membrane may be

such that tones are not adequately spaced along the cochlear duct in

some people to facilitate clear separation oftones. The reﬂex from

the superior olive to the spiral organ may have a very narrow

“window offunction” for people with perfect pitch but may not be

functioning in some other people.The auditory cortex may not be

able to translate as accurately in some people to distinguish

differences in tones.

11. It is much easier to perceive subtle musical tones when music is

played somewhat softly as opposed to very loudly because loud

sounds have sound waves with a greater amplitude,which causes the

basilar membrane to vibrate more violently over a wider range.The

spreading ofthe wave in the basilar membrane to some extent

counteracts the reﬂex from the superior olive that is responsible for

enabling a person to hear subtle tone differences.

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