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12. Spinal Cord and Spinal

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Thecentral nervous system (CNS) con-

sistsof the brain and spinal cord, with

the division between these two partsof

the CNSplaced somewhat arbitrarily at

the levelof the foramen magnum. The pe-

ripheral nervous system (PNS)consists of

nerves and ganglia outside the cranial cavity

and vertebralcolumn (see chapter 11). Nerves are

bundlesof axonsand their schwann cells, surrounded by

connective tissue sheaths. Gangliaare accumulations of cell bodies in the PNS.

The PNSincludes 12 pairs of cranial nerves and 31 pairs of spinal nerves.

The CNSreceives sensory information, evaluates that information, stores

some information, and initiatesreactions. The PNS collectsinformation from nu-

meroussources both inside and outside the body and relays it through axonsof

sensoryneurons to the CNS. Axonsof motor neurons in the PNS relay information

from the CNS to variouspar ts ofthe body, primarily to muscles and glands,

therebyregulating activity in those structures.

The spinalcord and spinal nerves are described in this chapter. The brain

and cranialnerves are considered in the next chapter. The speciﬁc topics of this

chapter are the spinal cord(402), reﬂexes (405), spinal cord pathways(410),

structure of peripheral nerves(410), and spinal nerves (410).

Spinal Cord

and Spinal

Nerves

Colorized SEM of nerve fasciclescontaining

bundlesof axons.

CHAPTER

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12. Spinal Cord and Spinal

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Part3 Integration and ControlSystems402

cording to the portion of the vertebral column from which their

nerves enter and exit.The spinal cord gives rise to 31 pairs of spinal

nerves,which exit the vertebral column through the intervertebral

foramina (see ﬁgure 7.15).The nerves from the lower segments de-

scend some distance in the vertebral canal before they exit because

the spinal cord is shorter than the vertebral column.

The spinal cord is not uniform in diameter throughout its

length.It’s larger in diameter at its superior end, and it gradually

decreases in diameter toward its inferior end.Two enlargements

occur where nerves supplying the upper and lower limbs enter

and leave the cord (see ﬁgure 12.1).The cervical enlargement in

the inferior cervical region corresponds to the location where ax-

ons that supply the upper limbs enter and leave the cord.T he

lumbar enlargement in the inferior thoracic and superior lum-

bar regions is the site where the axons supplying the lower limbs

enter or leave the cord.

PREDICT

Whyis the cord enlarged in the cervical and lumbar areas?

Immediately inferior to the lumbar enlargement, the spinal

cord tapers to form a conelike region called the conus medullaris.

Its tip is the inferior end of the spinal cord and extends to the level

of the second lumbar vertebra. The nerves supplying the lower

limbs and other inferior structures of the body arise from the sec-

ond lumbar to the ﬁfth sacral nerves.They exit the lumbar enlarge-

ment and conus medullaris,course inferiorly through the vertebral

canal,and exit through the intervertebral foramina from the second

lumbar to the ﬁfth sacral vertebrae.The conus medullaris and the

numerous nerves extending inferiorly from it,within the vertebral

canal, resemble a horse’s tail and are therefore called the cauda

(kaw⬘da˘,tail) equina (e¯-kwı¯⬘na˘,horse; see ﬁgure 12.1).

Meningesof the SpinalCord

The spinal cord and brain are surrounded by connective tissue

membranes called meninges (me˘-nin⬘je¯z; figure 12.2). The

most superficial and thickest membrane is the dura mater

(doo⬘ra˘ ma¯⬘ter; tough mother). The dura mater surrounds the

spinal cord and is continuous with the epineurium ofthe spinal

nerves (discussed on p.410). The dura mater around the spinal

cord is separated from the periosteum of the vertebral canal by

the epidural space. This is a true space around the spinal cord

that contains blood vessels, areolar connective tissue,and fat.

Epidural anesthesiaof the spinal ner ves is induced by injecting

anesthetics into this space.Epidural anesthesia is often given to

women during childbirth.

The next meningeal membrane is a very thin,w ispy arach-

noid (a˘-rak⬘noyd; spiderlike; i.e., cobwebs) mater.The space be-

tween this membrane and the dura mater is the subdural space

and contains only a very small amount ofserous ﬂuid.

The third meningeal layer,the pia (pı¯⬘a˘;affectionate) mater

is bound very tightly to the surface ofthe brain and spinal cord. Be-

yond the conus medullaris,the pia mater forms the ﬁlum termi-

nale (f ı¯⬘lu˘m ter⬘mi-nal⬘e¯), a connective tissue ﬁlament, which

extends inferiorly to the coccyx where it anchors the spinal cord.

Spinal Cord

Objectives

■ Describe the general structure and location of the spinal

cord.

■ Describe the spinal cord in crosssection, and explain the

functionsof each area.

Thespinal cord is extremely important to the overall func-

tion of the nervous system.It is the communication link between

the brain and the PNS inferior to the head;it integrates incoming

information and produces responses through reﬂex mechanisms.

GeneralStructure

The spinal cord (ﬁgure 12.1) extends from the foramen magnum

to the level ofthe second lumbar vertebra. It’s considerably shorter

than the vertebral column because it doesn’t grow as rapidly as the

vertebral column during development. The spinal cord is com-

posed ofcervical, thoracic, lumbar, and sacral segments, named ac-

Rootlets of

spinal nerves

Spinal nerves

Cervical

enlargement

Lumbar

enlargement

Coccygeal

nerve

Conus

medullaris

Cauda

equina

Filum

terminale

Figure 12.1

SpinalCord and Spinal Nerve Roots

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Chapter 12 Spinal Cord and Spinal Nerves 403

Between the arachnoid mater and the pia mater is the subarach-

noid space, which contains weblike strands of the arachnoid

mater, blood vessels,and cerebrospinal (ser⬘e˘-bro¯-spı¯-na˘l,se˘-

re¯⬘bro¯-spı¯-nal) ﬂuid (CSF),which is described in chapter 13.

The spinal cord is held in place within the vertebral canal by

a series ofconnective tissue strands connecting the pia mater to the

dura mater.This causes the arachnoid mater to form points be-

tween the upper spinal nerves. Because the points create a

“toothed” appearance,these attachments are called denticulate

(den-tik⬘u¯-la¯t)ligaments (see ﬁgure 12.2).

Introduction ofNeedles into the Subarachnoid Space

Severalclinical procedures involve the insertion of a needle into the

subarachnoid space inferior to the levelof the second lumbar

vertebra. The needle doesn’tcontact the spinal cord because it

extendsonly approximately to the second lumbar vertebra of the

vertebralcolumn, but the subarachnoid space extends to level 52 of

the vertebralcolumn. Nor does the needle damage the nerves of the

cauda equina located in the subarachnoid space, because the needle

quite easilypushes the nerves aside. In spinal anesthesia, or spinal

block, drugsthat block action potentialtransmission are introduced

into the subarachnoid space to preventpain sensations in the lower

halfof the body. A spinal tap is the removal ofCSF from the

subarachnoid space. A spinaltap may be performed to examine the

CSFfor infectious agents (meningitis), for the presence of blood

(hemorrhage), or for the measurementof CSF pressure. A radiopaque

substance mayalso be injected into this area, and a myelogram

(radiograph ofthe spinal cord) may be taken to visualize spinalcord

defectsor damage.

CrossSection of the Spinal Cord

A cross section ofthe spinal cord reveals that the cord consists of a

peripheral white portion and a central gray portion (ﬁgure 12.3).

The white matter consists of myelinated axons forming nerve

tracts,and the gray matter consists of neuron cell bodies, dendrites,

and axons.An anterior median ﬁssure and a p osterior median

sulcus are deep clefts partially separating the two halves of the

cord.The white matter in each half of the spinal cord is organized

into three columns, or funiculi (fu¯-nik⬘u¯-lı¯),called the ventral

(anterior),dorsal (posterior),and lateral columns. Each column

is subdivided into nerve tracts or fasciculi (fa˘-sik⬘u¯-lı¯); also re-

ferred to as pathways.Individual axons ascending to the brain or

descending from the brain are usually grouped together within the

nerve tracts. Axons within a given nerve tract carry basically the

same type of information, although they may overlap to some ex-

tent.For example, one ascending nerve tract carries action poten-

tials related to pain and temperature sensations,whereas another

functions to carry action potentials related to light touch.

The central gray matter is organized into horns.Each half of

the central gray matter of the spinal cord consists of a relatively

thinposterior (dorsal) horn and a larger anter ior (ventral) horn.

Small lateral horns exist in levels of the cord associated with the

autonomic nervous system (see chapter 16).The two halves of the

spinal cord are connected by grayand white commissures (see

ﬁgure 12.3).The white and gray commissures contain axons that

cross from one side of the spinal cord to the other.The central

canalis in the center of the gray commissure.

Spinal nerves arise from numerous rootlets along the dorsal and

ventral surfaces ofthe spinal cord (see ﬁgure 12.3). About six to eight

ofthese rootlets combine to form each ventral root on the ventral (an-

terior) side ofthe spinal cord, and another six to eight form each dor-

sal root on the dorsal (posterior) side ofthe cord at each segment. The

ventral and dorsal roots join one another just lateral to the spinal cord

to form a spinal nerve.Each dorsal root contains a ganglion, called the

dorsal root,or spinal, ganglion (gang⬘gle¯-on;a swelling or knot).

Organization of Neuronsin the Spinal Cord and

Spinal Nerves

The cell bodies of sensory neurons are in the dorsal root ganglia

(ﬁgure 12.3c). The axons of these unipolar neurons extend from

various parts of the body and pass through spinal nerves to the

dorsal root ganglia. The axons do not synapse in the dorsal root

ganglion but pass through the dorsal root and project into the pos-

terior horn ofthe spinal cord gray matter. The axons either synapse

with interneurons in the posterior horn or pass into the white mat-

ter and ascend or descend in the spinal cord.

The cell bodies ofmotor neurons, which supply muscles and

glands, are located in the anterior and lateral horns of the spinal

cord gray matter (see ﬁgure 12.3c).Multipolar somatic motor neu-

rons are in the anterior horn,also called the motor horn, and auto-

nomic neurons are in the lateral horn. Axons from the motor

neurons form the ventral roots and pass into the spinal nerves.

Thus,dorsal roots contain sensory axons, ventral roots contain mo-

tor axons,and spinal nerves have both sensory and motor axons.

Dura mater

Pia mater

Denticulate

ligament

Subdural space

Epineurium of

spinal nerve

Subarachnoid

space

Dorsal root

ganglion

Spinal nerve

Arachnoid mater

Ventral root

Figure 12.2

MeningealMembranes Surrounding the Spinal

Cord

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Posterior horn

Posterior median

sulcus

Dorsal root

Posterior

horn

Central

canal

horn

Peripheral

white portion

Grey

commissure

White

commissure

median

fissure

Anterior horn

Lateral horn

Dorsal root

Ventral root

Dorsal root ganglion

Sensory neuron

Somatic motor neuron

Spinal nerve

Autonomic neuron

Posterior horn

Lateral horn

Anterior horn

Gray

matter

Dorsal (posterior) column

Ventral (anterior) column

Lateral column

White

matter

Rootlets

Gray commissure

Ventral

root

Spinal

nerve

Dorsal root

ganglion

Dorsal root

Central canal

White commissure

Anterior median fissure

Posterior median

sulcus

Figure 12.3

CrossSection of the Spinal Cord

(a) A 3-D drawing ofa segment of the spinal cord showing one dorsal and one ventralroot on each side and the rootlets that form them. (b) Photograph of a cross

section through the midlumbar region. The darker-colored areasare white matter, where tractsare located. The lighter area is gray matter, where neuron cellbodies

are located. (c) Relationship ofsensory and motor neurons to the spinal cord.

(a)

(b)

(c)

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Chapter 12 Spinal Cord and Spinal Nerves 405

1. Describe the cervical and lumbar enlargements of the

spinal cord, the conusmedullaris, and the cauda equina.

Howmany pairs of spinal nerves exit the spinal cord?

2. Name the meninges surrounding the spinal cord. What is

found within the epidural, subdural, and subarachnoid

spaces?

3. How is the spinal cord held within the vertebral canal?

4. Explain the arrangement of white matter in the spinal cord.

Whatare commissures?

5. Describe the spinal cord gray matter. Where are sensory,

somaticmotor, and autonomic neuron cell bodies located in

the graymatter?

6. Where do dorsal and ventral roots exit the spinal cord?

Whatkinds of axons are in the dorsal and ventral roots and

in the spinal nerves?

PREDICT

Explain whythe dorsal root ganglia are larger in diameter than the

dorsalroots.

Reﬂexes

Objective

■ Listthe components and characteristics of a reﬂex.

The basic structural unit ofthe ner vous system is the neuron.

Thereﬂex arc is the basic functional unit of the nervous system and

is the smallest,simplest portion capable of receiving a stimulus and

producing a response. The reﬂex arc has ﬁve basic components:

(1) a sensory receptor,(2) a sensor y neuron,(3) an interneuron,

(4)a motor neuron, and (5) an effector organ (ﬁgure 12.4).

Action potentials initiated in sensory receptors are transmit-

ted along the axons ofsensor y neurons to the CNS,where the ax-

ons usually synapse with interneurons.Interneurons synapse with

motor neurons, which send axons out of the spinal cord and

through the PNS to muscles or glands,where the action potentials

of the motor neurons cause effector organs to respond. The re-

sponse produced by the reﬂex arc is called a reﬂex.It’s an auto-

matic response to a stimulus that occurs without conscious

thought.

Reﬂexes are,in general, homeostatic. Some function to re-

move the body from painful stimuli that would cause tissue dam-

age,and others function to keep the body from suddenly falling or

moving because of external forces. A number of reﬂexes are re-

sponsible for maintaining relatively constant blood pressure,blood

carbon dioxide levels,and water intake.

Individual reﬂexes vary in their complexity.Some involve

simple neuronal pathways and few or even no interneurons,

whereas others involve complex pathways and integrative centers.

Many are integrated within the spinal cord,and others are inte-

grated within the brain. Some reﬂexes involve excitatory neurons

and result in a response, such as when a muscle contracts (see

chapter 11).Other reﬂexes involve inhibitory neurons and result in

inhibition of a response, such as when a muscle relaxes.In addi-

tion,hig her brain centers inﬂuence reﬂexes by either suppressing

or exaggerating them.Major spinal cord reﬂexes include the stretch

reﬂex, the Golgi tendon reﬂex, the withdrawal reﬂex, and the

crossed extensor reﬂex.

Stretch Reﬂex

The simplest reﬂex is the stretch reﬂex(ﬁgure 12.5), a reﬂex in

which muscles contract in response to a stretching force applied

to them.The sensory receptor of this reﬂex is the muscle spin-

dle, which consists of 3–10 small, specialized skeletal muscle

cells. The cells are contractile only at their ends and are inner-

vated by speciﬁc motor neurons called gamma motor neurons

(the term gamma refers to motor neurons with small diameter

axons) originating from the spinal cord and controlling contrac-

tion of the ends of the muscle spindle cells. Sensory neurons

Ventral root

Motor neuron

Effector organ

Sensory neuron

Sensory

receptor

Interneuron

Spinal cord

Dorsal root

Dorsal root ganglion

Skin

Skeletal

muscle

Spinal

nerve

ProcessFigure 12.4

ReﬂexArc

The partsof a reﬂex arc are labeled in the order in which action potentials pass through them. The ﬁve componentsare the (1) sensory receptor, (2) sensory neuron,

(3) interneuron, (4) motor neuron, and (5) effector organ.

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innervate the noncontractile centers of the muscle spindle cells.

Axons of these sensory neurons synapse directly with motor

neurons in the spinal cord called alpha motor neurons (the

term alpha refers to motor neurons with large diameter axons),

which in turn innervate the muscle in which the muscle spindle

is embedded.The stretch reﬂex is unique because there is no in-

terneuron between the sensory and motor neurons.

Stretching a muscle also stretches muscle spindles located

among the muscle ﬁbers.The stretch stimulates the sensory neu-

rons that innervate the center ofeach of the muscle spindles.The

increased frequency of action potentials in the sensory neurons

stimulates the alpha motor neurons in the spinal cord.The alpha

motor neurons transmit action potentials to skeletal muscle,caus-

ing a rapid contraction ofthe stretched muscle, which opposes the

stretch ofthe muscle. The postural muscles demonstrate the adap-

tive nature of this reﬂex.If a person is standing upright and then

bends slightly to one side,the postural muscles associated with the

vertebral column on the other side are stretched.As a result,stretch

reﬂexes are initiated in those muscles,which cause them to con-

tract and reestablish normal posture.

Part3 Integration and ControlSystems406

Collateral axons from the sensory neurons of the muscle

spindles also synapse with neurons whose axons contribute to as-

cending nerve tracts,which enable the brain to perceive that a mus-

cle has been stretched (see p.412). Descending neurons within the

spinal cord synapse with the neurons ofthe stretch reﬂex modify-

ing their activity.This activity is important in maintaining posture

and in coordinating muscular activity.

Gamma motor neurons are responsible for regulating the

sensitivity of the muscle spindles. As a skeletal muscle contracts,

the tension on the centers ofmuscle spindles within the muscle de-

creases because the muscle spindles passively shorten as the muscle

shortens.The decrease in tension in the centers of the muscle spin-

dles cause them to be less sensitive to stretch.Sensitivity is main-

tained because at the same time alpha motor neurons are

stimulating the muscle to contract,gamma motor neurons stimu-

late the muscle spindles to contract.The contraction of the muscle

ﬁbers at the ends ofthe muscle spindles pulls on the center part of

the muscle spindles and maintains the proper tension.The activity

ofthe muscle spindles help control and coordinate muscular activ-

ity,such as posture, muscle tension, and muscle length.

1. Muscle spindles detect stretch of the muscle.

4. Stimulation of the alpha motor neurons causes the muscle to contract

and resist being stretched.

2. Sensory neurons conduct action potentials to the spinal cord.

3. Sensory neurons synapse with alpha motor neurons.

Muscle fiber of muscle

Muscle fiber of muscle spindle

Gamma motor neuron

Sensory neuron

Stretch

Sensory

neuron endings

Gamma motor

neuron endings

Muscle spindle

To brain

Alpha

motor

neuron

Neuromuscular

junction

Sensory

neuron

Muscle

spindle

Stretch

reflex

ProcessFigure 12.5

Stretch Reﬂex

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Chapter 12 Spinal Cord and Spinal Nerves 407

Knee-JerkReﬂex

Theknee-jerk reﬂex, or patellar reﬂex,is a classic example of the stretch

reﬂex. Cliniciansuse this reﬂexto determine whether the higher CNS

centersthat normally inﬂuence thisreﬂex are functional. When the patellar

ligamentis tapped, the tendons and muscles of the quadricepsfemoris

muscle group are stretched. The muscle spindle ﬁberswithin these

musclesare also stretched, and the stretch reﬂexis activated.

Consequently, contraction ofthe musclesextends the leg, thus producing

the characteristicknee-jerkresponse. When the stretch reﬂex is greatly

exaggerated, itindicates that the neuronswithin the brain that innervate

the gamma motor neuronsand enhance the stretch reﬂexare overly active.

On the other hand, ifthe neurons that innervate the gamma motor neurons

are depressed, the stretch reﬂexcan be suppressed or absent. Absence of

the stretch reﬂexmay indicate thatthe reﬂex pathway is not intact.

Golgi Tendon Reﬂex

TheGolgi tendon reﬂex prevents contracting muscles from apply-

ing excessive tension to tendons.Golgi tendon organs are encap-

sulated nerve endings that have at their ends numerous terminal

branches with small swellings associated with bundles of collagen

ﬁbers in tendons.The Golgi tendon organs are located within ten-

dons near the muscle–tendon junction (ﬁgure 12.6).As a muscle

contracts,the attached tendons are stretched,resulting in increased

tension in the tendon.The increased tension stimulates action po-

tentials in the sensory neurons from the Golgi tendon organs.

Golgi tendon organs have a high threshold and are sensitive only to

intense stretch.

The sensory neurons of the Golgi tendon organs pass through

the dorsal root to the spinal cord and enter the posterior gray matter,

where they branch and synapse with inhibitory interneurons.The in-

terneurons synapse with alpha motor neurons that innervate the

muscle to which the Golgi tendon organ is attached.When a great

amount of tension is applied to the tendon,the sensor y neurons of

the Golgi tendon organs are stimulated.The sensory neurons stimu-

late the interneurons to release inhibitory neurotransmitters which

inhibit the alpha motor neurons ofthe associated muscle and causes

it to relax. This reﬂex protects muscles and tendons from damage

caused by excessive tension.The sudden relaxation of the muscle re-

duces the tension applied to the muscle and tendons.A weight lifter

who suddenly drops a heavy weight after straining to lift it does so,in

part,because of the effect of the Golgi tendon reﬂex.

Golgi

tendon

organ

Muscle contraction

increases tension

applied to tendons. In

response, action

potentials are conducted

to the spinal cord.

Sensory

neuron

Tendon

Muscle

1. Golgi tendon organs detect tension applied to a tendon.

4. Inhibition of the alpha motor neurons causes muscle relaxation,

relieving the tension applied to the tendon.

2. Sensory neurons conduct action potentials to the spinal cord.

3. Sensory neurons synapse with inhibitory interneurons

that synapse with alpha motor neurons.

To brain

Alpha motor

neuron

Inhibitory

interneuron

Sensory

neuron

Golgi

tendon

reflex

Golgi tendon organ

ProcessFigure 12.6

Golgi Tendon Reﬂex

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Tremendous amounts oftension can be applied to muscles

and tendons in the legs.Frequently an athlete’s Golgi tendon reﬂex

is inadequate to protect muscles and tendons from excessive ten-

sion.The large muscles and sudden movements of football players

and sprinters can make them vulnerable to relatively frequent

hamstring pulls and calcaneal (Achilles) tendon injuries.

WithdrawalReﬂex

The function ofthe withdrawal, or ﬂexor,reﬂex is to remove a limb

or other body part from a painful stimulus.The sensor y receptors

are pain receptors (see chapter 15).Action potentials from painful

stimuli are conducted by sensory neurons through the dorsal root to

the spinal cord, where they synapse with excitatory interneurons,

which in turn synapse with alpha motor neurons (ﬁgure 12.7).The

alpha motor neurons stimulate muscles,usually ﬂexor muscles, that

remove the limb from the source ofthe painful stimulus. Collateral

branches ofthe sensory neurons synapse with ascending ﬁbers to the

brain,providing conscious awareness of the painful stimuli.

Reciprocal Innervation

Reciprocal innervation is associated with the withdrawal reﬂex

and reinforces its efﬁciency (ﬁgure 12.8).Collateral axons of sen-

sory neurons that carry action potentials from pain receptors

Part3 Integration and ControlSystems408

synapse with inhibitory interneurons in the dorsal horn of the

spinal cord,which synapse with and inhibit alpha motor neurons

of extensor (antagonist) muscles. When the withdrawal reﬂex is

initiated, flexor muscles contract, and reciprocal innervation

causes relaxation of the extensor muscles.This reduces the resis-

tance to movement that the extensor muscles would otherwise

generate.

Reciprocal innervation is also involved in the stretch reﬂex.

When the stretch reﬂex causes a muscle to contract,reciprocal in-

nervation causes opposing muscles to relax.In the patellar reﬂex,

for example, the quadriceps femoris muscle contracts and the

hamstring muscles relax.

Crossed ExtensorReﬂex

Thecrossed extensor reﬂex is another reﬂex associated with the

withdrawal reﬂex (ﬁgure 12.9). Interneurons that stimulate al-

pha motor neurons,resulting in withdrawal of a limb, have col-

lateral axons that extend through the white commissure to the

opposite side of the spinal cord and synapse with alpha motor

neurons that innervate extensor muscles in the opposite side of

the body.When a withdrawal reflex is initiated in one lower

limb,the crossed extensor reﬂex causes extension of the opposite

lower limb.

Withdrawal reflex

1. Pain receptors detect a painful stimulus.

2. Sensory neurons conduct action

potentials to the spinal cord.

3. Sensory neurons synapse with excitatory

interneurons that synapse with

alpha motor neurons.

4. Excitation of the alpha motor neurons

results in contraction of the flexor

muscles and withdrawal of the limb from

the painful stimulus.

Alpha motor

neuron

Neuromuscular

junction

Excitatory

interneuron

Sensory neuron

Stimulus

To brain

Sensory neuron

ProcessFigure 12.7

WithdrawalReﬂex

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1. During the withdrawal reflex,

sensory neurons conduct action

potentials to the spinal cord.

2. Sensory neurons synapse with

excitatory interneurons

that are part of the withdrawal

reflex.

3. Collateral branches also

synapse with inhibitory

interneurons that are

part of reciprocal innervation.

4. Inhibition of the alpha motor

neurons supplying the extensor

muscles causes them to relax

and not oppose the flexor

muscles of the withdrawal

reflex.

Reciprocal innervation

Withdrawal reflex with reciprocal innervation

Collateral branch

from sensory

neuron

Inhibitory

interneuron

Sensory

neuron

Neuromuscular

junction

Alpha motor

neuron

Excitatory

interneuron

To brain

ProcessFigure 12.8

WithdrawalReﬂex with Reciprocal Innervation

3. Stimulation of the alpha motor neurons cause

contraction of flexor muscles and stimulation of

alpha motor neurons supplying extensor

muscles in the opposite limb causes them to

contract and support body weight during

the withdrawal reflex.

2. Sensory neurons synapse with excitatory

interneurons that are part of the

withdrawal reflex. Collateral branches also

synapse with excitatory interneurons

that cross over to the opposite side of the spinal

cord as part of the crossed extensor reflex.

Sensory

neuron

To brain

Alpha motor

neuron

Neuromuscular

junction

Excitatory

interneuron

Alpha motor

neuron

Crossed

extensor

reflex

Withdrawal

reflex

Neuro-

muscular

junction

1. During the withdrawal reflex, sensory neurons

conduct action potentials to the spinal cord.

ProcessFigure 12.9

WithdrawalReﬂex with Crossed Extensor Reﬂex

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The crossed extensor reﬂex is adaptive in that it helps prevent

falls by shifting the weight ofthe body from the affected to the un-

affected limb.For example, when a person steps on a sharp object,

the affected limb is withdrawn from the stimulus (withdrawal re-

ﬂex) while the other limb is extended (crossed extensor reﬂex).

Therefore, when a person steps on a sharp object with the right

foot,the body weight is shifted from the right to the left lower limb.

Initiating a withdrawal reﬂex in both legs at the same time would

cause one to fall.

7. Contrast and give the functions of a stretch reﬂex and a

Golgi tendon reﬂex. Describe the sensoryreceptors for each.

8. Describe the operation of gamma motor neurons. What do

theyaccomplish?

9. What is a withdrawal reﬂex? How do reciprocal innervation

and the crossed extensorreﬂex assist the withdrawal reﬂex?

Spinal Cord Pathways

Objective

■ Describe and give examplesof how convergent and

divergentpathways interact with reﬂexes.

Reﬂexes do not operate as isolated entities within the nervous

system because of divergent and convergent pathways (see chapter

11).Diverging branches of the sensory neurons or interneurons in a

reﬂex arc send action potentials along ascending nerve tracts to the

brain (ﬁgure 12.10).A pain stimulus, for example, not only initiates

a withdrawal reﬂex, which removes the affected part of the body

from the painful stimulus, but also causes perception of the pain

sensation as a result ofaction potentials sent to the brain.

Axons within descending tracts from the brain carry action

potentials to motor neurons in the anterior horn ofthe spinal cord,

converging with neurons ofreﬂex arcs. The neurotransmitters re-

leased from the axons ofthese tracts either stimulate or inhibit the

motor neurons in the anterior horn.Neurotransmitters change the

sensitivity of the reﬂex by stimulating (EPSP) or inhibiting (IPSP)

the motor neurons.Various ascending and descending tracts oc-

cupy speciﬁc areas ofthe spinal cord (ﬁgure 12.11).

10. How do ascending and descending pathways relate to

reﬂexesand other neuron functions?

Part3 Integration and ControlSystems410

Structure of PeripheralNerves

Objective

■ Describe the structure of a peripheral nerve.

Peripheral nerves consist of axon bundles Schwann cells,

and connective tissue (ﬁgure 12.12).Each axon, or nerve ﬁber,and

its Schwann cell sheath are surrounded by a delicate connective tis-

sue layer,the endoneurium (en-do¯-noo⬘re¯-u˘m). A heavier con-

nective tissue layer, the perineurium (per-i-noo⬘re¯-u˘m),

surrounds groups of axons to form nerve fascicles (fas⬘i-klz). A

third layer of dense connective tissue, the epineurium (ep-i-

noo⬘re¯-u˘m),binds the ner ve fascicles together to form a nerve.The

connective tissue layers of nerves make them tougher than the

nerve tracts of the CNS.

11. Describe the structure of peripheral nerves.

Spinal Nerves

Objectives

■ Describe the structure and explain the naming of the spinal

nerves.

■ Describe dorsal roots, ventral roots, dorsal rami, and

ventral rami of spinal nerves.

■ Describe plexuses, and outline the pattern and distribution

of intercostal nerves.

■ Describe the structure, distribution, and function of the

cervical, brachial, lumbosacral, and coccygeal plexuses.

All ofthe 31 pairs of spinal nerves,except the ﬁrst pair and

those in the sacrum,exit the vertebral column through interver-

tebral foramena located between adjacent vertebrae. The first

pair ofspinal ner ves exits between the skull and the ﬁrst cervical

vertebra.The nerves of the sacrum exit from the single bone of

the sacrum through the sacral foramina (see chapter 7). Eight

spinal nerve pairs exit the vertebral column in the cervical re-

gion,12 in the thoracic reg ion,5 in the lumbar region, 5 in the

sacral region, and 1 in the coccygeal region (ﬁgure 12.13).For

convenience,each of the spinal ner ves is designated by a letter

and number. The letter indicates the region of the vertebral

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Skeletal

muscle

Effector organ

Motor neuron

Skin

Sensory

receptor

Sensory neuron

brain

From

brain

Descending

axon

Descending

tract

Ascending

axon

Ascending

tract

Site of

divergence

Site of

convergence

Figure 12.10

SpinalReﬂex, with Ascending and Descending Axons

Ascending

nerve tracts

Descending

nerve tracts

Figure 12.11

CrossSection of the Spinal Cord at the

CervicalLevel Depicting the Pathways

Ascending nerve tractsare blue, descending nerve tracts are pink. The arrows

indicate the direction ofeach pathway.

Fat

Epineurium

Perineurium

Endoneurium

Schwann

cell

Axon

Fascicle

Artery

and vein

Figure 12.12

Structure ofa Peripheral Nerve

Nerve structure illustrating axonssurrounded by various layersof connective

tissue: epineurium around the whole nerve, perineurium around nerve

fascicles, and endoneurium around Schwann cellsand axons.

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Clinical Focus SpinalCord Injury

Damage to the spinalcord can disrupt as-

cending tractsfrom the spinal cord to the

brain, resulting in the loss of sensation,

and/or descending tractsfrom the brain to

motor neuronsin the spinal cord, resulting

in the loss of motor functions. About

10,000 new casesof spinal cord injury oc-

cur each year in the United States. Automo-

bile and motorcycle accidents are leading

causes, followed bygunshot wounds, falls,

and swimming accidents. Spinalcord in-

juryis classiﬁed according to the vertebral

levelat which the injury occurred, whether

the entire cord isdamaged at that level or

onlya portion of the cord, and the mecha-

nism ofinjury. Most spinal cord injuries oc-

cur in the cervical region or at the

thoracolumbar junction and are incom-

plete. The primary mechanisms include

concussion (an injury caused by a blow),

contusion (an injury resulting in hemor-

rhage), or laceration (a tear or cut) and in-

volve excessive flexion, extension,

rotation, or compression of the vertebral

column. The majorityof spinal cord injuries

are acute contusions of the cord due to

bone or disk displacement into the cord

and involve a combination ofexcessive di-

rectional movements, such assimultane-

ousﬂexion and compression.

At the time of spinal cord injury, two

types of tissue damage occur: (1) primary,

mechanicaldamage and (2) secondary, tis-

sue damage. Secondary spinalcord dam-

age, which begins within minutes of the

primary damage, is caused by ischemia,

edema, ion imbalances, the release of“exci-

totoxins” such asglutamate, and inﬂamma-

tory cell invasion. Secondary damage

extendsinto a much larger region of the cord

than the primary damage. Itis the primary

focus of current research in spinalcord in-

jury. The onlytreatment for primary damage

is prevention, such as wearing seatbelts

when riding in automobiles and not diving

in shallow water. Once an accidentoccurs,

however, little can be done atpresent about

the primarydamage. On the other hand, it’s

now known that much of the secondary

damage can be prevented or reversed.

Until the 1950s, spinal cord injuries

were often ultimatelyfatal. Now, with quick

treatment, directed at the mechanisms of

secondarytissue damage, much of the total

damage to the spinalcord can be prevented.

Treatmentof the damaged spinal cord with

large doses of methylprednisolone, a syn-

thetic steroid, within 8 hoursof the injur y,

can dramaticallyreduce the secondary dam-

age to the cord. The objectivesof these treat-

ments are to reduce inﬂammation and

edema. Currenttreatment includes anatomic

realignment and stabilization ofthe ver te-

bral column, decompression ofthe spinal

cord, and administration of methylpred-

nisolone. Rehabilitation isbased on retrain-

ing the patient to use whatever residual

connectionsexist across the site of damage.

Ithad long been thought that the spinal

cord is incapable ofregeneration following

severe damage. It’snow known that follow-

ing injury, most neuronsof the adult spinal

cord survive and begin to regenerate, grow-

ing about1 mm into the site of damage, but

then they regress to an inactive, atrophic

state. In addition, fetusesand newborns ex-

hibit considerable regenerative ability and

functionalimprovement. The major block to

adult spinal cord regeneration isthe forma-

tion ofa scar, consisting mainlyof myelin and

astrocytes, atthe site of injury. Myelin in the

scar isapparently the primary inhibitor of re-

generation. Implantation of peripheral

nerves, Schwann cells, or fetalCNStissue can

bridge the scar and stimulate some regenera-

tion. Certain growth factorscan also stimu-

late some regeneration. Current research

continuesto look for the right combination of

chemicalsand other factors to stimulate re-

generation ofthe spinal cord following injury.

column from which the nerve emerges:C, cervical; T,thoracic; L,

lumbar; and S, sacral. The single coccygeal nerve is often not

designated, but when it is, the symbol often used is Co.The

number indicates the location in each region where the nerve

emerges from the vertebral column, with the smallest number

always representing the most superior origin.For example, the

most superior nerve exiting from the thoracic region of the ver-

tebral column is designated T1. The cervical nerves are desig-

nated C1–C8, the thoracic nerves T1–T12, the lumbar nerves

L1–L5,and the sacral nerves S1–S5.

Each of the spinal nerves except C1 has a speciﬁc cutaneous

sensory distribution.Figure 12.14 illustrates the dermatomal (der-

ma˘-to¯⬘ma˘l)map for the sensory cutaneous distribution of the

spinal nerves.A dermatome is the area ofskin supplied with sen-

sory innervation by a pair of spinal nerves.

Part3 Integration and ControlSystems412

PREDICT

The dermatomalmap is important in clinicalconsiderations of nerve

damage. Lossof sensation in a dermatomalpattern can provide valuable

information aboutthe location of nerve damage. Predict the possible site

ofnerve damage for a patient who suffered whiplash in an automobile

accidentand subsequently developed anesthesia (no sensations) in the

leftarm, forearm, and hand (see ﬁgure 12.14 for help).

Figure 12.15 depicts an idealized section through the trunk.

Each spinal nerve has a dorsal and a ventral ramus (ra¯⬘mu˘s;

branch). Additional rami (ra¯⬘mı¯), called communicating rami,

from the thoracic and upper lumbar spinal cord regions carry ax-

ons associated with the sympathetic division (see chapter 16).The

dorsal rami (ra¯⬘mı¯) innervate most ofthe deep muscles of the

dorsal trunk responsible for movement of the vertebral column.

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They also innervate the connective tissue and skin near the midline

ofthe back.

The ventral rami are distributed in two ways. In the tho-

racic region, the ventral rami form intercostal (between ribs)

nerves, which extend along the inferior margin of each rib and

innervate the intercostal muscles and the skin over the thorax.

The ventral rami of the remaining spinal nerves form five

plexuses (plek⬘su˘s-e¯z).The term plexus means braid and de-

scribes the organization produced by the intermingling of the

nerves. The ventral rami of different spinal nerves, called the

rootsof the plexus,join with each other to form a plexus. These

roots should not be confused with the dorsal and ventral roots

from the spinal cord,which are more medial. Nerves that arise

from plexuses usually have axons from more than one spinal

nerve and thus more than one level ofthe spinal cord. The ventral

rami of spinal nerves C1-C4 form the cervical plexus, C5-T1

form the brachial plexus,L1-L4 form the lumbar plexus, L4-S4

form the sacral plexus,and S4, S5, and the coccygeal nerve (Co)

form the coccygeal plexus.

Several smaller somatic plexuses, such as the pudendal

plexus in the pelvis,are derived from more distal branches of the

spinal nerves.Some of the somatic plexuses are mentioned where

appropriate in this chapter. Autonomic plexuses (described in

chapter 16) also exist in the thorax and abdomen.

Dura mater

Cauda equina

Conus medullaris

Cervical

plexus

(C1–4)

Brachial

plexus

(C5–T1)

Sacral

plexus

(L4–S4)

Coccygeal

plexus

(S4–Co)

Lumbar

plexus

(L1–4)

Cervical

nerves

Thoracic

nerves

Lumbar

nerves

Sacral

nerves

Coccygeal

nerves

Cervical

nerves

Thoracic

nerves

Lumbar

nerves

Sacral

nerves

Coccygeal

nerves

Lumbo-

sacral

plexus

(L1–S4)

Functions

Diaphragm

movement

Neck and shoulder

movement

Upper limb

movement

Rib movement

in breathing,

vertebral column

movement, and

tone in postural

back muscles

Hip movement

Lower limb

movement

Head movement

Figure 12.13

SpinalNerves

(a) Spinalcord, the spinal nerves, their plexuses, and their branches. (b) Regionsof the spinal cord and their general functions.

(a) (b)

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12. Describe the connective tissue layers within and

surrounding spinal nerves.

13. Differentiate between rootlet, dorsal root, ventral root, and

spinal nerve. Indicate whethereach contains motor ﬁbers,

sensoryﬁbers, or both.

14. List all of the spinal nerves by name and number. Where do

theyexit the vertebral column?

15. What is a dermatome? Why are dermatomes clinically

important?

16. Contrast dorsal and ventral rami of spinal nerves. What

musclesdo the dorsal rami innervate?

17. Describe the distribution of the ventral rami of the thoracic

region.

18. What is a plexus? What happens to the axons of spinal

nervesas they pass through a plexus?

19. Name the main spinal plexuses and the spinal nerves

associated with each one.

CervicalPlexus

The cervical plexus is a relatively small plexus originating from

spinal nerves C1–C4 (ﬁgure 12.16). Branches derived from this

plexus innervate superﬁcial neck structures, including several of

the muscles attached to the hyoid bone.The cervical plexus inner-

Part3 Integration and ControlSystems414

vates the skin of the neck and posterior portion of the head (see

ﬁgure 12.14).

One of the most important derivatives of the cervical

plexus is the phrenic (fren⬘ik) nerve, which originates from

spinal nerves C3–C5, derived from both the cervical and

brachial plexus. The phrenic nerves descend along each side of

the neck to enter the thorax.They descend along the sides of the

mediastinum to reach the diaphragm, which they innervate.

Contraction ofthe diaphr agm is largely responsible for the abil-

ity to breathe.

PhrenicNerve Damage

Damage to the phrenicnerve severely limits a person’s ability to breathe.

Care mustbe taken not to damage the phrenic nerve during open

thoracicsurgery or open heart surgery. Cancer of the bronchusis the

mostcommon type of cancer in men, accounting for about30% of all

male cancers, and mostoften occurs in men who smoke cigarettes.

Tumorsat the base of the lung can compressthe phrenic nerve.

PREDICT

Explain how damage to or compression ofthe right phrenic nerve

affectsthe diaphragm. Describe the effect on breathing of a

completelysevered spinal cord at the level of C2 versus C6.

T10

T11

T12

T10

T11

T12

Figure 12.14

DermatomalMap

Lettersand numbers indicate the spinal nerves innervating a given region of skin.

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Chapter 12 Spinal Cord and Spinal Nerves 415

Dorsal root

(spinal) ganglion

Dorsal ramus of

spinal nerve

Ventral ramus of

spinal nerve

(intercostal nerve)

Spinal nerve

Dorsal root of

spinal nerve

Communicating rami

Ganglion of

sympathetic chain

Roots of

splanchnic nerve

Rootlets

Ventral root of

spinal nerve

Dorsal

root

ganglion

Dorsal rootlets

Spinal

nerve

Transverse

process of

vertebra

(cut)

Intervertebral

foramen

Figure 12.15

SpinalNerves

(a) Typicalthoracic spinal nerves. (b) Photograph of four dorsal roots in place along the vertebralcolumn.

(a)

(b)

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Roots (ventral rami)

brachial

plexus

Branches

Other nerves (not part

of cervical plexus)

Hypoglossal nerve (XII)

Accessory nerve (XI)

Lesser occipital nerve

Nerve to

sternocleidomastoid muscle

Nerve to

trapezius muscle

Greater auricular nerve

Superior root of

ansa cervicalis

Transverse cervical

nerve

Ansa cervicalis

Inferior root

of ansa cervicalis

Supraclavicular nerves

Phrenic nerve

Part3 Integration and ControlSystems416

BrachialAnesthesia

The entire upper limb can be anesthetized byinjecting an anesthetic

near the brachialplexus. This iscalled brachial anesthesia. The

anestheticcan be injected between the neck and the shoulder posterior

to the clavicle.

Figure 12.16

CervicalPlexus, Anterior View

The rootsof the plexus are formed by the ventral rami of the spinalner ves

C1–C4.

Roots: C5, C6, C7, C8, T1

Anterior divisions

Posterior divisions

Cords: posterior, lateral, medial

Branches: Axillary nerve

Radial nerve

Musculocutaneous nerve

Median nerve

Ulnar nerve

Lower

trunk

Middle

trunk

Upper

trunk

Long

thoracic

nerve

Trunks: upper, middle, lower

Dorsal scapular nerve

Suprascapular nerve

Subclavian nerve

Musculo-

cutaneous

nerve

Medial and

lateral

pectoral

nerves

Median

nerve

Ulnar nerve

Lateral cord

Posterior cord

Radial nerve

Axillary nerve

Medial brachial

cutaneous nerve

Medial cord

Figure 12.17

BrachialPlexus, Anterior View

The rootsof the plexus are formed by the ventral rami of the spinalner ves

C5–T1 and join to form an upper, middle, and lower trunk. Each trunkdivides

into anterior and posterior divisions. The divisionsjoin together to form the

posterior, lateral, and medialcords from which the major brachialplexus

nervesarise.

BrachialPlexus

Thebr achial plexus originates from spinal nerves C5–T1 (ﬁgure

12.17).A connection is also present from C4 of the cervical plexus

to the brachial plexus. The ﬁve ventral rami that constitute the

brachial plexus join to form three trunks,which separate into six

divisionsand then join again to create three cords (posterior,lat-

eral,and medial) from which ﬁve branches, or nerves of the upper

limb,emerge.

The ﬁve major nerves emerging from the brachial plexus to

supply the upper limb are the axillary,radial, musculocutaneous,

ulnar,and median nerves. The axillary nerve innervates part of the

shoulder; the radial nerve innervates the posterior arm, forearm,

and hand; the musculocutaneous nerve innervates the anterior

arm;and the ulnar and median nerves innervate the anterior fore-

arm and hand.Smaller ner ves from the brachial plexus innervate

the shoulder and pectoral muscles.

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Chapter 12 Spinal Cord and Spinal Nerves 417

AxillaryNerve

The axillary (ak⬘sil-a¯r-e¯)nerve innervates the deltoid and teres

minor muscles (ﬁgure 12.18).It also provides sensory innervation

to the shoulder joint and to the skin over part ofthe shoulder.

Lateral cord

Medial cord

Posterior cord

Teres minor

Axillary nerve

Deltoid

Figure 12.18

AxillaryNer ve

Route ofthe axillary nerve and the muscles it innervates. The inset depicts the cutaneous distribution of the nerve (shaded area).

AxillaryNerve

Origin

Posterior cord of brachial plexus, C5–C6

Movements/MusclesInnervated

Laterally rotates arm

• Teres minor

Abducts arm

• Deltoid

CutaneousInnervation

Inferior lateral shoulder

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Lateral head of

triceps brachii

Brachioradialis

Extensor carpi

radialis longus

Extensor carpi

radialis brevis

Supinator

Adductor pollicis

longus

Extensor pollicis

brevis and longus

Radial nerve

Posterior cord

Lateral cord

Medial cord

Long head of

triceps brachii

Medial head of

triceps brachii

Anconeus

Extensor digitorum

Extensor digiti minimi

Extensor carpi ulnaris

Extensor indicis

Figure 12.19

RadialNerve

Route ofthe radial nerve and the muscles it innervates. The insets depict the

cutaneousdistribution of the nerve (shaded area).

Part3 Integration and ControlSystems418

Radial Nerve

Ther adial nerve emerges from the posterior cord of the brachial

plexus and descends within the deep aspect of the posterior arm

(ﬁgure 12.19).About midway down the shaft of the humerus, it lies

against the bone in the radial groove.The radial nerve innervates

all ofthe extensor muscles of the upper limb, the supinator muscle,

and the brachioradialis.Its cutaneous sensory distribution is to the

posterior portion of the upper limb, including the posterior sur-

face ofthe hand.

RadialNerve Damage

Because the radialnerve lies near the humerus in the axilla, itcan be

damaged ifit’s compressed againstthe humerus. Improper use of crutches

(i.e., when the crutch ispushed tightly into the axilla) can resultin “crutch

paralysis.”In this disorder, the radial nerve is compressed between the top

ofthe crutch and the humerus. As a result, the radial nerve is damaged,

and the musclesit innervates lose their function. The major symptom of

crutch paralysisis“wrist drop” in which the extensor muscles of the wrist

and ﬁngers, which are innervated bythe radial nerve, fail to function; asa

result, the elbow, wrist, and ﬁngersare constantlyﬂexed.

PREDICT

Wristdrop can also result from a compound fracture ofthe humerus.

Explain how and where damage to the nerve mayoccur.

RadialNer ve

Origin

Posterior cord of brachial plexus, C5–T1

Movements/MusclesInnervated

Extends elbow

• Triceps brachii

• Anconeus

Flexes elbow

• Brachialis (part; not shown; sensory only)

• Brachioradialis

Extends and abducts wrist

• Extensor carpi radialis longus

• Extensor carpi radialis brevis

Supinates forearm

• Supinator

Extends ﬁngers

• Extensor digitorum

• Extensor digiti minimi

• Extensor indicis

Extends and adducts wrist

• Extensor carpi ulnaris

Abducts thumb

• Abductor pollicis longus

Extends thumb

• Extensor pollicis longus

• Extensor pollicis brevis

CutaneousInnervation

Posterior surface of arm and forearm,

lateral two-thirds of dorsum of hand

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Chapter 12 Spinal Cord and Spinal Nerves 419

MusculocutaneousNerve

The musculocutaneous (mu˘s⬘ku¯-lo¯-ku¯-ta¯⬘ne¯-u˘s)nerve pro-

vides motor innervation to the anterior muscles of the arm as

well as cutaneous sensory innervation to part of the forearm

(ﬁgure 12.20).

Posterior cord

Lateral cord

Medial cord

Coracobrachialis

Musculocutaneous nerve

Biceps brachii

Brachialis

Figure 12.20

MusculocutaneousNerve

Route ofthe musculocutaneous nerve and the muscles it innervates. The inset depicts the cutaneous distribution of the nerve (shaded area).

MusculocutaneousNerve

Origin

Lateral cord of brachial plexus, C5–C7

Movements/MusclesInnervated

Flexes shoulder

• Biceps brachii

• Coracobrachialis

Flexes elbow and supinates forearm

• Biceps brachii

Flexes elbow

• Brachialis (also small amount of

innervation from radial nerve)

CutaneousInnervation

Lateral surface of forearm

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Part3 Integration and ControlSystems420

UlnarNerve

The ulnar nerve innervates two forearm muscles plus most of

the intrinsic hand muscles, except some associated with the

thumb.Its sensor y distribution is to the ulnar side of the hand

(ﬁgure 12.21).

Posterior cord

Lateral cord

Medial cord

Ulnar nerve

Flexor carpi ulnaris

Flexor digitorum

profundus

Hypothenar muscles

Adductor pollicis

All dorsal and

palmar interossei The two medial

(ulnar) lumbricales

Figure 12.21

Ulnar Nerve

Route ofthe ulnar nerve and the muscles it innervates. The inset depicts the cutaneous distribution of the nerve (shaded area).

Ulnar Nerve Damage

The ulnar nerve isthe most easily damaged of all the peripheralner ves,

butsuch damage is almost always temporary. Slightdamage to the ulnar

nerve mayoccur where it passes posterior to the medial epicondyle of

the humerus. The nerve can be feltjust below the skin at this point, and,

ifthis region of the elbow is banged against a hard object, temporary

ulnar nerve damage mayoccur. This damage resultsin painful tingling

sensationsradiating down the ulnar side of the forearm and hand.

Because ofthis sensation, this area ofthe elbow is often called the

“funnybone” or “crazy bone.”

Ulnar Nerve

Origin

Medial cord of brachial plexus, C8–T1

Movements/MusclesInnnervated

Flexes and adducts wrist

• Flexor carpi ulnaris

Flexes ﬁngers

• Part of the ﬂexor digitorum profundus

controlling the distal phalanges of little

and ring ﬁngers

Adducts thumb

• Adductor pollicis

Controls hypothenar muscles

• Flexor digiti minimi brevis

• Abductor digiti minimi

• Opponens digiti minimi

Flexes metacarpophalangeal joints and

extends interphalangeal joints

• Two medial (ulnar) lumbricales

Abducts and adducts ﬁngers

• Interossei

CutaneousInnervation

Medial third of hand, little ﬁnger, and

medial half of ring ﬁnger

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Chapter 12 Spinal Cord and Spinal Nerves 421

Posterior cord

Lateral cord

Medial cord

Median nerve

Palmaris longus

Pronator teres

Flexor carpi radialis

Flexor digitorum superficialis

Flexor digitorum profundus

Flexor pollicis longus

Pronator quadratus

Thenar muscles

The two lateral

(radial) lumbricales

Figure 12.22

Median Nerve

Route ofthe median nerve and the muscles it innervates.

The insetdepicts the cutaneous distribution of the nerve (shaded area).

Median Nerve

Themedian nerve innervates all but one of the ﬂexor muscles of the

forearm and most of the hand muscles at the base of the thumb,

called the thenar area ofthe hand. Its cutaneous sensory distribution

is to the radial portion ofthe palm of the hand (ﬁgure 12.22).

Median Nerve Damage

Damage to the median nerve occursmost commonly where it enters the

wristthrough the carpal tunnel. This tunnelis created by the concave

organization ofthe carpal bones and the ﬂexor retinaculum on the

anterior surface ofthe wrist. None of the connective tissue componentsof

the carpaltunnel expands readily. Inﬂammation in the wristor an

increase in the size ofthe tendons in the carpal tunnelcan produce

pressure within it, therebycompressing the median nerve and resulting in

numbness, tingling, and pain in the ﬁngers. Thiscondition is referred to

ascarpal tunnel syndrome. Carpaltunnel syndrome is common among

people who perform repetative movementsof the wrists and ﬁngers, such

askeyboard operators. Surgery is often required to relieve the pressure.

People attempting suicide bycutting the wrists commonly cut the

median nerve proximalto the carpal tunnel.

OtherNerves of the Brachial Plexus

Several nerves, other than the ﬁve just described, arise from the

brachial plexus (see ﬁgure 12.16).They supply most of the muscles

acting on the scapula and arm and include the pectoral,long tho-

racic,thoracodorsal, subscapular, and suprascapular nerves. In ad-

dition,brachial plexus nerves supply the cutaneous innervation of

the medial arm and forearm.

20. Name the structures innervated by the cervical plexus.

Describe the innervation of the phrenicnerve.

21. Name the ﬁve major nerves that emerge from the brachial

plexus. Listthe muscles they innervate and the areas of the

skin theysupply. In addition to these ﬁve nerves, name the

musclesand skin areas supplied by the remaining brachial

plexusnerves.

Median Nerve

Origin

Medial and lateral cords of brachial

plexus, C5–T1

Movements/MusclesInnervated

Pronates forearm

• Pronator teres

• Pronator quadratus

Flexes and abducts wrist

• Flexor carpi radialis

Flexes wrist

• Palmaris longus

Flexes ﬁngers

• Part of ﬂexor digitorum profundus

controlling the distal phalanx of the

middle and index ﬁngers

• Flexor digitorum superﬁcialis

Controls thumb muscle

• Flexor pollicis longus

Controls thenar muscles

• Abductor pollicis brevis

• Opponens pollicis

• Flexor pollicis brevis

Flexes metacarpophalangeal joints and

extends interphalangeal joints

• Two lateral (radial) lumbricales

CutaneousInnervation

Lateral two-thirds of palm of hand,

thumb, index and middle ﬁngers,

and the lateral half of ring ﬁnger

and dorsal tips of the same ﬁngers

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Lumbar and SacralPlexuses

The lumbar plexus originates from the ventral rami of spinal

nerves L1-L4 and the sacral plexus from L4-S4. Because of

their close, overlapping relationship and their similar

distribution,however, the two plexuses often are considered to-

gether as a single lumbosacral plexus (L1-S4; figure 12.23).

Four major nerves exit the lumbosacral plexus and enter the

Part3 Integration and ControlSystems422

lower limb: the obturator,femoral, tibial, and common ﬁbular

(peroneal).The obturator nerve innervates the medial thigh; the

femoral nerve innervates the anterior thigh; the tibial nerve in-

nervates the posterior thigh,the leg ,and foot; and the common

ﬁbular nerve innervates the posterior thigh,the anterior and lat-

eral leg,and the foot. Other lumbosacral nerves supply the lower

back,the hip, and the lower abdomen.

Figure 12.23

LumbosacralPlexus, Anterior View

The rootsof the plexus are formed by the ventral rami of the spinalner ves L1–S4 and form anterior and posterior divisions, which give rise to the lumbrosacral

nerves. The lumbo sacraltrunk joins the lumbar and sacralplexuses.

Iliohypogastric

Ilioinguinal

Lateral femoral

cutaneous

Genitofemoral

Lumbosacral trunk

Superior gluteal

Inferior gluteal

Common

fibular

(peroneal)

Tibial

Femoral

Pudendal

Obturator

Anterior divisions

Nerves

Posterior divisions

Posterior femoral cutaneous

Ischiadic

(sciatic)

Roots

Anterior divisions

Nerves

Posterior divisions

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Chapter 12 Spinal Cord and Spinal Nerves 423

ObturatorNerve

The obturator (ob⬘too-ra˘-to¯r)nerve supplies the muscles that

adduct the thigh.Its cutaneous sensor y distribution is to the me-

dial side ofthe thigh (ﬁgure 12.24).

Adductor magnus

Adductor brevis

Adductor longus

Gracilis

Obturator externus

Obturator nerve

Figure 12.24

Obturator Nerve

Route ofthe obturator nerve and the muscles it innervates. The inset depicts the cutaneous distribution of the nerve (shaded area).

Obturator Nerve

Origin

Lumbosacral plexus, L2–L4

Movements/MusclesInnervated

Rotates thigh laterally

• Obturator externus

Adducts thigh

• Adductor magnus (partial)

• Adductor longus

• Adductor brevis

Adducts thigh and ﬂexes knee

• Gracilis

CutaneousInnervation

Superior medial side of thigh

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Part3 Integration and ControlSystems424

Psoas major

Iliacus

Sartorius

Vastus

lateralis

Rectus femoris

Vastus

intermedius

Femoral nerve

Pectineus

Vastus medialis

Femoral Nerve

Thefemoral ner ve innervates the iliopsoas and sartorius muscles

and the quadriceps femoris group.Its cutaneous sensory distribu-

tion is the anterior and lateral thigh and the medial leg and foot

(ﬁgure 12.25).

FemoralNerve

Origin

Lumbosacral plexus, L2–L4

Movements/MusclesInnervated

Flexes hip

• Psoas major

• Iliacus

• Pectineus

Flexes hip and ﬂexes knee

• Sartorius

Extends knee

• Vastus lateralis

• Vastus intermedius

• Vastus medialis

Extends knee and ﬂexes hip

• Rectus femoris

CutaneousInnervation

Anterior and lateral branches supply the

anterior and lateral thigh;

saphenous branch supplies the

medial leg and foot

Figure 12.25

FemoralNerve

Route ofthe femoral nerve and the muscles it innervates. The inset depicts the cutaneous distribution of the nerve (shaded area).

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Chapter 12 Spinal Cord and Spinal Nerves 425

Semimembranosus

Semitendinosus

Adductor

magnus

(partial)

Popliteus

Flexor digitorum

longus

Biceps femoris long head

Tibial nerve

Soleus

Flexor hallucis

longus

Lateral plantar nerve

to plantar muscles

Gastrocnemius

Tibialis posterior

Medial plantar nerve

to plantar muscles

Figure 12.26

TibialNerve

Route ofthe tibial nerve and the muscles it innervates. The inset depicts the

cutaneousdistribution of the nerve (shaded area).

Tibial and Common FibularNerves

The tibial and common ﬁbular (peroneal) (per-o¯-ne¯⬘a˘l)nerves

originate from spinal segments L4–S3 and are bound together

within a connective tissue sheath for the length ofthe thigh (ﬁgures

12.26 and 12.27; see ﬁgure 12.23). These two nerves, combined

within the same sheath,are referred to jointly as the ischiadic (is-

ke¯-ad⬘ik) nerve (see ﬁgure 12.23). The ischiadic nerve is com-

monly called the sciatic (sı¯-at⬘ik) nerve. The term sciatic

originated as a degenerate form of ischiadic,and the International

Conference ofAnatomists has recently decided to begin using the

correct term. The ischiadic nerve, by far the largest peripheral

nerve in the body,passes through the greater ischiadic notch in the

pelvis and descends in the posterior thigh to the popliteal fossa,

where the two portions ofthe ischiadic nerve separate.

The tibial nerve innervates most ofthe posterior thigh and leg

muscle (see ﬁgure 12.26).It branches in the foot to form the medial

and lateral plantar (plan⬘ta˘r) nerves, which innervate the plantar

muscles of the foot and the skin over the sole of the foot.Another

branch,the sural (soo⬘ ra˘l)nerve,supplies part of the cutaneous in-

nervation over the calfof the leg and the plantar surface of the foot.

TibialNerve

Origin

Lumbosacral plexus, L4–S3

Movements/MusclesInnervated

Extends hip and ﬂexes knee

• Biceps femoris (long head)

• Semitendinosus

• Semimembranosus

Adducts thigh and extends hip

• Adductor magnus (partial)

Plantar ﬂexes foot

• Plantaris

• Gastrocnemius

• Soleus

• Tibialis posterior

Flexes knee

• Popliteus

Flexes toes

• Flexor digitorum longus

• Flexor hallucis longus

CutaneousInnervation

None

Medialand Lateral

PlantarNerves

Origin

Tibial nerve

Movements/MusclesInnervated

Flex and adduct toes

• Plantar muscles of foot

CutaneousInnervation

Sole of foot

SuralNerve (not shown)

Origin

Tibial nerve

Movements/MusclesInnervated

None

CutaneousInnervation

Lateral and posterior one-third of leg

and lateral side of foot

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Part3 Integration and ControlSystems426

The common fibular nerve divides into the deep and su-

perficial fibular (peroneal) nerves. These branches innervate

the anterior and lateral muscles of the leg and foot. The cuta-

neous distribution of the common fibular nerve and its

branches is the lateral and anterior leg and the dorsum of the

foot (see figure 12.27).

Fibularis

tertius

Extensor

digitorum

brevis

Fibularis

longus

Fibularis

brevis

Biceps femoris

short head

Tibialis anterior

Extensor hallucis

longus

Extensor digitorum

longus

Common fibular

(peroneal) nerve

Deep fibular

(peroneal) nerve

Superficial

fibular (peroneal)

nerve

Figure 12.27

Fibular Nerve

Route ofthe common ﬁbular (peroneal) nerve and the muscles it innervates. The inset depicts the cutaneous distribution ofthe ner ve (shaded area).

Common Fibular

(Peroneal) Nerve

Origin

Lumbosacral plexus, L4–S2

Movements/MusclesInnervated

Extends hip and ﬂexes knee

• Biceps femoris (short head)

CutaneousInnervation

Lateral surface of knee

Deep Fibular

(Peroneal) Nerve

Origin

Common ﬁbular (peroneal) nerve

Movements/MusclesInnervated

Dorsiﬂexes foot

• Tibialis anterior

• Peroneus tertius

Extends toes

• Extensor digitorum longus

• Extensor hallucis longus

CutaneousInnervation

Great and second toe

SuperﬁcialFibular

(Peroneal) Nerve

Origin

Common ﬁbular (peroneal) nerve

Movements/MusclesInnervated

Plantar ﬂexes and everts foot

• Peroneus longus

• Peroneus brevis

Extends toes

• Extensor digitorum brevis

CutaneousInnervation

Dorsal anterior third of leg and dorsum

of foot

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Clinical Focus Nerve Replacement

Patients paralyzed by strokes or spinal

cord lesionsare now able to regain certain

limited functions. Microcomputersare be-

ing perfected that stimulate certain pro-

grammed activities, such asgrasping and

walking. The microcomputer initiateselec-

tric impulses that are conveyed through

ﬁne wire leadsto either peripheral nerves

or directly to the muscles responsible for

the desired movement. The subtle move-

mentof muscles not affected by the paral-

ysis initiate the program. Sensors con-

nected to the microcomputer are attached

to the skin overlying functional muscles

and are able to detect electrical activity

associated with movementof the underly-

ing muscles. For example, a person with

both legsparalyzed may have such a sen-

sor attached to the abdomen. The abdom-

inal muscles normally involved in

stabilizing and moving the pelvis during

walking are stimulated by descending

tracts when walking is initiated by CNS

centers. The resultant abdominal muscle

activity is detected by the sensor, which

activatesthe program that stimulates the

appropriate sequence of muscles in the

lower limbs, and the paralyzed person

walks. Similarly, a quadriplegicusing sub-

tle movements of the shoulder, neck, or

face, where specific sensors can be

placed, can initiate certain upper limb and

grasping actions.

Chapter 12 Spinal Cord and Spinal Nerves 427

IschiadicNerve Damage

Ifa person sits on a hard surface for a considerable time, the ischiadic

(sciatic) nerve maybe compressed against the ischial portion of the

coxa. When the person standsup, a tingling sensation described as

“pinsand needles” can be felt throughout the lower limb, and the limb

issaid to have “gone to sleep.”

The ischiadicnerve may be seriously injured in a number of ways.

A ruptured intervertebraldisk or pressure from the uterus during

pregnancymay compress the roots of the ischiadicner ve. Other

possibilitiesfor causing ischiadic nerve damage include hip injury or an

improperlyadministered injection in the hip region.

OtherLumbosacral Plexus Nerves

In addition to the nerves just described, the lumbosacral plexus

gives rise to nerves that supply the lower abdominal muscles (ilio-

hypogastric nerve),the hip muscles that act on the femur (gluteal

nerves),and the muscles of the abdominal ﬂoor (pudendal ner ve;

see ﬁgure 12.23). The iliohypogastric (il⬘e¯-o¯-hı¯-po¯-gas⬘trik),il-

ioinguinal (il⬘e¯-o¯-ing⬘gwi-na˘l), genitofemoral (jen⬘i-to¯-fem⬘o˘-

ra˘l), cutaneous femoral, and pudendal (pu¯-den⬘da˘l)nerves

innervate the skin ofthe suprapubic area, the external genitalia, the

superior medial thigh,and the posterior thigh. The pudendal nerve

plays a vital role in sexual stimulation and response.

PudendalNerve Anesthesia

Branchesof the pudendal nerve are anesthetized before a doctor

performsan episiotomy for childbirth. An episiotomy (e-piz-e¯-ot⬘o¯-me¯,

e-pis-e¯-ot⬘o¯-me¯) is a cut in the perineum that makesthe opening of the

birth canallarger.

CoccygealPlexus

Theco ccygeal (kok-sij⬘e¯-a˘l)plexusis a very small plexus formed

from the ventral rami of spinal nerves S4, S5, and the coccygeal

nerve.This small plexus supplies motor innervation to muscles of

the pelvic ﬂoor and sensory cutaneous innervation to the skin over

the coccyx.The dorsal rami of the coccygeal nerves innervate some

skin over the coccyx.

22. Name the four major nerves that arise from the lumbosacral

plexus, and describe the musclesand skin area they

supply. Whatis the name applied to the tibial and common

ﬁbularnerves bound together?

23. Describe the structures innervated by the remaining

lumbosacral nerves.

24. What structures are innervated by the coccygeal plexus?

Seeley−Stephens−Tate:

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Clinical Focus PeripheralNer vousSystem Disorders_Spinal Nerves

GeneralTypes of PNS Disorders

Anesthesia is the loss of sensation (the

Greek word esthesis meanssensation). It

may be a pathologic condition if it hap-

pensspontaneously, or it may be induced

to facilitate surgeryor some other medical

treatment.

Hyperesthesia (hı¯⬘per-es-the¯⬘ze¯-a˘) is

an abnormalacuteness to sensation, espe-

ciallyan increased sensitivity to pain, pres-

sure, or light.

Paresthesia (par-es-the⬘ze¯-a˘) isan ab-

normalspontaneous sensation, such astin-

gling, prickling, or burning.

Neuralgia (noo-ral⬘je¯-a˘) consists of se-

vere spasmsof throbbing or stabbing pain re-

sulting from inﬂammation or damage along

the pathwayof a nerve.

Ischiadica(is⬘ke¯-ad⬘i-ka˘), or sciatica,is

a neuralgia ofthe ischiadic nerve, with pain

radiating down the backofthe thigh and leg.

The mostcommon cause is a herniated lum-

bar disk, resulting in pressure on the spinal

nervescontributing to the lumbar plexus. Is-

chiadica mayalso be produced by ischiadic

neuritisarising from a number of causes, in-

cluding mechanicalstretching of the nerve

during exertion, vitamin deﬁciency, or meta-

bolicdisorders (such as gout or diabetes).

Neuritis (noo-rı¯⬘tis) is a generalterm

referring to inﬂammation ofa nerve that has

a wide varietyof causes, including mechani-

cal injury or pressure, viral or bacterial

infection, poisoning bydrugs or other chem-

icals, and vitamin deﬁciencies. Neuritisin

sensorynerves is characterized by neuralgia

or may result in anesthesia and lossof re-

ﬂexesin the affected area. Neuritis in motor

nervesresults in loss of motor function.

Infections

Herpesis a family of diseases characterized

by skin lesions, which are caused by a

group ofclosely related viruses (the herpes

viruses). The term isderived from the Greek

word herpo, meaning to creep, and indi-

catesa spreading skin eruption. The viruses

apparentlyreside in the ganglia of sensory

nervesand cause lesions along the course

of the nerve. Herpes simplex II, or genital

herpes, isusually responsible for a sexually

transmitted disease causing lesionson the

externalgenitalia.

The varicella-zoster virus causes the

diseaseschicken pox in children and shin-

gles in older adults, a disease also called

herpeszoster. Normally, thisvirus ﬁrst en-

tersthe body in childhood to cause chicken

pox. The virus then lies dormant in the

spinal ganglia for manyyears and can be-

come active during a time ofreduced resist-

ance to cause shingles, a unilateral patch

ofskin blisters and discoloration along the

path of one or more spinal nerves, most

commonlyaround the waist. The symptoms

can persistfor 3–6 months.

Poliomyelitis (po¯⬘le¯-o¯-mı¯⬘e˘-lı¯⬘tis; “po-

lio” or infantile paralysis; the Greekword

polio means gray matter) is a disease

caused byan Enterovirus. It’sactually a CNS

infection, butits major effect is on the pe-

ripheral nerves and the muscles they sup-

ply. The virusinfects the motor neurons in

the anterior horn ofthe spinal cord. The in-

fection causes degeneration of the motor

neurons, which resultsin paralysis and at-

rophy of the muscles innervated by those

nerves.

Anestheticleprosy is a bacterial infec-

tion of the peripheral nerves caused by

Mycobacterium leprae. The infection re-

sults in anesthesia, paralysis, ulceration,

and gangrene.

Geneticand Autoimmune

Disorders

Myotonicdystrophy is an autosomal domi-

nant hereditary disease characterized by

muscle weakness, dysfunction, and atro-

phyand by visual impairment as a result of

nerve degeneration.

Myasthenia (mı¯-as-the¯⬘ne¯-a˘) gravisis

an autoimmune disorder resulting in a re-

duction in the number offunctional acetyl-

choline receptors in neuromuscular

junctions. T cells of the immune system

breakdown acetylcholine receptor proteins

into two fragments, which trigger antibody

production by the immune system. Myas-

thenia gravisresults in fatigue and progres-

sive muscular weakness because of the

neuromuscular dysfunction.

Part3 Integration and ControlSystems428

SpinalCord

(p. 402)

GeneralStructure

1. The spinal cord gives rise to 31 pairs ofspinal nerves. The spinal

cord has cervical and lumbar enlargements where nerves ofthe

limbs enter and leave.

2. The spinal cord is shorter than the vertebral column.Nerves from

the end ofthe spinal cord form the cauda equina.

Meningesof the Spinal Cord

Three meningeal layers surround the spinal cord: the dura mater, arach-

noid mater,and pia mater.

CrossSection of the Spinal Cord

1. The cord consists ofperipheral white matter and central gray matter.

2. White matter is organized into funiculi,which are subdivided into

fasciculi,or nerve tracts, which carry action potentials to and from

the brain.

3. Gray matter is divided into horns.

• The dorsal horns contain sensory axons that synapse with

interneurons.The ventral horns contain the neuron cell bodies of

somatic motor neurons,and the lateral horns contain the neuron

cell bodies ofautonomic neurons.

• The gray and white commissures connect each half of the spinal cord.

4. The dorsal root conveys sensory input into the spinal cord,and the

ventral root conveys motor output away from the spinal cord.

SUMMARY

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Chapter 12 Spinal Cord and Spinal Nerves 429

Reﬂexes

(p. 405)

1. A reﬂex arc is the functional unit ofthe nervous system.

• Sensory receptors respond to stimuli and produce action

potentials in sensory neurons.

• Sensory neurons propagate action potentials to the CNS.

• Interneurons in the CNS synapse with sensory neurons and with

motor neurons.

• Motor neurons carry action potentials from the CNS to effector

organs.

• Effector organs such as muscles or glands respond to the action

potentials.

2. Reﬂexes don’t require conscious thought,and they produce a

consistent and predictable result.

3. Reﬂexes are homeostatic.

4. Reﬂexes are integrated within the brain and spinal cord.Higher

brain centers can suppress or exaggerate reﬂexes.

Stretch Reﬂex

Muscle spindles detect stretch ofskeletal muscles and cause the muscle to

shorten reﬂexively.

Golgi Tendon Reﬂex

Golgi tendon organs respond to increased tension within tendons and

cause skeletal muscles to relax.

WithdrawalReﬂex

1. Activation ofpain receptors causes contraction of muscles and the

removal ofsome part of the body from a painful stimulus.

2. Reciprocal innervation causes relaxation ofmuscles that would

oppose the withdrawal movement.

3. In the crossed extensor reﬂex,during ﬂexion of one limb caused by

the withdrawal reﬂex,the opposite limb is stimulated to extend.

SpinalCord Pathways

(p. 410)

Convergent and divergent pathways interact with reﬂexes.

Structure ofPeripheral Nerves

(p. 410)

In the PNS,individual axons are surrounded by the endoneurium.Groups

ofaxons, called fascicles, are bound together by the perineurium. The fas-

cicles form the nerve and are held together by the epineurium.

SpinalNerves

(p. 410)

1. Eight cervical,12 thoracic, 5 lumbar, 5 sacral pairs, and 1 coccygeal

pair make up the spinal nerves.

2. Spinal nerves have speciﬁc cutaneous distributions called dermatomes.

3. Spinal nerves branch to form rami.

• The dorsal rami supply the muscles and skin near the midline of

the back.

• The ventral rami in the thoracic region form intercostal nerves,

which supply the thorax and upper abdomen.The remaining ventral

rami join to form plexuses (see following summary sections).

Communicating rami supply sympathetic nerves (see chapter 16).

CervicalPlexus

Spinal nerves C1–C4 form the cervical plexus,which supplies some mus-

cles and the skin of the neck and shoulder.The phrenic nerves innervate

the diaphragm.

BrachialPlexus

1. Spinal nerves C5–T1 form the brachial plexus,which supplies the

upper limb.

2. The axillary nerve innervates the deltoid and teres minor muscles

and the skin ofthe shoulder.

3. The radial nerve supplies the extensor muscles ofthe arm and forearm

and the skin ofthe posterior surface of the arm, forearm, and hand.

4. The musculocutaneous nerve supplies the anterior arm muscles and

the skin ofthe lateral surface of the forearm.

5. The ulnar nerve innervates most ofthe intrinsic hand muscles and

the skin on the ulnar side ofthe hand.

6. The median nerve innervates the pronator and most ofthe ﬂexor

muscles ofthe forearm, most of the thenar muscles, and the skin of

the radial side ofthe palm of the hand.

7. Other nerves supply most ofthe muscles that act on the arm, the

scapula,and the skin of the medial arm and forearm.

Lumbar and SacralPlexuses

1. Spinal nerves L1–S4 form the lumbosacral plexus.

2. The obturator nerve supplies the muscles that adduct the thigh and

the skin ofthe medial thigh.

3. The femoral nerve supplies the muscles that ﬂex the thigh and

extend the leg and the skin ofthe anterior and lateral thigh and the

medial leg and foot.

4. The tibial nerve innervates the muscles that extend the thigh and

ﬂex the leg and the foot.It also supplies the plantar muscles and the

skin ofthe posterior leg and the sole of the foot.

5. The common ﬁbular nerve supplies the short head ofthe biceps

femoris,the muscles that dorsiﬂex and plantar ﬂex the foot, and the

skin ofthe lateral and anterior leg and the dorsum of the foot.

6. In the thigh,the tibial nerve and the common ﬁbular nerve are

combined as the ischiadic (sciatic) nerve.

7. Other lumbosacral nerves supply the lower abdominal muscles,the

hip muscles,and the skin of the suprapubic area, external genitalia,

and upper medial thigh.

CoccygealPlexus

Spinal nerves S4,S5, and Co form the coccygeal plexus, which supplies the

muscles ofthe pelvic ﬂoor and the skin over the coccyx.

1. The spinal cord extends from the

a. medulla oblongata to the coccyx.

b. level ofthe third cervical vertebra to the coccyx.

c. level ofthe axis to the lowest lumbar vertebra.

d. medulla oblongata to the level ofthe second lumbar vertebra.

e. axis to the sacral hiatus.

2. The structure that anchors the inferior end ofthe spinal cord to the

coccyx is the

a. conus medullaris.

b. cauda equina.

c. ﬁlum terminale.

d. lumbar enlargement.

e. posterior median sulcus.

REVIEW AND COMPREHENSION

Seeley−Stephens−Tate:

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III. Integration and Control

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12. Spinal Cord and Spinal

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3. Axons ofsensory neurons synapse with the cell bodies of

interneurons in the ofspinal cord gray matter.

a. anterior horn

b. lateral horn

c. posterior horn

d. gray commissure

e. lateral funiculi

4. Cell bodies for sensory neurons are located in the

a. anterior horn ofspinal cord gray matter.

b. lateral horn ofspinal cord gray matter.

c. posterior horn ofspinal cord gray matter.

d. dorsal root ganglia.

e. posterior columns.

5. Given these components ofa reﬂex arc:

1. effector organ

2. interneuron

3. motor neuron

4. sensory neuron

5. sensory receptor

Choose the correct order an action potential follows after a sensory

receptor is stimulated.

a. 5,4,3,2,1

b. 5,4,2,3,1

c. 5,3,4,1,2

d. 5,2,4,3,1

e. 5,3,2,1,4

6. A reﬂex response accompanied by the conscious sensation ofpain is

possible because of

a. convergent pathways.

b. divergent pathways.

c. a reﬂex arc that contains only one neuron.

d. sensory perception in the spinal cord.

7. Several ofthe events that occurred between the time that a physician

struck a patient’s patellar tendon with a rubber hammer and the

time the quadriceps femoris contracted (knee-jerk reﬂex) are listed

below:

1. increased frequency ofaction potentials in sensory neurons

2. stretch ofthe muscle spindles

3. increased frequency ofaction potentials in the alpha motor

neurons

4. stretch ofthe quadriceps femoris

5. contraction ofthe quadriceps femoris

Which ofthese lists most closely describes the sequence of events as

they normally occur?

a. 4,1,2,3,5

b. 4,1,3,2,5

c. 1,4,3,2,5

d. 4,2,1,3,5

e. 4,2,3,1,5

8. are responsible for regulating the sensitivity ofthe

muscle spindle.

a. Alpha motor neurons

b. Sensory neurons

c. Gamma motor neurons

d. Golgi tendon organs

e. Inhibitory interneurons

Part3 Integration and ControlSystems430

9. Which ofthese events occurs when a person steps on a tack with the

right foot?

a. The right foot is pulled away from the tack because ofthe Golgi

tendon reﬂex.

b. The left leg is extended to support the body because ofthe

stretch reﬂex.

c. The ﬂexor muscles ofthe right thigh contract, and the extensor

muscles ofthe right thigh relax because of reciprocal

innervation.

d. Extensor muscles contract in both thighs because ofthe crossed

extensor reﬂex.

10. Which ofthese is a correct count of the spinal nerves?

a. 9 cervical,12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

b. 8 cervical,12 thoracic, 5 lumbar,5 sacral, 1 coccygeal

c. 7 cervical,12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

d. 8 cervical,11 thoracic, 4 lumbar,6 sacral, 1 coccygeal

e. 7 cervical,11 thoracic, 5 lumbar, 6 sacral, 1 coccygeal

11. Given these structures:

1. dorsal ramus

2. dorsal root

3. plexus

4. ventral ramus

5. ventral root

Choose the arrangement that lists the structures in the order that an

action potential passes through them,given that the action potential

originates in the spinal cord and propagates to a peripheral nerve.

a. 2,1,3

b. 2,3,1

c. 3,4,5

d. 5,3,4

e. 5,4,3

12. Damage to the dorsal ramus ofa spinal nerve results in

a. loss ofsensation.

b. loss ofmotor function.

c. both a and b.

13. A collection ofspinal nerves that join together after leaving the

spinal cord is called a

a. ganglion.

b. nucleus.

c. projection nerve.

d. plexus.

14. A dermatome

a. is the area ofskin supplied by a pair of spinal nerves.

b. may be supplied by more than one nerve from a plexus.

c. can be used to locate the site ofspinal cord injury.

d. all ofthe above.

15. Which ofthese nerves arises from the cervical plexus?

a. median

b. musculocutaneous

c. phrenic

d. obturator

e. ulnar

16. The skin on the posterior surface ofthe hand is supplied by the

a. median nerve.

b. musculocutaneous nerve.

c. ulnar nerve.

d. axillary nerve.

e. radial nerve.

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

III. Integration and Control

Systems

12. Spinal Cord and Spinal

Nerves

Companies, 2004

Chapter 12 Spinal Cord and Spinal Nerves 431

17. The thenar muscles and most ofthe ﬂexor muscles of the forearm

are supplied by the

a. musculocutaneous nerve.

b. radial nerve.

c. median nerve.

d. ulnar nerve.

e. axillary nerve.

18. The intrinsic hand muscles,other than those that move the thumb,

are supplied by the

a. musculocutaneous nerve.

b. radial nerve.

c. median nerve.

d. ulnar nerve.

e. axillary nerve.

19. The ischiadic (sciatic) nerve is actually two nerves combined within

the same sheath. The two nerves are the

a. femoral and obturator.

b. femoral and gluteal.

c. common ﬁbular (peroneal) and tibial.

d. common ﬁbular (peroneal) and obturator.

e. tibial and gluteal.

20. The muscles ofthe anterior compartment of the thigh are supplied

by the

a. obturator nerve.

b. gluteal nerve.

c. ischiadic (sciatic) nerve.

d. femoral nerve.

e. ilioinguinal nerve.

Answers in Appendix F

1. Describe how stimulation ofa neuron that has its cell body in the

cerebrum could inhibit a reﬂex that is integrated within the spinal cord.

2. A cancer patient has his left lung removed.To reduce the space

remaining where the lung is removed,the diaphragm on the left side

is paralyzed to allow the abdominal viscera to push the diaphragm

upward.What nerve is cut? Where is a good place to cut it,and

when would the surgery be done?

3. Based on sensory response to pain in the skin ofthe hand, how

could you distinguish between damage to the ulnar,median,and

radial nerves?

4. During a difﬁcult delivery,the baby’s arm delivered ﬁrst.The

attending physician grasped the arm and forcefully pulled it.Later a

nurse observed that the baby could not abduct or adduct the medial

four ﬁngers and ﬂexion ofthe wrist was impaired. What nerve was

damaged?

5. Two patients are admitted to the hospital.According to their charts,

both have herniated disks that are placing pressure on the roots of

the ischiadic nerve.One patient has pain in the buttocks and the

posterior aspect ofthe thigh. The other patient experiences pain in

the posterior and lateral aspects ofthe leg and the lateral part of the

ankle and foot.Explain how the same condition, a herniated disk,

could produce such different symptoms.

6. In an automobile accident a woman suffers a crushing hip injury.

For each ofthe conditions given here, state what nerve is damaged.

a. Unable to adduct the thigh

b. Unable to extend the leg

c. Unable to ﬂex the leg

d. Loss ofsensation from the skin of the anterior thigh

e. Loss ofsensation from the skin of the medial thig h

Answers in Appendix G

CRITICAL THINKING

1. The cord is enlarged in the inferior cervical and superior lumbar

regions because ofthe large numbers of motor nerve ﬁbers exiting

from the cord to the limbs and sensory nerve ﬁbers entering the cord

from the limbs.Also, more neuron cell bodies in the spinal cord regions

are associated with the increased numbers ofsensory and motor ﬁbers.

2. Dorsal root ganglia contain neuron cell bodies,which are larger in

diameter than the axons ofthe dorsal roots.

3. Nerves C5–T1,which innervate the left arm, forearm, and hand, are

damaged.

4. Damage to the right phrenic nerve results in the absence of

muscular contraction in the right halfof the diaphr agm.Because

the phrenic nerves originate from C3–C5,damage to the upper

cervical region ofthe spinal cord eliminates their functions; damage

in the lower cord below the point where the spinal nerves originate

doesn’t affect the nerves to the diaphragm.Breathing is affected,

however,because the intercostal nerves to the intercostal muscles,

which move the ribs,are paralyzed.

5. The radial nerve lies along the shaft ofthe humerus about midway

along its length.If the humerus is fractured, the radial nerve can be

lacerated by bone fragments or,more commonly,pinched between

two fragments ofbone, decreasing or eliminating the function of the

nerve.

ANSWERS TO PREDICT QUESTIONS

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