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Copyright (c) The McGrawCopyright (c) The McGraw--HillHillCompanies, Inc. Permission requiredCompanies, Inc. Permission required

for reproduction or display.for reproduction or display. 11

Chapter FourteenChapter Fourteen

The Spinal Cord andThe Spinal Cord andSpinal NervesSpinal Nerves

Lecture OutlineLecture Outline

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The Spinal CordThe Spinal Cord -- FunctionsFunctions

The spinal cord serves threeThe spinal cord serves three

principal functions:principal functions:

1.1. ConductionConduction

2.2. LocomotionLocomotion

3.3. ReflexesReflexes


Conduction.Conduction.

The spinal cord contains bundles of nerveThe spinal cord contains bundles of nerve

fibers that conduct information up and downfibers that conduct information up and down

the body, connecting different levels of thethe body, connecting different levels of the

trunk with each other and with the braintrunk with each other and with the brain

It enables sensory information to reach theIt enables sensory information to reach the

brain, motor commands to reach thebrain, motor commands to reach the

effectors, and input received at one level ofeffectors, and input received at one level ofthe cord to affect output from another levelthe cord to affect output from another level

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Locomotion.Locomotion.

Walking involves repetitive, coordinated contractionsWalking involves repetitive, coordinated contractionsof several muscle groups in the limbsof several muscle groups in the limbs

Motor neurons in the brainMotor neurons in the brain initiateinitiate walking, but thewalking, but thesimple repetitive muscle contractions that put onesimple repetitive muscle contractions that put onefoot in front of another, over and over, arefoot in front of another, over and over, arecoordinated by groups of neurons calledcoordinated by groups of neurons called centralcentralpattern generatorspattern generators in the cord.in the cord.

These neuronal circuits produce the sequence of outputsThese neuronal circuits produce the sequence of outputs

to the extensor and flexor muscles that cause alternatingto the extensor and flexor muscles that cause alternatingmovements of the legsmovements of the legs

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Reflexes.Reflexes.

Reflexes are involuntary stereotypedReflexes are involuntary stereotyped

responses to stimuliresponses to stimuli

They involve the brain, spinal cord, andThey involve the brain, spinal cord, and

peripheral nervesperipheral nerves

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Surface AnatomySurface Anatomy

TheThe spinal cordspinal cord beginsbegins

at the foramenat the foramen

magnum of the skullmagnum of the skull

and passes through theand passes through the

vertebral canal as farvertebral canal as far

as the inferior marginas the inferior margin

of the first lumbarof the first lumbar

vertebra (L1) or slightlyvertebra (L1) or slightly

beyond.beyond.

FIGURE 14.1

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In adults, it averages about 1.8 cm thick and 45 cmIn adults, it averages about 1.8 cm thick and 45 cmlonglong

It occupies only the upper twoIt occupies only the upper two--thirds of thethirds of thevertebral canalvertebral canal

The cord gives rise to 31 pairs of spinal nervesThe cord gives rise to 31 pairs of spinal nerves

The first pair of nerves pass between the skull andThe first pair of nerves pass between the skull andvertebra C1, and the rest pass through thevertebra C1, and the rest pass through theintervertebral foramina.intervertebral foramina.

The part supplied by each pair of spinal nerves isThe part supplied by each pair of spinal nerves iscalled acalled a segment.segment.

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The cord is divided intoThe cord is divided intocervicalcervical,, thoracicthoracic,, lumbarlumbar,,

andand sacral regionssacral regions, named, namedfor the levels of thefor the levels of the

vertebral column throughvertebral column through

which the spinal nerveswhich the spinal nerves

emergeemerge

A bundle of nerve rootsA bundle of nerve roots

called thecalled the cauda equinacauda equinaoccupies the canal from L2occupies the canal from L2

to S5to S5

FIGURE 14.1

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Meninges of the Spinal CordMeninges of the Spinal Cord TheThe dura materdura mater forms a toughforms a tough collagenouscollagenous

membrane sleeve called themembrane sleeve called the dural sheathdural sheath around thearound thespinal cordspinal cord

The space between the sheath and vertebral bone,The space between the sheath and vertebral bone,called thecalled the epidural spaceepidural space, is occupied by blood vessels,, is occupied by blood vessels,adipose tissue, and loose connective tissueadipose tissue, and loose connective tissue

TheThe arachnoidarachnoid matermater consists of a simpleconsists of a simple squamoussquamousepithelium, theepithelium, the arachnoidarachnoidmembranemembrane,, adhering toadhering tothe inside of thethe inside of the duradura, and a loose mesh of, and a loose mesh ofcollagenouscollagenous and elastic fibers spanning the gapand elastic fibers spanning the gapbetween thebetween the arachnoidarachnoid membrane and themembrane and the piapiamatermater

TheThe piapia matermater is a delicate, translucent membraneis a delicate, translucent membranethat closely follows the contours of the spinal cord.that closely follows the contours of the spinal cord.

Anatomy of the Spinal CordAnatomy of the Spinal Cord

The spinal cord isThe spinal cord is

made up of regionsmade up of regions

of gray & whiteof gray & white

matter, and servesmatter, and serves

as anas aninformationinformation

highwayhighway..

The spinal cord canThe spinal cord can

range from 40range from 40--45 cm45 cm

in length for anin length for an

adult.adult.

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Cross Sectional AnatomyCross Sectional Anatomy

The spinal cord, like the brain, consists of twoThe spinal cord, like the brain, consists of two

kinds of nervous tissue called gray and whitekinds of nervous tissue called gray and white

matter.matter.1.1. Gray MatterGray Matter: consists of neuronal cell bodies, dendrites,: consists of neuronal cell bodies, dendrites,

unmyelinatedunmyelinated axons, axon terminals,axons, axon terminals, neuroglianeuroglia, and blood, and blood

vessels.vessels.

2.2. White MatterWhite Matter: consists of: consists ofmyelinatedmyelinated &&unmyelinatedunmyelinated nervenerve

axons, and blood vessels.axons, and blood vessels.

Gray MatterGray Matter

Gray matterGray matter has a relatively dull colorhas a relatively dull color

because it contains little myelinbecause it contains little myelin

It contains the somas, dendrites, andIt contains the somas, dendrites, and

proximal parts of the axons of neurons.proximal parts of the axons of neurons.

It is the site of synaptic contact betweenIt is the site of synaptic contact between

neurons, and therefore the site of all informationneurons, and therefore the site of all information

processing in the central nervous systemprocessing in the central nervous system

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White MatterWhite Matter

White matterWhite matter contains an abundance of myelinatedcontains an abundance of myelinated

axons, which give it a bright, pearly whiteaxons, which give it a bright, pearly white

appearanceappearance

It is composed of bundles of axons, calledIt is composed of bundles of axons, called tractstracts,,

that carry signals from one part of the CNS tothat carry signals from one part of the CNS to

another.another.

Gray & White MatterGray & White Matter

Internal Anatomy of the Spinal CordInternal Anatomy of the Spinal Cord

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The central gray matter consists of twoThe central gray matter consists of two dorsaldorsal

((posteriorposterior)) hornshorns, and two thicker, and two thicker ventralventral

((anterioranterior)) hornshorns..

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In the thoracic and lumbar regions, an additionalIn the thoracic and lumbar regions, an additional

lateral hornlateral horn is visible on each side of the gray matteris visible on each side of the gray matter

It contains neurons of the sympathetic nervousIt contains neurons of the sympathetic nervous

system, which send their axons out of the cord by waysystem, which send their axons out of the cord by way

of the ventral root along with the somatic efferentof the ventral root along with the somatic efferent

fibersfibers 1818

Spinal Nerve RootsSpinal Nerve Roots

As a spinal nerve approaches the cord, it branches into aAs a spinal nerve approaches the cord, it branches into adorsal rootdorsal rootandand ventral rootventral root

The dorsal root carries sensory nerve fibers, which enter theThe dorsal root carries sensory nerve fibers, which enter the

dorsal horn of the cord.dorsal horn of the cord.

The ventral horns contain the large somas of the somaticThe ventral horns contain the large somas of the somatic

motor neurons, which send their axons out to the body.motor neurons, which send their axons out to the body.

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White MatterWhite Matter

The white matter of the spinal cord consists of bundles ofThe white matter of the spinal cord consists of bundles ofaxons that course up and down the cord and providesaxons that course up and down the cord and providesavenues of communication between different levels of theavenues of communication between different levels of theCNS.CNS.

These bundles are arranged in three pairs of columnsThese bundles are arranged in three pairs of columns;; dorsaldorsal((posteriorposterior)) columnscolumns, a, a lateral columnslateral columns, and, and ventralventral ((anterioranterior))columns.columns.

Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

FIGURE 14.2b

2020

FIGURE 14.3 Tracts of the Spinal Cord. All of the

illustrated tracts occur on both sides of the cord,but only the ascending sensory tracts are shownon the left(red), and only the descending motor

tracts on the right(green).

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Spinal TractsSpinal Tracts

Ascending tractsAscending tracts carry sensory information up the cordcarry sensory information up the cord

andand descending tractsdescending tracts conduct motor impulses downconduct motor impulses down

All nerve fibers in a given tract have a similar origin,All nerve fibers in a given tract have a similar origin,

destination, and functiondestination, and function


FIGURE 14.3

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Spinal TractsSpinal Tracts

Several of these tracts cross over from the left side of theSeveral of these tracts cross over from the left side of the

body to the right, or vice versa, as they pass up or down thebody to the right, or vice versa, as they pass up or down thebrainstem and spinal cord.brainstem and spinal cord.

As a result, the left side of the brain receives sensoryAs a result, the left side of the brain receives sensoryinformation from the right side of the body and sends itsinformation from the right side of the body and sends its

motor commands to that side, while the right side of themotor commands to that side, while the right side of thebrain senses and controls the left side of the bodybrain senses and controls the left side of the body

When the origin and destination of a tract are on oppositeWhen the origin and destination of a tract are on oppositesides of the body, we say they aresides of the body, we say they are contralateralcontralateral to eachto each

other.other.

When a tract does not cross, so the origin and destination ofWhen a tract does not cross, so the origin and destination of

its fibers are on the same side of the body, we say they areits fibers are on the same side of the body, we say they are

ipsilateralipsilateral..


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Ascending TractsAscending Tracts

Ascending tracts carry sensory signals up the spinalAscending tracts carry sensory signals up the spinal

cordcord

Sensory signals typically travel across three neuronsSensory signals typically travel across three neurons

from their origin in the receptors to their destination infrom their origin in the receptors to their destination in

the sensory areas of the brain.the sensory areas of the brain.


FIGURE 14.3

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Descending TractsDescending Tracts

Descending tracts carry motor signals down theDescending tracts carry motor signals down the

brainstem and spinal cordbrainstem and spinal cord

A descending motor pathway typically involves twoA descending motor pathway typically involves two

neurons called the upper and lower motor neuronneurons called the upper and lower motor neuron


FIGURE 14.3

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Descending TractsDescending Tracts

TheThe upper motor neuronupper motor neuron begins with a soma in thebegins with a soma in the

cerebral cortex or brainstem and has an axon thatcerebral cortex or brainstem and has an axon that

terminates on aterminates on a lower motor neuronlower motor neuron in the brainstem orin the brainstem or

spinal cordspinal cord

The axon of the lower motor neuron then leads the restThe axon of the lower motor neuron then leads the rest

of the way to the muscle or other target organof the way to the muscle or other target organ


FIGURE 14.3

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General Anatomy ofGeneral Anatomy ofNerves and GangliaNerves and Ganglia

A nerve is a cordlikeA nerve is a cordlike

organ composed oforgan composed of

nerve fibers (axons)nerve fibers (axons)

and connective tissueand connective tissue

FIGURE 14.7a

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FIGURE 14.7a Anatomy of a Nerve.A spinal nerve and its association with the spinal cord.

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General Anatomy ofGeneral Anatomy of

Nerves and GangliaNerves and Ganglia

Each nerve fiber isEach nerve fiber isenclosed in its ownenclosed in its ownfibrous sleeve calledfibrous sleeve calledanan endoneuriumendoneurium

Nerve fibers areNerve fibers arebundled in groupsbundled in groupscalledcalled fasciclesfascicles

They areThey are separatedseparatedfrom each other by afrom each other by aperineuriumperineurium

A fibrousA fibrous epineuriumepineuriumcovers the entirecovers the entirenervenerve

FIGURE 14.7a

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General Anatomy ofGeneral Anatomy ofNerves and GangliaNerves and Ganglia

AAsensory nervesensory nerve isis

composed of afferentcomposed of afferent

fibers onlyfibers only

AAmotormotor nervenerve ofof

efferent fibers onlyefferent fibers only

AAmixedmixed nervenerve isis

composed of bothcomposed of both

Most nerves areMost nerves are

mixedmixed FIGURE 14.7a

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General Anatomy ofGeneral Anatomy of

Nerves and GangliaNerves and Ganglia

AAganglionganglion is ais a

swelling along theswelling along the

course of a nervecourse of a nerve

containing the cellcontaining the cell

bodies of thebodies of the

peripheral neuronsperipheral neurons

FIGURE 14.8

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Spinal NervesSpinal Nerves

Within the vertebralWithin the vertebralcanal, each branchescanal, each branchesinto ainto a dorsal rootdorsal rootwhich carries sensorywhich carries sensorysignals to the dorsalsignals to the dorsalhorn of the spinalhorn of the spinalcord, and acord, and a ventralventralrootroot which receiveswhich receivesmotor signals frommotor signals from

the ventral hornthe ventral horn FIGURE 14.9FIGURE 14.10

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The dorsal root has aThe dorsal root has aswelling, theswelling, the dorsaldorsalroot ganglionroot ganglion,,containing unipolarcontaining unipolarsomatic sensorysomatic sensoryneuronsneurons

FIGURE 14.9FIGURE 14.10

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There are 31 pairs ofThere are 31 pairs ofspinal nervesspinal nerves,, whichwhichenter and leave theenter and leave thespinal cord andspinal cord andemerge mainlyemerge mainlythrough thethrough theintervertebralintervertebralforaminaforamina

FIGURE 14.9


Cervical NervesCervical Nerves

(8 Pairs)(8 Pairs)

Thoracic NervesThoracic Nerves

(12 Pairs)(12 Pairs)

Lumbar NervesLumbar Nerves

(5 Pairs)(5 Pairs)

Sacral NervesSacral Nerves

(5 Pairs)(5 Pairs)

CoccygealCoccygeal NervesNerves(1 Pair)(1 Pair)

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FIGURE 14.9

The Spinal Nerve Roots.

Dorsal View.

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Cutaneous InnervationCutaneous Innervationand Dermatomesand Dermatomes

Each spinal nerve except C1Each spinal nerve except C1receives sensory input from areceives sensory input from aspecific area of skin called aspecific area of skin called adermatome,dermatome, derived from thederived from theembryonic dermatomesembryonic dermatomes

AAdermatome mapdermatome mapis ais adiagram of the cutaneousdiagram of the cutaneousregions innervated by eachregions innervated by eachspinal nervespinal nerve

Such a map is verySuch a map is verysimplified, however, becausesimplified, however, becausethe dermatomes overlap atthe dermatomes overlap attheir edges by as much astheir edges by as much as50%50% FIGURE 14.18

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Cutaneous InnervationCutaneous Innervationand Dermatomesand Dermatomes

Therefore, severance of oneTherefore, severance of onesensory nerve root does notsensory nerve root does notentirely deaden sensationentirely deaden sensationfrom a dermatomefrom a dermatome

It is necessary to sever orIt is necessary to sever oranesthetize three successiveanesthetize three successivespinal nerves to produce aspinal nerves to produce atotal loss of sensation fromtotal loss of sensation fromone dermatomeone dermatome

Spinal nerve damage isSpinal nerve damage isassessed by testing theassessed by testing thedermatomes with pinpricksdermatomes with pinpricksand noting areas in whichand noting areas in whichthe patient has no sensationthe patient has no sensation FIGURE 14.18

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FIGURE 14.18 A Dermatome Map of the Body.Anterior and posterior views. Each zone of the skin is

innervated by sensory branches of the spinal nerves indicatedby the labels. Nerve C1 does not innervate the skin.

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Somatic ReflexesSomatic Reflexes

AAreflexreflex is a quick, involuntary,is a quick, involuntary,stereotyped reaction of a gland or musclestereotyped reaction of a gland or muscleto a stimulusto a stimulus

Visceral reflexesVisceral reflexesare reactions of glands,are reactions of glands,cardiac muscle, and smooth muscle,cardiac muscle, and smooth muscle,controlled by the autonomic nervouscontrolled by the autonomic nervoussystemsystem

SomaticSomatic((spinalspinal))reflexesreflexesare responses ofare responses ofskeletal muscles, controlled by the somaticskeletal muscles, controlled by the somaticnervous systemnervous system

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Somatic ReflexesSomatic Reflexes

A somatic reflex employsA somatic reflex employs

a simple neural pathwaya simple neural pathway

called acalled a reflex arcreflex arc,, inin

which signals travel from:which signals travel from: a somatic receptor through ana somatic receptor through an

afferent nerve fiber to theafferent nerve fiber to the

spinal cord or brainstemspinal cord or brainstem

an integrating center in thean integrating center in theCNSCNS

an efferent nerve fiber leavingan efferent nerve fiber leavingthe CNS, and finally to athe CNS, and finally to askeletal muscleskeletal muscle

FIGURE 14.19

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FIGURE 14.19 A Representative Reflex Arc.

The monosynaptic reflex arc of the patellar tendon reflex.

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Clinical PerspectivesClinical Perspectives

TraumaTrauma is the most common disorder of theis the most common disorder of thespinal cord, usually resulting from accidentsspinal cord, usually resulting from accidents

Complete transection of the spinal cordComplete transection of the spinal cordimmediately abolishes sensation and motorimmediately abolishes sensation and motorcontrol in areas below the injurycontrol in areas below the injury

SpinalSpinal shockshocktypically lasts up to 20 days fromtypically lasts up to 20 days fromthe injurythe injury

Somatic and autonomic reflexes then begin toSomatic and autonomic reflexes then begin toreappear, and may be exaggeratedreappear, and may be exaggerated

((hyperreflexiahyperreflexia).).

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Clinical PerspectivesClinical Perspectives

Flaccid paralysis is typically replaced by spasticFlaccid paralysis is typically replaced by spastic

paralysis as reflex functions returnparalysis as reflex functions return

ParaplegiaParaplegia (paralysis of both lower limbs) and(paralysis of both lower limbs) and

quadriplegiaquadriplegia (paralysis of all four limbs) are(paralysis of all four limbs) are

common consequences of spinal cord injury,common consequences of spinal cord injury,

whilewhile hemiplegiahemiplegia (paralysis of one side of the(paralysis of one side of the

body) usually results from a brain lesionbody) usually results from a brain lesion

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