Lecture 13a
Transcript of Lecture 13a
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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
The Spinal CordThe Spinal Cord -- FunctionsFunctions
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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The Spinal CordThe Spinal Cord -- FunctionsFunctions
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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The Spinal CordThe Spinal Cord -- FunctionsFunctions
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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Surface AnatomySurface Anatomy
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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Surface AnatomySurface Anatomy
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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Gray MatterGray Matter
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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Gray MatterGray Matter
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.
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FIGURE 14.2b
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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
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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.
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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
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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
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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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Spinal NervesSpinal Nerves
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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Spinal NervesSpinal Nerves
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
Spinal NervesSpinal Nerves
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