Chapter 13

Copyright © John Wiley & Sons, Inc. All rights reserved.
Chapter 13
The Spinal
Cord and
Spinal Nerves
Lecture slides prepared by Curtis DeFriez, Weber State University

Introduction to the Spinal Cord
 About 100 million neuronsand even moreneuroglia
comprisethespinal cord, thepart of thecentral nervous
system that extendsfrom thebrain.
 Thespinal cord and itsassociated spinal nervescontain
reflex circuits that control someof your most rapid
reactionsto environmental changes.
 Thegray matter of thecord isasitefor integrationof
postsynaptic potentials(IPSPsand EPSPs).
 Thewhitematter of thecord containsmajor sensory and
motortracts to and from thebrain.

 Thespinal cord beginsasacontinuation of themedulla
oblongata(themost inferior portion of the
brain stem) extending
from theforamen
magnum of the
occipital boneto its
termination asthe
conus medullaris
between L1 - L2.
External Cord Anatomy

 Thespinal cord isoval in shapeand slightly flattened
anteriorly and posteriorly.
 Two typesof connectivetissue
coveringsprotect thecord
and providephysical stability:
Thebony vertebral column
providesthebackbone.
Thespinal meninges surround
thecord asacontinuation of
thecranial meningesthat encirclethebrain.

 Thesethreemembranes(each called ameninx) are
labeled from superficial to deep asfollows:
Theoutermost dura mater(tough mother) formsasac that
enclosestheentirecord.
Themiddlemeninx isadelicateavascular covering
called thearachnoid mater. It isattached to theinside
of theduraand formstheroof of thesubarachnoid space
(SAS) in which cerebral spinal fluid (CSF) circulates.
Thetransparent pia materispressed-up against thecord and
isfilled with blood vesselsthat supply nutrientsto it.

 Theepidural space runsbetween theduramater and the
moresuperficial ligamentum flavum (which linesthe
undersideof thebony vertebral lamina).
 Thesubdural space liesbetween theduraand the
arachnoid. In thespinal column, theduraand arachnoid
membranes
areheld firmly together
so that thesubdural
spaceisoften no more
than apotential space.

 Thepiamater has21 pairsof denticulate ligaments which
attach it to thearachnoid and duramaters. Named for their
tooth-likeappearance, thedenticulateligamentsare
traditionally believed to providestability for thespinal cord
against sudden
shock and
displacement within
thevertebral column.

 Arising from theconusmedullarisisthefilum terminale,
an extension of thepiamater that extendsinferiorly and
blendswith the
arachnoid and durato
anchor thespinal cord to
thecoccyx.
 Thecaudaequinaor “horses
tail” aretherootsof the
lower spinal nervesthat angle
down alongsidethefilum terminale.

 Thespinal cord hastwo enlargements, onein thecervical
areafrom C4–T1, and another in thelumbar areabetween
T9–T12.
 Thecervical enlargement
correlateswith thesensory
input and motor output to the
upper extremities.
 Thelumbar enlargement handles
motor output and sensory input
to and from thelegs.

 From superior to inferior, thespinal
cord becomesprogressively smaller.
 Thereislessand lesswhitematter
aswedescend becausethereare
fewer sensory tractsgoing up
(they haven’t “jumped on” yet),
and therearefewer motor tracts
going down (they’ve“jumped
off” already.)

 Two bundlesof axons, called roots, connect each spinal
nerveto asegment of thecord by even smaller bundlesof
axonscalled rootlets.
 The posterior(dorsal) root and rootletscontain only
sensory axons, which conduct
nerveimpulsesfrom sensory
receptorsin theskin, muscles,
and internal organsinto the
central nervoussystem.

 Each posterior root hasaswelling, theposterior(dorsal)
root ganglion, which containsthecell bodiesof sensory
neurons. Theanterior(ventral) root and rootlets contain
axonsof motorneurons, which conduct nerveimpulsesfrom
theCNSto
effectors(muscles
and glands).

Epidural Anesthesia
 Epidural anesthesiaiscommonly administered to women
about to go into labor. In thisprocedure, aneedleisplaced
between thebonesof theposterior spineuntil it just
penetratestheligamentum flavum yet remainssuperficial to
theduramater.
 Local anesthetic isused to providepain relief –
even completeanesthesiaif acesarean section isrequired.

Lumbar Puncture
 A needleinserted into thesubarachnoid spacefor thepurpose
of withdrawing CSF (for diagnosisor to reducepressure) or
to introduceadrug or contrast agent iscalled alumbar
puncture.
 CSF isoften collected to diagnosemeningitisor some
other diseaseof theCNS.
 Agentsinjected into theSASincludedrugssuch as
antibiotics, chemotherapeutic agents, or analgesics, or
contrast mediafor radiographic procedures.
• Thepressureof CSF in theSAScan also bemeasured
during alumbar puncture.

Lumbar Puncture
 Thesiteused for most lumbar puncturesisbetween the3rd
and 4th
(or 4th
and 5th
) lumbar vertebrae- below thetermination
of theactual cord in theregion of thecaudaequina. With the
needlein theSAS, CSF can besampled.
 Anestheticscan also
begiven in thisway,
but using 1/10 the
doserequired for
epidural anesthesia.

Internal Cord Anatomy
 In thespinal cord, thewhitematter ison theoutside, and the
gray matter ison theinside. In thebrain thewhitematter is
on theinside, and thegray matter ison theoutside.

 Thewhite matterof thecord consistsof millionsof nerve
fibers which transmit electrical information between the
limbs, trunk and organsof thebody, and thebrain.
 Internal to thisperipheral region, and surrounding the
central canal, isthe
butterfly-shaped
central region made
up of nerve cell
bodies (gray matter–
herestained brown).

 Anterior(ventral) gray horns consist of somatic motor
neurons.
 Posterior(dorsal) gray horns consist of somatic and
autonomic sensory nuclei.
 Theposteriorgray horn isthesiteof synapsebetween
first-order sensory neuronscoming in from theperiphery,
and second-order neuronswhich either ascend in thecord
or exit back out aspartsof reflex arcs.

 Thelateral gray horns arefound only in thethoracic,
upper lumbar, and sacral segmentsof thecord.
 They contain cell bodiesof autonomic motor neurons
• Someof thesefibersascend outsidethedurabut closeto
thecord to supply sympathetic
innervation to thehead.
• Otherstravel in sympathetic
trunksto theorgansand
glandsof thethorax, abdomen,
and pelvis.

 Other notablefeaturesvisualized on atransected cord arethe
anterior median fissure, theposterior median sulcus, thegray
and whitecommissures, and thecentral canal.
 Thecentral canal extendstheentirelength of thespinal
cord and
isfilled
with CSF.

 A tract isabundleof
neuronal axonsthat areall
located in aspecific areaof
thecord and all traveling to
thesameplace(higher or
lower in thebrain or cord).

 Thewhitematter of thecord isdivided into anterior,
posterior, and lateral columnsin which ascending sensory
tractsaretraveling to someplacein thebrain and descending
motor tracts(red) aretraveling to alocation in thecord.
The afferent tracts are noted here in blue, while the efferent tracts are shown in red.

 Names of tracts areformed by using compound wordsthat
denotetheoriginof thetract, and theplacewhereit ends.
 Thespinothalamic tract goesfrom thespinal cord to the
brain – it isan afferent tract.
 Thecorticospinal tract goesfrom thecortex of thebrain to
thespinal cord – it isan efferent tract.
 Thevestibulospinal tract originatesfrom an areain the
brain which you probably don’t recognize; however, you
can recognizethedestination in thespine, and therefore
deducethat it isamotor tract.

 Theposteriorcolumns areafferent tractsthat convey
nerveimpulsesfor discriminativetouch, light pressure,
vibration, and consciousproprioception (awarenessof
tendon and joint position in spaceand their
relativemovements).
 Thespinothalamic tract isan
afferent tract that transmitssensations
of pain, warmth, coolness, itching,
tickling, deep pressure, and
crudetouch.

 Thelateral and anteriorcorticospinal tracts aremajor
pathwaysfor carrying signalsfrom thecerebral cortex
that result in voluntary movement of skeletal muscles.
 Other motortracts convey nerveimpulsesfrom thebrain
stem that coordinate
visual stimuli with body
movements, maintain
postureand regulate
skeletal muscletone.

 A dermatomeisan areaof skin that is
innervated by asinglespinal nerve,
indicated by thelettersand number of a
particular segmental nerve.
 Important dermatomesinclude:
 C6/C7 - thumb and index
finger (“six-shooter”)
 T4 - nippleline
 T10 - umbilicus
 L1-L5 - lower extremities
“L for legs”)
Dermatomes

Damage to the Cord
 “Transection” of the spinal cord meansthat ascending
and descending tractsarepartially or completely severed.
 If transection occurs, say in amotor vehicleor diving
accident, paralysiswill occur depending on thelevel of the
injury. Transection:
 at thebaseof skull resultsin death by asphyxiation
 in theupper cervical arearesultsin quadriplegia
 between thecord enlargementsresultsin someform of
paraplegia

Peripheral Nerves

Peripheral Nerves
 Spinal nerves arethepathsof communication between the
spinal cord and specific regionsof thebody.
 Nervesarearranged in fasciclessurrounded by a
perineurium, with theentirenervesheathed by aCT
epineurium.

Peripheral Nerves
 31 left-right pairs of spinal nerves emergefrom the
cord at regular intervals(called segments). Except for the
first cervical pair thespinal nervesleavethevertebral
column from theintervertebral foramen between adjoining
vertebrae– thefirst pair leavesbetween the
skull and thefirst cervical vertebrae.
Cervical – 8 pairs, C1-C8
Thoracic – 12 pairs, T1-T12
Lumbar – 5 pairs, L1-L5
Sacral - 5 pairs, S1-S5


 Piercing the dura, thesegmental (spinal) nervesexit the
central nervoussystem into theperipheral nervoussystem
and almost immediately split into 3 major branches: An
anterior ramus, posterior ramus, and rami communicantes
(connectionsto
sympathetic
ganglia).
Peripheral Nerves

Peripheral Nerves
 Theanterior rami of thesegmental nervesmay travel alone
(such astheintercostal nerveswhich run underneath each of
the12 ribs), or they can join together to form large“braided
ropes” – a plexus of nerves.
 Thereareanumber of major nerveplexuses, all formed
from anterior rami of spinal nerves, and all located
anterior to thespine: Thecervical plexus, brachial
plexus, celiac (solar) plexus, lumbarplexus, sacral
plexus, and coccygeal plexus

Nerve Plexuses
 Thecervical plexus, formed by theanterior rami of C1-C5,
servesthehead, neck, and diaphragm.
 Thephrenic nerves arisefrom thecervical plexusto
supply themajor muscleof
respiration (“C3,4,5 keep
thediaphragm alive”).

Nerve Plexuses
 The brachial plexus isformed
by theanterior rami of C5-C8 and
T1. It isdivided into roots→
trunks→ divisions→ cords
→ nerves.
 Thenervesfrom thebrachial
plexussupply theshoulders
and upper limbs.

 Someof themajor nervesthat arisefrom thebrachial
plexusesarethe:
 musculocutaneous
nerve
 axillary nerve
 radial nerve
 median nerve
 ulnarnerve
 long thoracic nerve
Nerve Plexuses

Nerve Plexuses
 Injuriesto thebrachial plexusarenot uncommon:
 Erb’s palsy isaparalysisof thearm that most often
occursasan infant'shead and neck arepulled toward
thesideat thesametimeastheshoulderspass
through thebirth canal.
• A similar injury may beobserved at any age,
including adults, following atraumatic fall
or other traumawhereby
thenervesof theplexus
areviolently stretched.

Nerve Plexuses
 Injuriesto thebrachial plexusor peripheral nerves:
 Median nerve injury, either at theplexusor occurring
moredistally, resultsin numbness, tingling and pain in the
palm and fingers.
• Carpal tunnel syndromeisacommon typeof median
nerveinjury that isseen in peoplewho perform
repetitivemotionsof thehand and
wrist liketyping on acomputer
keyboard.

 Theulnarnerve isthelargest unprotected (by muscleor
bone) nervein thehuman body. It emergesfrom themedial
and lateral cordsof thebrachial plexusto supply themedial
half of thehand. Striking themedial epicondyleof the
humeruswherethenerveisexposed
isreferred to asbumping one’s“funny bone”.
 Damageto thenerveleadsto
abnormal sensationsin the4–5th
fingersand an inability to abduct
or adduct thelittleand ring fingers.
Nerve Plexuses

Nerve Plexuses
 Thelong thoracic nerve emergesfrom thecordsof the
brachial plexusto supply theserratusanterior muscle.
 Becauseof itslong, relatively superficial course, it is
susceptibleto injury either through direct traumaor stretch
of theplexus. Injury (resulting in
a“winged scapula” in which the
arm cannot beabducted beyond the
horizontal position) hasbeen reported
in almost all sports.

Nerve Plexuses
 Thelumbarplexus isformed by theanterior rami of L1-L4
to supply theanterolateral abdominal wall, external genitalia,
and part of thelower limbs.
 Thefemoral and obturator
nerves comefrom the
lumbar plexus.

Nerve Plexuses
 Thesacral plexus isformed by theanterior rami of L4-L5
and S1-S4. It suppliesthebuttocks, perineum, and part of the
lower limbs.
 It givesriseto thelargest
nervein thebody, the
sciatic nerve.

Nerve Plexuses
 Thecoccygeal plexus isformed by theanterior rami of S4-
S5 and thecoccygeal nerves. It isasmall plexusfrom which
theanococcygeal nerveexitsto supply asmall areaof skin in
the
coccygeal region.

Nerve Terminology Summary
 Rootlets roots(ant. and post.)  segmental nerves
 anterior ramus form largenerveplexuses
 posterior ramus
 rami communicantes

Reflexes
 A reflex isafast, involuntary responseto astimulus. In a
spinal reflex theintegration takesplacein thespinal cord,
not thebrain.
 Spinal reflexescan bemonosynaptic (sensory neuron
with motor neuron) or polysynaptic (involving
interneurons), and they can go in and out on thesame, or
on theopposite
sideof the
cord.

Reflexes
 A reflex arc isapathway that anerveimpulsefollowsto
produceareflex. Componentsof areflex arc includea
sensory receptorand asensory neuron, an integrating
centerinsidethecord, an exiting motorneuron, and an
effector(which isusually some
sort of muscleor agland
which makessome-
thing moveor secrete
“involuntarily”).

Reflexes
 Sensory receptors involved in reflex arcsarespecifically
adapted to perceivethestimulusand initiatean impulse.
 Themotor neuron
becomesstimulated
without any processing
in thebrain (that comes
later)… first you kick the
person examining you
(patellarreflex), then you feel
thetap of thehammer!

Reflexes
Interactions Animation
 Componentsof aReflex Arc Animations
You must be connected to the internet to run this animation

 Reflex arcscan beipsilateral (all neuronsand effectorson
thesamesideof thebody) or contralateral (thereceptors
and afferent neuronsareon theopposite
sideof thebody astheefferent
neuronsand effectors.)
Theflexor (withdrawal)
reflex isagood example
of acontralateral reflex
(stepping on atack).
Reflexes

Reflexes
 In addition to initiating theflexor
reflex that causesyou to
withdraw thelimb, thepain
impulsesfrom stepping
on thetack also
initiateacrossed-
extensorreflex to
help you maintain
your balance.

Reflexes
 Someimportant spinal reflexes include:
 Thepatellarreflex in which theleg extendsin response
to stretch of thepatellar tendon. Thisreflex can be
blocked by damagein thecorticospinal tractsfrom
diabetes, neurosyphilis, or damageto thelumbar region
of thespinal cord.
 TheAchilles reflex causescontraction of thecalf when
aforceisapplied to theAchillestendon. It isabsent after
damageto thelower cord or lumbosacral plexus.

Reflexes
 Someimportant spinal reflexes include:
 TheBabinski, orplantarflexion reflex isconsidered
normal in adultsif they flex (curl) thebig toewhen the
soleof thefoot isstimulated. If thesoleof thefoot is
stimulated and thepatient extendsthebig toe, it would
indicatedamagein thecorticospinal tract.
 Infantsnormally extend their toeswhen stimulated in this
way; so an “abnormal Babinski” doesnot indicateany
diseaseor damagein thisagegroup.

 Summary of ReflexesAnimation
Reflexes
Interactions Animation
You must be connected to the internet to run this animation

End of Chapter 13
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