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Inthe cranial vault, the dura mater consists of two layers: anouter periosteal
layeradherenttotheinnersurfaceofthecraniumandaninnermeningeallayer.
At various sites, the two layers of the cranial dura are separated by venous
sinuses.By contrast, the spinal duramateris single layered, devoidofvenous
sinuses,andcontinuousonlywiththemeningeallayerofthecranialdura.There
isaseparateperiostealliningagainsttheboneofthevertebralcanal.Thesmall
spacebetweenthisliningandthespinalduramateriscalledtheepiduralspace.
Thisspacecontainsepiduralfat,areolartissue,andtheinternalvertebralvenous
plexus. The spinal dura forms dural root sleeves that follow the dorsal and
ventralrootsintoeachintervertebralforamen(Figure2.8,lower).Atthesesites,
thesleevesendbyblendingwiththeepineuriumofeachspinalnerve.Thedura
isadherenttotheperiosteumliningtheforaminaatthesesites.Atthelevelofthe
secondsacralvertebra,thetubularduralsactapersdowntoaslender covering
forthefilum terminale (aspecializationof the pia mater describedlater).This
covering,referredtoasthecoccygealligament,anchorsthespinalcordwiththe
vertebralcanalwhere itdescends andattachestotheperiosteum ofthe coccyx
(Figure2.6).CSFpushesthearachnoiddirectlyupagainsttheduramater.Thus,
there is actually no space between the dura and arachnoid under normal
conditions.Several authorsofanatomy have referredtoa “potential”subdural
space, which may be increased under pathologic conditions. For instance,
rupture of bridging veins between the dura and arachnoid may result in the
accumulation of blood and the expansion of thisspace, a condition knownas
subduralhematoma.
Thearachnoidiscontinuouswiththesamelayersurroundingthebrain.This
delicate avascular membrane completely lines the dural sac and its dural root
sleevesand thuscovers thenerverootsanddorsal rootganglia. Likethedura,
the arachnoid blends with the epineurium of the spinal nerves. The arachnoid
followsthedurainferiorly,whereitendsasasacatthelevelofthesecondsacral
vertebraenclosingthecaudaequina(Figure2.6).Thesubarachnoidspace,filled
withCSF, surroundsthe spinalcord andpiamaterincludingitsspecializations
(thedenticulateligaments and filum terminale are describedinthefollowing).
Thespaceis largestinferiorlybetween thelevelofthesecondlumbarvertebra
andthesecondsacralvertebrainaregionknownasthelumbarcistern(Figure
2.6).The lumbarcistern containsthecauda equinaand filumterminale,which
arefree-floatingwithintheCSF.Needlesmaybeinsertedintothelumbarcistern
attheintervertebralspacebetweenL3andL4orL4andL5todrawoffCSFfor
sampling(a“spinaltap”)ortoinjectanesthetics (a“spinalblock”)withoutthe
riskofinjuringthespinal cord,whichendsastheconusmedullaris atthefirst
lumbar vertebral level. The nerve roots of the cauda equina are generally not
injuredbytheprocedurebecause,beingsuspendedinfluid,theyrollawayfrom
theneedlepoint.
Thepia mater istheinnermostofthethreemeningealinvestmentsandthe
onlyonethatdirectlycontactsthesurfaceofthespinalcord(Figure2.8).Thepia
alsodirectlycoverstherootsofthespinalnervesandthespinalbloodvessels.At
theconusmedullaris,thepiamatercontinuesinferiorlyintothelumbarcisternas
aslenderfilamentcalledthefilumterminale(Figures2.6and 2.7).Surrounded
bythenerverootsofthecaudaequina,thefilumterminalepiercesthearachnoid
at the level of the second sacral vertebra and then becomes invested by dura
mater to become the coccygeal ligament (Figure 2.7). By attaching to the
periosteumofthecoccyx,thecoccygealligamentcentersthespinalcordwithin
thelumbarcistern andserves asananchortomaintainthatposition toprevent
cord injury, especially under conditions of sudden jarring that could force
delicatespinaltissueupagainstthebonywallsofthevertebralcanal.
FIGURE2.8Spinalmeningesandnerveroots.
Bilaterally thickened lateral extensions of the pia mater give rise to the
denticulateligaments(Figure 2.8). The two denticulateligaments—one onthe
rightandanotherontheleft—arelocatedbetweenthedorsalandventralrootson
eachsideofthespinalcord. Fromeachligament20to22tooth-liketriangular
processesextendfurtherlaterally,piercethearachnoid,andfusewiththedura.
Theattachment oftheprocesses oftheligament alternateswiththe passageof
thenerverootsthroughtheduramater(Figure2.8).Thefirstattachmentisatthe
levelof theforamen magnum,whereasthe mostcaudal attachmentisbetween
theT12andL1nerveroots.Asinthefilumterminaleandcoccygealligaments,
the attachments of the denticulate ligaments to the dura mater provide an
importantfixationofthespinalcord,protectingitfromsuddendisplacements.
OrganizationofaSpinalNerve
Sensoryinformation (i.e., pain, temperature, touch, etc.) is conveyed from the
body into the spinal cord via the dorsal roots, which enter the spinal cord
posteriorlyalongtheposterolateralsulcus(Figure2.8).The neuroncell bodies
responsible for conveying the sensory (afferent) information are located in a
distal swelling of the dorsal root known as the spinal ganglion. The spinal
ganglion is generally surrounded by bone and is found at the level of the
intervertebral foramen. The ventral roots of the spinal cord convey motor
(efferent)informationfromthespinalcordtoskeletalmuscle.Ventralrootfibers
exit the spinal cord at the anterolateral sulcus and also contain axons from
pregangionicautonomicneurons(Figure2.8).Thedorsalandventralrootsthat
delineate a spinal cord segmental level fuse at the level of the intervertebral
foramen just distal to the spinal ganglion to form the spinal nerve associated
with that spinal cord segment. The spinal nerve is a mixed nerve, in that it
contains both sensory and motor fibers from the dorsal and ventral roots,
respectively.Thefirstcervicalnervelacksdorsalrootsin50%ofpeople,andthe
coccygealnerves maybe absententirely(7).Itshould benoted thatthespinal
nervesare relatively short. Almost immediately after exiting the intervertebral
foramen,eachnervebifurcatesintoadorsalandventralramus(Figure2.8).The
dorsal ramus provides the cutaneous innervation of the back and the motor
innervation of the intrinsic or deep back muscles. All the appendicular and
remaining trunk muscle innervation and sensory innervation of the body
(excluding the motor and sensory innervation provided by cranial nerves) is
providedbytheventralramusofeachspinalnerve.Itisimportanttoappreciate
thatwhereasthedorsalandventralrootsconveypuresensoryandmotorfibers,
respectively, the dorsal and ventral rami are mixed nerves, and each contains
bothsensoryandmotorfibers.
NEUROANATOMICORGANIZATIONOFTHE
SPINALGRAYANDWHITEMATTER
Inacrosssection,thespinalcordiscomposedofacentralportionofbutterflyshapedgraymatterandperipherallyorientedwhitematter(Figure2.9).Thegray
mattercomprisespredominantlyneurons,theirprocesses,andglialcellsandhas
anenrichedbloodsupply.Thewhitemattercontainsascendinganddescending
fiber tracts and glial cells and appears white in unfixed tissue because of a
predominanceofmyelin.Thetwohalvesofthegraymatterareconnectedacross
themidlinebyadorsalandventralgraycommissure,whichislocatedaboveand
below the central canal respectively (Figure 2.5). The gray matter is further
subdividedintoaposterior(dorsal)horn(column)andananterior(ventral)horn
(column).Thethoracicanduppertwolumbarspinalcordsegmentsalsodisplay
a wedge-shaped, intermediate lateral horn (intermediolateral cell column)
(Figure2.5).
Thewhitematteroneachsideofthespinalcordisorganizedintothreelarge
areasorfuniculi(Figure2.5).Theposteriorfuniculusistheareaofwhitematter
betweenthe posterior median sulcus and the posterolateral sulcus. The lateral
funiculusisdefinedasthewhitematterbetweentheposterolateralsulcusandan
imaginarylinefromthemedialborderoftheanteriorhorntotheanteriorsurface
ofthespinalcord.Theanteriorfuniculusistheremainingwhitemattermedialto
theimaginarylinenotedaboveandlateraltotheanteriormedianfissure(Figure
2.5). Each funiculus of the spinal cord comprises bundles of fibers that run
togetherandsubservethesamefunction.Eachbundleisreferredtoasatractor
fasciculus.Forinstance,atcervicallevelsandtheuppersixthoraciclevelsofthe
spinal cord, the posterior funiculus is subdivided into two major fasciculi
(gracilisandcuneatus)bytheposteriorintermediatesulcusandseptum(Figure
2.9).(Thefunctionalsignificanceofthesefibertractsisdiscussedinthesection
“WhiteMatter”).
FIGURE 2.9 Diagram of the spinal cord showing the somatotopic
organizationoffibersintheposteriorfuniculus.
GrayMatter
Theposterior(dorsal)horncontainsclustersofneuronsconcernedwithsensory
function. The central processes of spinal ganglion cells generally synapse on
neuronsinthedorsalhorn,wheresensoryinformationisrelayedeithertohigher
centersinthebrainorsegmentallywithinthespinalcord.
The lateral (intermediate) horn is limited to the thoracic and upper two
lumbar spinal cord segments. It contains preganglionic sympathetic neurons
whose axons exit the spinal cord via the ventral roots. Preganglionic
parasympatheticneuronsarelocatedinacomparableregionofthegraymatterat
theS2–S4levelsofthespinalcord.However,awell-definedlateralhornisnot
foundatthesesacrallevels.Axonsofthepreganglionicparasympatheticneurons
distributetothedescendingcolon,sigmoidcolon,rectum,andallpelvicviscera
viathepelvicsplanchnicnerves.
The anterior (ventral) horn contains both interneurons and motoneurons.
Motoneuronaxonsinnervatingskeletalmusclecomprisethemajorcomponentof
the ventral root. Alpha motoneurons of the anterior horn are somatotopically
organizedsuchthatneuronssupplyingflexormusclesarelocateddorsallyinthe
anteriorhorn and neurons supplyingextensormuscles are located ventrally. In
addition,neuronssupplyingthetrunkmusculaturearelocatedmedially,whereas
neurons innervating the limbs are located laterally within the anterior horn
(Figure2.10).
FIGURE 2.10 Diagram of the spinal cord showing the somatotopic
organizationofventralhornmotorneurons.
WhiteMatter
Heavily myelinated nerve fibers in the posterior funiculus are concerned
primarilywithtwogeneralmodalitiesrelatedtoconsciousproprioception.These
are kinesthesia (sense of position and movement) and discrimative touch
(discriminatingtwo points and localizingtouchsensation). Injury to thespinal
cord in the regionof the posterior funiculusmay result clinically ina loss or
diminution of vibratory sense, position sense, two-point tactile discrimination,
andtouchandweightperceptioninthebodyipsilateralandcaudaltothespinal
cordlesion. The central processes ofneuronswhose cell bodies arelocatedin
spinalgangliaformthefibersoftheposteriorfuniculus.Thefibersthatenterthe
spinalcordbelowthesixththoracicsegmentformthefasciculusgracilis(gracile
tract), whereas fibers that enter the cord above the sixth thoracic segment are
located laterally and form the fasciculus cuneatus (cuneate tract). These two
tracts are separated by the posterior intermediate sulcus and septum, which is
foundatallcervicallevelsofthespinalcordandtheuppersixthoracic levels.
Thus, the nerve fibers in the posterior funiculus are somatotopically arranged
withthegreatestnumberofmedialfibersarisingfromthesacrallevelsandthe
greatest number of lateral fibers comingfrom thecervical levels (Figure 2.9).
Fibersinthegracileandcuneatetractascendipsilaterallytothecaudalmedulla
where they synapse on neurons inthe gracile and cuneate nucleirespectively.
Axonsofthese medullary neurons cross the midline and ascendasthemedial
lemniscus,which terminates onneuronsin the thalamus(ventralposterolateral
(VPL) nucleus). Thalamic neurons, in turn, project to the ipsilateral cerebral
cortex.
Afunctionalrelationshipexistsbetweentheposteriorfuniculusandthetracts
inthe lateral funiculus of the spinalcord.Forexample, injury to the posterior
funiculus augments all forms of sensation conveyed by the spinothalamic
pathways in the lateral funiculus. Thus, painful stimuli are triggeredbylower
stimulationthresholdsandnonpainfulstimuliareinterpretedasbeingpainful.In
both man and animals, there have been reports of lesions in the posterior
funiculusthatresultinnolossofthesensorymodalitiespresumablycarriedby
thiswhitematterregion:thepresenceofthe spinocervicalthalamictractin the
lateralfuniculusmaycompensateforsomeposteriorfuniculusdeficits.
Sensorystimulitransmittedviatheposteriorfuniculiareofthreetypes:those
impressed passively on the organism, those that have temporal or sequential
components added to a spatial cue, and those that are not perceived without
manipulationand activeexplorationby thedigits.The firsttypeof stimulusis
exemplifiedbyavibratingtuningfork,two-pointdiscrimination,oratouchbya
pieceofcotton.Thesepassivestimuliaretransmittednotonlybythe posterior
funiculi, but also by a number of parallelpathways,such as the spinocervical
thalamic tract. Thus, such passive sensations are often preserved following
lesionsoftheposteriorfuniculi.Thesecondtypeofstimulusisexemplifiedbya
determinationofthedirectionoflinesthataredrawnontheskinorthedirection
ofmovementofadigitortoe.Thistypeofstimulus,whichcontainstemporalor
sequential factors added to a spatial cue, is transmitted exclusively by the
posteriorfuniculi.Thus,theinformationthatconcernstherelativechangesina
stimulus over a period oftime or the direction ofa stimulus is transmitted to
higherCNScentersonlybytheposteriorfuniculi.Thisisalsothecasewiththe
third type of stimulus. Recognizing shapes and patterns by active exploration
withthedigits(i.e.,stereognosis)ismediatedonlybytheposteriorfuniculi,and
theabilitytorecognizetheseshapesisoftenlostwithlesionstothispartofthe
spinalcord.
Position and movement sense is severely affected following injury to the
posteriorfuniculi,especially inthedistalpartoftheextremities. Smallpassive
movements are not recognized as movements, but as touch or pressure. The
directionofmovement isseldom perceived.Thislossofposition sensegreatly
impairs motor function. The sensory loss causes movements to be clumsy,
uncertain,andpoorly coordinated—aconditionreferredtoasposterior column
orsensoryataxia.
AscendingTractsintheLateralandAnteriorFuniculi
PosteriorSpinocerebellarTract
This ascending tract is located in the dorsal aspect of the lateral funiculus
(Figure 2.11). The tract conveys to the cerebellum proprioceptive information
fromreceptorslocatedinmuscles,tendons,andjoints.Thecentralprocessesof
spinalganglioncellsenterthespinalcordviathedorsalrootandthenascendor
descendinthefasciculusgracilisforoneortwosegmentsbeforesynapsingon
neuronsin thenucleus dorsalisofClarke (Clarke’snucleus)locatedwithinthe
intermediategraymatterpredominantlyatthoracicspinalcordlevels.Axonsof
neurons in the nucleus dorsalis form the posterior spinocerebellar tract. Since
Clarke’snucleusisfoundonlybetweenC8andL2,theposteriorspinocerebellar
tractisnotfoundcaudaltoL2inthespinalcord.Thecentralprocessesofthose
spinalganglioncellsconveyingproprioceptiveinformationinto thespinalcord
belowL2 ascendto theL2levelinthe fasciculusgracilis beforesynapsingon
cells in Clarke’s nucleus. Similarly, incoming proprioceptive information
enteringthespinalcordrostraltoC8iscarriedbynervefibersascendinginthe
fasciculus cuneatus. The cuneate tract nerve fibers synapse on neurons of the
accessory cuneate nucleus in the medulla; this is homologous to the nucleus
dorsalis.Bothpathwaysareuncrossed.
FIGURE 2.11 Diagram of ascending (right) and descending (left)
pathwaysofthespinalcord.
The posterior spinocerebellar tract conveys to the cerebellum information
pertaining to muscle contraction, including phase, rate, and strength of
contraction.ClinicaleffectsofposteriorspinocerebellartractdestructioninSCI
aremaskedbytheeffectsofdestructionoftheadjacentlateralcorticospinaltract.
SpinocervicalThalamicTract
Althoughthespinocervical thalamictracthasnotbeendemonstratedin man,a
descriptionofthistractisincludedherebecauseofitspossiblerelevancetoan
explanation of some clinical cases involving preserved sensory function
followingSCI.Incats,thecentralprocessesofspinalganglioncellsconveying
stimuli into the spinal cord from low-threshold cutaneous receptors, pressure
receptors,andimpulsesfollowingpinchingoftheskinsynapseonneuronsinthe
ipsilateraldorsalhorn.Theseneuronssendtheiraxonsintothedorsalaspectof
the lateral funiculus, where they ascend to the upper two or three cervical
segments and synapse on neurons of the lateral cervical nucleus. The lateral
cervicalnucleusislocatedlateraltothedorsalhorninthelateralfuniculusofthe
upper three cervical segments of the cat spinal cord (8). The lateral cervical
nucleus gives rise to axons that cross the midline in the anterior white
commissure(Figure2.5)atspinallevelsC1andC2andascendwiththemedial
lemniscus to terminate in the contralateral VPLnucleus of the thalamus.This
spinocervicalthalamictracthasasomatotopicorganizationsimilartothatofthe
posteriorfuniculuspathways(i.e.,sacralfibersaremediallylocated,andcervical
fibersarethemostlaterallylocated;9).Itisimportanttoappreciatethatthereis
evidence that the pathways in the posterior funiculus (dorsal column–medial
lemniscalsystem)are functionally linked with the spinocervical thalamic tract
becausesomespinocervicalcollateralsterminateinthedorsalcolumnnucleiand
somecellsinthedorsalcolumnnucleiprojecttothelateralcervicalnucleus(10).
Theputative clinical relevance ofthisinformation is that thespinocervical
thalamic tract accounts for the persistance of kinesthesia and discriminative
touch sensation following total interruption of the posterior funiculus in
experimentalanimals.AccordingtoAfifiandBergman(11),thepresenceofthe
spinocervical thalamic tract has been assumed in man because of the similar
persistence of posterior funiculus sensations after total posterior funiculus
lesions in patients. Thus, the old concept of the absolute necessity of the
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