OperativeSpinalCordAnatomyLastUpdated:November9,2017

Thespinalcord(SC)isalongitudinalneuralstructurewithinthespinalcanal,whichactsmostlyasaconduitforinformationreachingorleavingtheperipheryofthebody.TheSCstartsattheforamenmagnumandextendstotheconusmedullarisatapproximatelythelevelofthefirstlumbarvertebra.

TheSCis45cminlengthandisshorterthanthespinalcanalduetotheclassicalgrowthmismatchbetweenitandthevertebralcolumnduringembryonicdevelopment.Thus,inordertomaintaintheestablishedsegmentalconnections,thoracolumbarrootsmustelongatetoreachtheirintervertebralforamina.

Asaconsequence,theuppercervicalrootsreachtheirexitalmosthorizontallytothevertebrallevel;lowercervicalandupperthoracicrootscourseobliquelyonetotwosegmentsbelow;andlumbosacralrootstravelalmostverticallyoverseveralsegmentsinthetypicalarrangementofthecaudaequina.

TheSChasacylindricalshape,whichisslightlyflattenedintheanteroposterioraxis.Itfollowsthecurvatureofthevertebralcolumnandshowstwocharacteristicenlargements,namelythecervicalandlumbarones,wheremotorneuronsrelatedtotheupperandlowerlimbsconcentrate.ThefirstisobservedatthecervicalspinalcordlevelfromC4toT2,whereasthelatterhasaclosecorrelationtothefirstthreelumbarvertebrae.Finally,theconusmedullarisisusuallyalignedtothefirstlumbarvertebraandgivesrisetomorethan50rootletsoveralengthof<3cm.

TheNeurosurgicalAtlas byAaronCohen-Gadol,M.D.

TheSCiscoveredbytheflexiblevertebralcolumnandthemeninges.Theduramateristhemostexternallayer,hasabout0.8mmthickness,andcontainscollagenandelasticfibers.Atthespinallevel,theduraisarrangedinthreelayers,contrarytothetwo-layerintracranialdura.Theinternallayerisincontinuitywiththeinnerdurallayerofthehead;themiddlelayerisconnectedtotheexternaldurallayerofthehead;andtheexternallayercontinuesastheperiosteumoftheskull.

IntheSC,theoutermostorperiosteallayerisnotattachedtothemiddlelayer;thisconfigurationcreatesananatomicalepiduralspacethatcontainsveinsandfattissue.TheduracoverstheentirespinalcanaluntilthelevelofS1-S2,wheretheduralsacisformed.

Underandlooselyattachedtotheduraisthearachnoidlayer,whichcontainsthesubarachnoidspacefilledwithcerebrospinalfluid.Italsoextendstotheduralsac.Thearachnoidcoversthespinalnervestowardtherootsleeves,whereitfuseswiththedura.

Withinthesubarachnoidspace,severalseptationshavebeendescribed,especiallyintheposteriorspace,wherethereisalongitudinaldorsalordorsolateralseptumfromthearachnoidtothespinalpialsurfacedividingthesubarachnoidspaceintoleftandrighthalves.Ittaperstoafenestratedappearanceinthecervicalandconusmedullarisareas.Fromthesurgicalstandpoint,suchdorsalorposteriorseptationscanfacilitatedissectionoftheextramedullarytumorsawayfromthespinalcord.

Thepiamateristheinnermostmeningeallayerandencasesthespinalcord.Itprovidesabarrierbetweenthesubarachnoidspaceandtheperivascularspaces.Thepiaisfirmlyattachedtotheduraby21pairsofextensions,calledthedenticulateligaments.Theyrunalongsidethespinalcordtotheleveloftheconusmedullaris,wheretheyendbetweenthelastthoracicandthefirstlumbarnerves.

Theseligamentsaretriangularinshapewiththeirbasesattachedtothesurfaceofthespinalcord,whiletheirapicesinsert2mmdorsaland3mmcranialtotheintervertebralforamina.Thedenticulateligamentsareopportunelylocatedbetweenventralanddorsalroots,arethickercraniallynearthecervicalspine,andtendtodecreaseinstrengthastheSCdescends.

Figure1:Diagramofthecrosssectionofthespinalcordandmeninges.Thearachnoidlayerisattachedtothedura.Theintermediatelayerattachestothearachnoidandpiamater.Aposteriordorsalseptumiseasilyidentified.Denticulateligamentsareshownbilaterally.

Figure2:Photographofacadavericspecimenafterthoracolumbarlaminectomyandmidlineduralincision.ThereisamarkedchangeinSCvascularityfromthethoracic(leftsideofphoto)tothelumbarSC(rightside).Thedorsalrootshaveanobliquetrajectorytowardtheintervertebralforamina.Denticulateligamentsarepositionedanteriortothedorsalrootsandhaveatypicalbrightwhiteappearance.

TheunderstoodroleofthedenticulateligamentsistopromotespinalcordstabilitywithintheSC.Converselytopreviousbelief,thedenticulateligamentslimitcranial-caudalmovementsoftheSC,beingespeciallyresistanttocaudalstress,buthaveminimalornosignificantcontributiontolimitinganterior-posteriormotion.

Thepiaendsasfilumterminale,afibrousbandthatconnectstheconusmedullariswiththecoccyx.Itoccursasaresultofdedifferentiationofthecaudal-mostpartsoftheSC.Typically,thefilumterminaleisdividedintotwodistinctsegments,namelytheintraduralpart(untilreachingtheduralsac;calledinternum)andtheextraduralpart(calledexternumorligamentumcoccigeale).

Histologically,thefilumterminaleinternumfuseswiththefibersoftheduralsacandcontinuesasfilumterminaleexternum.Fromthephysiologicpointofview,cadavericstrainandelasticitymeasurementshavedemonstratedthatthefilumtogetherwiththedenticulateligamentsexertaprotectiveroleontheconusmedullarisfromtraction.

Figure3:Photographofacadaverspecimendemonstratestheanatomicrelationshipsofthecaudaequina.Typicalanastomosesareobserved,especiallybetweenthedorsalrootsontheirwaytotheintervertebralforamina(duralsleeve).Thefilumterminaleinternumisalsomarked.Notethedifferenceinthecolorofthefilumversusthecaudaequina.

Figure4:Intraoperativephotographillustratingthecaudaequina.Radiculararteriesandthearteryofthefilumterminale(blackarrows)arealsonoted.

CrossSectionalAnatomy

TheSCcontainslongitudinalcolumnsofnuclei(graymatter),whichareenclosedbyascendinganddescendingtracts(whitematter).Intheaxialsection,thegraymatterdemonstratesthewidelyrecognizedH-shapedstructure.Itisdividedintoanterior(ventral),posterior(dorsal),and,atsomelevels,theintermediatehorns(lateral).Suchdivisionisextendedtothewhitematter,revealingtheanterior,posterior,andlateralcolumnsorfuniculi.

EachsegmentoftheSCcontainsaventralandadorsalroot.Thereare31-pairsofspinalnerves(8cervical,12dorsal,5lumbar,5sacral,and1coccygeal).Fromtheanteriorhorn,severalmotorventralfilamentsariseattheanterolateralsulcusandgathertoformtheventralroot.Fromthelateralhorn,atthelevelofthoracicandupperlumbarareas,preganglionicsympatheticfibersarisetoinnervatetheautonomicganglia.Similarly,attheS2-S4levels,preganglionicparasympatheticfibersarise.

Dorsalsensoryafferentsentertheposterolateralsulcusontheirwaytotheposteriorhorn.Deeperfissuresareconsistentlyencounteredattheventral(anteriormedianfissure)anddorsal(posteriormediansulcus)surfaces.Moreover,theposteriorcolumnisdividedbytheposteriorintermediatesulcusatupperthoraciclevels.

InthecenteroftheSCliesthecentralcanal.Itconsistsoftheremnantsoftheneuraltubecentralcavitylinedbyependymalcellsandfilledwithcerebrospinalfluid.Theanteriorandposteriorcommissuresenclosethecentralcanal.

Thegraymatterhornsaresomatotopicallyorganizedandcontaindifferentclassesoffunctionalneurons.Asaresult,motoneuronsthatinnervateaxialmusclesaremediallylocatedintheventralhorn,whereasmotoneuronsthatcontroldistallimbmovementsarelocatedmorelaterally.Finally,motoneuronsresponsibleforcontrollingproximallimbmuscleslieinbetween.

Theposteriorhornhasalayeredneuronalorganization,whichisbasedonsynapticinputsandoutputs.Thesuperficiallayersreceiveexteroceptivesensoryinformationaboutpain,temperature,andlighttouch,andgeneratethecontralateralspinothalamictracts.Thedeeplayersareinvolvedwithproprioceptiveinformationandcontributetotheipsilateralspinocerebellartracts.Theposteriorcervicalhornalsoincludesthespinalnucleusofthetrigeminalnerve.

Thewhitematterisorganizedintractsassociatedwithmajormotororsensoryfunctions.Theposteriorcolumnenlarges,astheSCascends,toincludemoreaxonscarryingfinetouch,vibration,andproprioceptiveinformationfromthelowerlimbsmedially(fasciculusgracilis)andtheupperlimbslaterally(fasciculuscuneatus).Thelateralcolumncontainstwomostprominentascendingtracts,namelythelateralspinothalamicandthespinocerebellarones,andone

descendingtract,thelateralcorticospinaltract.Finally,theanteriorspinothalamicandcorticospinaltractsarefoundintheanteriorcolumn.

Thespinothalamictractscarrycontralateralinformationaboutcrudetouch,pain,andtemperature.Twotopographicaldetailsinthesetractsareworthdiscussing.First,theinitialsynapseoccursattheposteriorhorn,wheresecond-orderneuronsgenerateaxonsthatcourseapproximatelytwotothreelevelsaboveinordertodecussatethroughtheanteriorcommissure.Second,sacralfibersareplacedmorelaterallythancervicalones,therebyjustifyingthephenomenonofsensorysacralsparingincentralSClesions.

Thelateralcorticospinaltractisformedatthecaudalmedulla,wheremostofthefiberscomingfromtheprimarymotorcortexdecussate.Theuncrossedfiberscontinueasanteriorcorticospinaltractanddecussateonlyatthelevelofthesynapsewiththeventralhornneurons.Thistractisinvolvedinthecontrolofproximallimbandaxialmuscles.

Thelateralcorticospinaltractshowsasimilartopographicaldistributiontothespinothalamictract,insuchawaythatsacralfibersaresituatedlaterally.Inclinicalscenarios,compressivecervicalconditionsinitiallyaffectmotorfunctionsofthelowerlimbs.Severalothertractsaredescribed,eventhoughtheyarelessclinicallyrelevant.

Figure5:Majorwhitemattertracts,ascending(topillustration)anddescending(bottomillustration),arenoted.Inotherwords,orangetractsaredescendingpathwaysandbluetractsareascendingpathways.

SpinalVascularAnatomy

ThearterialnetworkthatsuppliestheSCismuchmorecomplexand

extensivethanthatofthebrain.Thespinalvascularanatomystartsinthesegmentalextraspinalarteries,whichcorrespondtothepathwaysofbloodfromtheaortaandprovidenotonlythearterialsupplytothecord,butalsotothenerveroots,dura,andparaspinalmusculature.

Eachsegmentalarteryhasaventralandadorsalbranch.Thedorsaldivisiongivesoffaspinalbranch,whichsplitsintotheretrocorporeal(anteriorspinalcanal),prelaminar(posteriorspinalcanal),andradiculararteries.Theradiculararteryistermedtheradiculomeningealarterywhenitfeedsthenerverootsandduraateverylevel.

Ontheotherhand,ifthesearteriestakepartinthecordvascularnetwork,theyarebettertermedradiculomedullaryarteriesiftheysupplytheanteriorspinalartery(ASA),andradiculopialorposteriorradiculomedullaryarteries,iftheysupplytheposteriorspinalarteries(PSAs)andsurfacevasocoronaoftheSC.

DifferentsegmentalarteriesarefoundregionallythatprovidebloodsupplytotheSCandspinalganglia.Inthecervicalregion,thevertebral,ascendingcervicalanddeepcervicalarteries;inthethoracicregion,theintercostalarteries;inthelumbarregion,thelumbar,iliolumbarandthelowestlumbararteries;andinthesacralandcoccygealregions,thelateralsacralarteriesareinvolvedinthearterialnetwork.

Figure6:Diagramofthespinalcordvascularanatomy.Pleaseseetext.

Figure7:Diagramofthespinalcordvascularanatomyinacross-sectionalview.Inthissketch,anteriorandposteriorradiculomedullaryarteriescreateacomplexarterialnetworkwiththeanteriorandposteriorspinalarteries,aswellaswiththevasocorona.

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Figure8:DiagramoftheSCsegmentalarterialanatomy.

Inthecervicalregion,8to10unpairedradiculomedullaryarteries,includingatleast1medullaryarterytothecervicalenlargementatthelevelofC6,connectdirectlywithASA.

Thethoracicregionhasanotherwisepoorvascularitywithfrom2to4smallmedullaryarteries.Thelumbarenlargementatthethoracolumbarregionisrichlysuppliedby1singlevessel,calledthearteryofAdamkiewicz.ItcommonlyarisesbetweenT9andT12in75%ofthespecimens,eventhoughhigher(T5-T8)orlower(L1-L2)levelsmayalsocarrythisimportantarteryin15%and10%,respectively.

ThearteryofAdamkiewiczisalsoknownasthegreatradiculararteryorevenasarteriaradiculomedullarismagnaandhasaleft-sidedpredominance.Asthearterypiercesthedura,aslightcaudalturnmayoccasionallybeseen.ThearterythenjoinstheventralrootonitswaytotheventralsurfaceoftheSC,wherethearteryanastomoseswiththeASAatorjustbeforeitstypicalandcharacteristichairpinturn.ThePSAsreceiveapproximately10to28feeders,whichcanalsodemonstrateahairpinconfigurationintheparamedianlocations.

Figure9:Photographofacadavericspecimen(anteriorview)showingthetypicalconfigurationofthearteryofAdamkiewicz.Thisarteryentersthespinalcanalontheleftside,followstheventralroot,coursesoverthespinalcordthroughseveralsegments,andmakesahairpinturnatorjustbeforeencounteringtheanteriorspinalartery.(AdaptedfromAlleyneetal,1998withpermission.)

Forthespinalarterialnetwork,3superficialspinalarteriesareidentifiedallthewaydowntotheleveloftheconusmedullarisovertheanteriormedianfissureandposterolateralsulci,respectively.Theyreceivesignificantcontributionofseveralfeeders,asmentionedbefore.

TheASAsoriginateasmedialbranchesofthevertebralarteriesandbecomeasinglearteryatthemidcervicallevel.Theadditional2

superficialvessels(PSAs)originatefromthevertebralarteriesorevenfromtheposteriorinferiorcerebellararteriesattheleveloftheforamenmagnum.Theanteriortwo-thirdsoftheSCissuppliedbytheASA,whereastheposteriorone-thirdisfedbythePSAs.

Theanteriorandposteriorsystemsjoinattheconusmedullaristoformthearterialbasketwithextensionsalongsidethefilumterminale.Thearterialbasketconsistsofatleast1or2anastomoticbranchesbetweentheASAandPSAs.

OneadditionalanastomoticsystemisobservedovertheSCsurfaceformingaratherdistinctpialplexus,whichiscalledthevasocorona.Theintramedullarystageofthevascularsupplyisservedby2systems,namelythesulcalorcentralarteriesandtheradialperforatingarteries.

Figure10:Photographofacadavericspecimenillustratingthetypicalarterialanastomoticnetworkattheconusmedullaris.Theanteriorspinalartery(ASA)formsanarterialbasketwiththeposteriorspinalarteries(PSAs)viatheanastomicbranches(ABCM).Earlybranchingandunilateralconfigurationsmaybeobservedoccasionally.A:anteriorview;B:rightobliqueview;C:leftobliqueview;D:posteriorview.(AdaptedfromMartirosyanetal,2015,withpermission.)

ThevenousdrainageoftheSCismorevariablethanthearterial

supplyandhasbeendividedintointrinsicandextrinsicsystems.Theintrinsicsystemcontainscentrifugallyorientedperipheralorradialveins,whichemergefromcapillariesatthegray-whitematterjunction.

Sulcalorcentralveinsdraintheanteriorhorns,anteriorcommissure,andassociatedwhitematterandcompletetheintrinsicsystemofvenousdrainage.Theextrinsicsystemofvenousdrainagehasadirectcorrelationtothepatternsofthearterialsystem.Twodominantveinsareusuallyobserved,theanteriorandposteriormedianveins.Thefirstreceivesvenousdrainagefromthesulcalveins,whereasthelatterreceivesvenousdrainagefromtheperipheralveins.

Next,theanteriorandposteriormedianveinsdraininto8to14anteriorand5to10posteriorradiculomedullaryveins,whichinturnemptyintotheepiduralvenousplexithroughavalvelesssystem.Similarlytothatobservedinthearterialsystem,agreatanteriorradiculomedullaryveinownsacrucialroleindrainingtheanteriorthoracolumbarspine.ItiscommonlydetectedbetweenT11andL3.

Thenextstageistheconnectiontotheintervertebralveinsandsubsequentlytothesegmentalveins(ascendinglumbarandazygossystems),beforejoiningthesuperiorvenacava.

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Figure11:DiagramoftheSCsegmentalvenousdrainage.

Figure12:Diagramofthespinalcordvenousanatomyfromacross-sectionalperspective.

PearlsandPitfalls

TheSChasacomplexanatomicalorganizationforreceiving,processing,andtransferringinformation.Longtractsgenerallycrossthemidline,eventhoughdecussationoccursatdifferentlevels.Dependingontheaxialandlongitudinalextentofthelesion,differentmotorandsensorysyndromesmaydevelop.TheexistenceofsulciandfissuresovertheSCsurfaceprovidesrelativelysafeentryroutesforreachingintramedullarytumors.WhenapproachingthelowerthoracicSClevels,thesurgeonshouldinvestigatethelocationofthearteryofAdamkiewicz.

Contributor:MarcusAndréAcioly,MD,PhD

DOI:https://doi.org/10.18791/nsatlas.v9.ch01

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