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IntroductionTherearetwomajor,parallelsystemsthatconveysomaticsensoryinformationfromtheperipheryofthepost-cranialbodytothecortex,thedorsalcolumn-medial lemniscussystemandtheanterolateralsystem.Therearecomparableparallelsystemscarryinginformationfromthefaceassociatedwiththecentralprojectionsofthetrigeminalnerve. Inaddition,there isan importantsystemcarryingproprioceptive informationfromthemusclespindles to thecerebellum.This tutorialwill focuson thepathways takenby thecomponentsof thesystems for transmissionof neural signals pertaining to pain and temperature sensation. It is important foryourunderstandingofneurologicaldeficitsseenintheclinictoknowwherethesepathwaystravelrelativetoeachotherandtootherstructures(includingthecranialnervenuclei)inthebrain.
Theanterolateralsystem is responsibleforconveying informationaboutpain,temperatureandcrudetouch(i.e.,touchlackingthespatialresolutionofthedorsalcolumnsystem)fromthepost-cranialbody.Comparableinformationabout the face isprocessed in trigeminalpathways. Thesepathwaysare illustrated inFigures 1and2.Mostperipheralprocessesassociatedwiththedorsalrootganglioncellsthatcontributetothissystemare“free.”Thatis,theyarenotassociatedwithencapsulatedendingslikethoseinthedorsalcolumn-mediallemniscal system. In addition, the first-order fibers associated with the anterolateral system are generallymuch smaller indiameter than thoseassociatedwith thedorsal columnsystem. (Sowhatdoes this tell youabouttherelativeconductionvelocitiesofthesetwoimportantsomaticsensorypathways?)
Thefirst-orderneurons intheanterolateralsystem, likethoseinthedorsalcolumn-medial lemniscalsystem,havetheircellbodiesinthedorsalrootganglia.Thecentralprocessesoftheseneuronsterminateonsecond-orderneuronsinthedorsalhornofthespinalcord.Painandtemperatureinformationfromreceptorsinthefaceiscarriedintothebrainonthefifthnerve.Thecellbodiesofthefirstorderneuronsareinthetrigeminalganglionandthecentralprocessesofthecellsmakesynapsesinanucleusinthemedullaknownasthespinaltrigeminalnucleus (of the fifthnerve).Thisnucleus isactuallycontinuouswiththedorsalhornof thespinalcord.
Thesecond-orderneuronsinthedorsalhornofthespinalcordsendtheiraxonsacrossthemidline,wheretheyaccumulateintheanterolateral(ventrolateral)partofthewhitematter.Theyascendinthislocationthroughthe length of the cord.Many of these fibers continue through themedulla, the pons and themidbrain tocontact third-order neurons in theventral posterior lateral (VPL) nucleusof the thalamus (aswell as otherthalamicnuclei).Thisdirectpathway fromthespinal cord to the thalamus isoftencalled thespinothalamictract.Actually,thethalamusisonlyoneofthetargetsofthesecond-orderneuronsintheanterolateralsystem.These neurons also project to central parts of the medulla, pons and midbrain known collectively as thereticularformation(thiscomponentoftheanterolateralsystemisknownasthe“spinoreticulartract”)andtothe periaqueductal gray matter and the superior colliculus (this component is known as the“spinomesencephalictract”).Second-orderneurons located inthespinal trigeminalnucleussendtheiraxonsacrossthemidlinetoformtheventraltrigeminothalamictract,whichtravelstotheventralposteriormedial(VPM)nucleusofthethalamus.
Third-orderneuronsintheventralposteriornucleusandinotherthalamicnucleithenprojecttothecortexviathe internal capsule.Thepostcentral gyrusappears tobe important for theability todiscriminate theexactlocationofpainfulstimuli,butmanyother,lesswell-understoodcorticalareas(includingareasintheanteriorpartofthecingulategyrus)appeartobeimportantinthecompletesensationofpain, includingthecomplexaffectivedimensionsofpain.
Figure 3 presents a diagram of the major parallel pathways carrying somatic sensory information to thecerebralcortex(seetutorialnoteson“MechanosensoryPathways”).ThepathwaysformechanoreceptionandthepathwaysforpainandtemperaturesensationshowninFigureA1areshowntogetherbilaterally.
Figure 2. Location of the anterolateral system in the cervical cord and brainstem, with the ventraltrigeminothalamictract,asseenincross-sections.Notethatatalllevels,thefibersofbothtractsarelocatedintheanterolateralpartofthebrainstemtegmentum(second-orderneuronsare illustrated inwhite).(SectionsfromSylvius4)(Figurecontinuedonnextpage)
i InthisTable,thetermsipsilateralandcontralateralwillrefertothesideoftheperipheralorcentralnervoussystemrelativetothelocationofthesensoryreceptors;e.g.,