MEDULLAOBLONGATAPONSMIDBRAIN

MEDULLAOBLONGATA

Thebrainstemconsistsofthemedullaoblongata,ponsandmidbrain.Itissitedintheposteriorcranialfossa,anditsventralsurfaceliesontheclivus.

MEDULLAOBLONGATA

Themedullaoblongatabetweenthepyramidaldecussationandthelowerborderoftheponsrepresentsthetransitionfromthespinalcordtothebrain.

Theanteriormedianfissure,whichisinterruptedbythepyramidaldecussation,andtheanterolateralsulcusoneachsideextenduptothepons.

Theanteriorfuniculithickenbelowtheponstoformthepyramids.Lateraltothemoneachsidebulgetheolives.

MEDULLAOBLONGATA

Medullaoblongataandponstogetherformthehindbrain,alsoknownasrhombencephalon,namedafterthisfossa.

Themedullaoblongataextendsfromjustabovethefirstpairofcervicalspinalnervestothelowerborderofthepons.Itisapproximately3cminlengthand2cmindiameteratitswidest.

Theventralsurfaceofthemedullaisseparatedfromthebasilarpartoftheoccipitalboneandapexofthedensbythemeningesandoccipito-axialligaments.

MEDULLAOBLONGATA

Caudally,thedorsalsurfaceofthemedullaoccupiesthemidlinenotchbetweenthecerebellarhemispheres.

Caudally,theventralmedianfissureisinterruptedbytheobliquelycrossingfasciclesofthepyramidaldecussation.

Rostrally,itendsatthepontineborderintheforamencaecum.Immediatelylateraltotheventralmedianfissurethereisaprominentelongatedridge,thepyramid,whichcontainsdescendingpyramidal,orcorticospinal,axons.Thelateralmarginofthepyramidisindicatedbyashallowventrolateralsulcus.

MEDULLAOBLONGATA

Fromtheventrolateralsulcusemerges,inlinewiththeventralspinalnerveroots,alinearseriesofrootletswhichconstitutethehypoglossalnerve.

Theabducensnerveemergesattheslightlynarrowedrostralendofthepyramid,whereitadjoinsthepons.

Caudallythepyramidtapersintothespinalventralfuniculus.Lateraltothepyramidandtheventrolateralsulcusthereisanovalprominence,theolive,whichcontainstheinferiorolivarynucleus.

MEDULLAOBLONGATA

Lateraltotheoliveistheposterolateralsulcus.Theglossopharyngeal,vagusandaccessorynervesjointhebrainstemalongthelineofthissulcus,inlinewiththedorsalspinalnerveroots.

SomebrainstemcellgroupsarethenucleiofcranialnervesCNIII–CNXII:theyareconcernedwiththesensory,motorandautonomicinnervationoftheheadandneck.OtherautonomicfibresthatarisefromthebrainstemaredistributedmorewidelyviathevagusnerveCNX.

MEDULLAOBLONGATA

Thebrainstemalsocontainsthereticularformation,thatextendsthroughoutitslength,andiscontinuouscaudallywithitsspinalcounterpart.

Somereticularnucleiarereferredtoasvitalcentressincetheyareconcernedwithregulationofcardiacandrespiratoryactivities;otherpartsofthereticularformationareinvolvedintheregulationofmuscletone,postureandreflexactivities.

MEDULLAOBLONGATA

Thebrainstemisthesiteofterminationofnumerousascendinganddescendingfibresandistraversedbymanyothers:•  spinothalamictract(spinallemniscus),•  mediallemniscus,•  trigeminothalamictractsallascendthroughthebrainstemtoreachthethalamus.

MEDIALSTRUCTURESOFMEDULLA1.ThehypoglossalnucleusofCNXII2.Themediallemniscus,whichcontainscrossedfibersfromthegracileandcuneatenuclei3.Thepyramid(corticospinalfibers)

MEDULLAOBLONGATA

LATERALSTRUCTURESOFMEDULLA1.Thenucleusambiguus(CNIX,X,andXI)2.Thevestibularnuclei(CNVIII)3.Theinferiorcerebellarpeduncle,whichcontainsthedorsalspinocerebellar,cuneocerebellar,andolivocerebellartracts4.Thelateralspinothalamictract(spinallemniscus)5.Thespinalnucleusandtractoftrigeminalnerve

MEDULLAOBLONGATA

Inthecaudalmedulla,thecorticospinalanddorsalcolumn-mediallemniscalpathways-sendaxonsacrossthemidline.

MEDULLAOBLONGATA

Thenucleusgracilisandnucleuscuneatusgiverisetoaxonsthatdecussateinthecaudalmedulla(thecrossingaxonsaretheinternalarcuatefibers),whichthenformandascendinthemediallemniscus.

Thecorticospinal(pyramidal)tracts,whicharecontainedinthepyramids,courseventromediallythroughthemedulla.

Mostofthesefibersdecussateinthecaudalmedullajustbelowthecrossingofaxonsfromthedorsalcolumnnuclei,andthentraveldownthespinalcordasthe(lateral)corticospinaltract.

MEDULLAOBLONGATA

Theolivesarelocatedlateraltothepyramidsintherostraltwo-thirdsofthemedulla.Theolivescontaintheconvolutedinferiorolivarynuclei.

Theolivarynucleisendclimbing(olivocerebellar)fibersintothecerebellumthroughtheinferiorcerebellarpeduncle.Theolivesareakeydistinguishingfeatureofthemedulla.

ThespinothalamictractandthedescendinghypothalamicfiberscoursetogetherinthelateralpartofthemedullabelowtheinferiorcerebellarpeduncleandnearthespinalnucleusandtractofCNV.

MEDULLAOBLONGATA

Clinically,evensmalllesionscandestroyvitalcardiacandrespiratorycentres,disconnectforebrainmotorareasfrombrainstemandspinalmotorneurones.Irreversiblecardiacandrespiratoryarrestfollowscompletedestructionoftheneuralrespiratoryandcardiaccentresinthemedulla.

MEDULLARYLEVELCNIX,theglossopharyngeal,andCNX,thevagusnerve—ofitsseveralnuclei,onesuppliesthemusclesofthepharynxandlarynx;thevagusnerveisprimarilyaparasympatheticnervetotheorgansofthethoraxandabdomen.•CNXI,thespinalaccessorynerve,innervatessomeofthemusclesoftheneck.•CNXII,thehypoglossalnerve,suppliesmotorfiberstothemusclesofthetongue.

MEDULLAOBLONGATA

CRANIALNERVENUCLEISPINALNUCLEUSOFCNVThespinalnucleusofthetrigeminalnerve(CNV)islocatedinapositionanalogoustothedorsalhornofthespinalcord.CentralprocessesfromcellsinthetrigeminalganglionconveyingpainandtemperaturesensationsfromthefaceenterthebrainstemintherostralponsbutdescendinthespinaltractofCNVandsynapseoncellsinthespinalnucleus.SOLITARYNUCLEUSThesolitarynucleusreceivestheaxonsofallgeneralandspecialvisceralafferentfiberscarriedintotheCNSbyCNVII,IX,andX.Theseincludetaste,cardiorespiratory,andgastrointestinalsensationscarriedbythesecranialnerves.TasteandvisceralsensoryneuronsallhavetheircellbodiesingangliaassociatedwithCNVII,IX,andXoutsidetheCNS.

MEDULLAOBLONGATA

CRANIALNERVENUCLEINUCLEUSAMBIGUUSThenucleusambiguusisacolumnoflargemotoneuronssituateddorsaltotheinferiorolive.AxonsarisingfromcellsinthisnucleuscourseintheCNIXandCNX.IntheCNX,thesefiberssupplymusclesofthesoftpalate,larynx,pharynx,andupperesophagus.Aunilaterallesionwillproduceipsilateralparalysisofthesoftpalatecausingtheuvulatodeviateawayfromthelesionednerveandnasalregurgitationofliquids,weaknessoflaryngealmusclescausinghoarseness,andpharyngealweaknessresultingindifficultyinswallowing.

MEDULLAOBLONGATA

uvulAAwayCNX

CRANIALNERVENUCLEIDORSALMOTORNUCLEUSOFCNXThesevisceralmotoneuronsofCNXarelocatedlateraltothehypoglossalnucleusinthefloorofthefourthventricle.Thisisamajorparasympatheticnucleusofthebrainstem,anditsuppliespreganglionicfibersinnervatingterminalgangliainthethoraxandtheforegutandmidgutpartsofthegastrointestinaltract.

MEDULLAOBLONGATA

CRANIALNERVENUCLEIHYPOGLOSSALNUCLEUSThehypoglossalnucleusCNXIIissituatednearthemidlinejustbeneaththecentralcanalandfourthventricle.Thisnucleussendsaxonsintothehypoglossalnervetoinnervateallofthetonguemusclesexceptthepalatoglossus.Thelesionofthehypoglossalnerve:tonguepointingtowardsame(affected)sideonprotrusion.

MEDULLAOBLONGATA

TongueTowardsCNXII

CRANIALNERVENUCLEITHEACCESSORYNUCLEUSTheaccessorynucleusCNXIisfoundinthecervicalspinalcord.Theaxonsofthespinalaccessorynervearisefromtheaccessorynucleus,passthroughtheforamenmagnumtoenterthecranialcavity,andjointhefibersofthevagustoexitthecranialcavitythroughthejugularforamen.Asaresult,intramedullarylesionsdonotaffectfibersofthespinalaccessorynerve.Thespinalaccessorynervesuppliesthesternocleidomastoidandtrapeziusmuscles.Thelesionofthespinalaccessorynerve– droopingshoulder.

MEDULLAOBLONGATA

CRANIALNERVENUCLEITherootletsoftheglossopharyngeal(CNIX)andvagus(CNX)nervesexitbetweentheoliveandthefibersoftheinferiorcerebellarpeduncle.Thehypoglossalnerve(CNXII)exitsmoremediallybetweentheoliveandthemedullarypyramid.

MEDULLAOBLONGATA

PONS

PONSTheponsislocatedbetweenthemedulla(caudally)andthemidbrain(rostrally).Thecerebellumoverliesthepons.Ontheventralsurfaceofthebrainstem,thetransitionbetweenmedullaandponsisclearlydemarcatedbyatransversesulcus.Laterally,inaregionknownasthecerebellopontineangle,thefacialCNVII,vestibulocochlearCNVIIIandglossopharyngealCNIXnervesemerge.

PONS

PONSTheventralsurfaceoftheponsisseparatedfromtheclivus(basisphenoidanddorsumsellae)bythecisternapontis.Thedorsalsurfaceoftheponsishiddenbythecerebellumwhichcoverstherostralhalfoftherhomboidfossa,intowhichtheaqueductofthemidbrainempties.Intotherhomboidfossa,theaqueductofthemidbrainempties.Theroofofthefossaisformedbyathinsheetoftissue,thesuperior(anterior)medullaryvelum,andisoverlainbythelingulaofthevermisofthecerebellum.

PONS

PONSThevelumisattachedoneachsidetothesuperiorcerebellarpedunclesandisenclosedbypiamateraboveandependymabelow.Thetrochlearnervesdecussateinthevelum.Thesurfacehasashallowmediansulcus,inwhichthebasilararteryruns.Theponsisconnectedtothebrainstemby3pairsofcerebellarpeduncles.Thefourthventricleisfoundbetweenthedorsalsurfaceoftheponsandthecerebellum.

PONS

PONSTheventralsurfaceoftheponsisdominatedbyfibers,whichformalargeventralenlargementthatcarriesfibersfrompontinenucleitothecerebelluminthemiddlecerebellarpeduncle.Thisventralenlargementisthekeydistinguishingfeatureofthepons.Transversesectionsrevealthattheponsconsistsofadorsaltegmentum,whichisacontinuationofthemedulla(excludingthepyramids),andaventral(basilar)part.Thelattercontainsbundlesoflongitudinaldescendingfibres,someofwhichcontinueintothepyramids;othersendinthemanypontineormedullarynuclei.Italsocontainsnumeroustransversefibresandscatteredpontinenuclei.

PONS

PONSThecorticospinaltractsaremorediffuseintheponsthaninthemedullaandareembeddedinthetransverselycoursingfibersthatenterthecerebelluminthemiddlecerebellarpeduncle.Themediallemniscusisstillsituatednearthemidlinebutisnowseparatedfromthecorticospinaltractsbythefibersformingthemiddlecerebellarpeduncle.Themediallemniscushaschangedfromadorsoventralorientationinthemedullatoamorehorizontalorientationinthepons.Thespinothalamictractandthedescendinghypothalamicfiberscontinuetocoursetogetherinthelateralpons.

PONS

PONSThelaterallemniscus,anascendingauditorypathway,islateralandjustdorsaltothemediallemniscus.Thelaterallemniscuscarriesthebulkofascendingauditoryfibersfrombothcochlearnucleitotheinferiorcolliculusofthemidbrain.Themediallongitudinalfasciculus(MLF)islocatednearthemidlinejustbeneaththefourthventricle.

PONS

CRANIALNERVESV,VI,VII,ANDVIIIFourcranialnervesemergefromthepons:CNV,VI,VII,VIII.CranialnervesVI,VII,andVIIIemergefromthepontomedullaryjunction.Thefacialnerveislocatedmedialtothevestibulocochlearnerve.Theabducensnerve(CNVI)emergesnearthemidlinelateraltothecorticospinaltract.Thetrigeminalnerve(CNV)emergesfromthemiddleofthepons

PONS

CRANIALNERVENUCLEI•  abducensnucleus•  facialmotornucleus•  superiorolivarynucleus•  vestibularnuclei•  cochlearnuclei•  trigeminalnucleiABDUCENSNUCLEUSTheabducensnucleusisfoundnearthemidlineinthefloorofthefourthventriclejustlateraltotheMLF.

PONS

CRANIALNERVENUCLEIFACIALMOTORNUCLEUSThefacialmotornucleusislocatedventrolateraltotheabducensnucleus.Fibersfromthefacialnucleuspassventrolaterallytoexitthebrainstematthepontomedullaryjunction.SUPERIOROLIVARYNUCLEUSThesuperiorolivarynucleusliesimmediatelyventraltothenucleusofCNVIIandreceivesauditoryimpulsesfrombothearsbywayofthecochlearnuclei.

PONS

CRANIALNERVENUCLEIThecochlearnucleiarefoundatthepontomedullaryjunctionjustlateraltotheinferiorcerebellarpeduncle.VESTIBULARNUCLEIThevestibularnucleiarelocatedneartheposteriorsurfaceoftheponslateraltotheabducensnucleus,andextendintothemedulla.COCHLEARNUCLEIThedorsalandventralcochlearnucleiarefoundatthepontomedullaryjunction.AllofthefibersofthecochlearpartofCNVIIIterminatehere.

PONS

CRANIALNERVENUCLEITRIGEMINALNUCLEIMotorNucleus-PonsThemotornucleusofCNVislocatedintheponsjustmedialtothemainsensorynucleusofthetrigeminal.Thesemotorfiberssupplythemusclesofmastication:masseter,temporalis,andmedialandlateralpterygoid.MainSensoryNucleus-PonsThemainsensorynucleusislocatedjustlateraltothemotornucleus.Themainsensorynucleusreceivestactileandpressuresensationsfromtheface,scalp,oralcavity,nasalcavity,anddura.SpinalTrigeminalNucleus-SpinalcordtoponsThespinaltrigeminalnucleusisacaudalcontinuationofthemainsensorynucleus.

PONS

MIDBRAIN

Themidbrain(mesencephalon)islocatedbetweentheponsanddiencephalon.Itistheshortestbrainstemsegment,nomorethan2cmlong.Themidbraincanbedividedintoadorsaltectumandrightandleftcerebralpeduncles,eachofwhichisfurtherdividedintoaventralcruscerebriandadorsaltegmentumbyapigmentedlamina,thesubstantianigra.

MIDBRAIN

Thecerebralaqueduct(ofSylvius),anarrowchannelthatconnectsthethirdandfourthventricles,passesthroughthemidbrain.

Theinferiorcolliculiandsuperiorcolliculiarefoundonthedorsalaspectofthemidbrain.

MIDBRAIN

Theinferiorcolliculiandsuperiorcolliculiareabovethecerebralaqueduct.

Theinferiorcolliculusprocessesauditoryinformationreceivedbilaterallyfromthecochlearnucleibyaxonfibersofthelaterallemniscus.Thesuperiorcolliculihelpdirectmovementsofbotheyesingaze.

Thepretectalregionislocatedjustbeneaththesuperiorcolliculiandinfrontoftheoculomotorcomplex.Thisareacontainsinterneuronsinvolvedinthepupillarylightreflex.

MIDBRAIN

Themassivecerebralpedunclesextendventrallyfromthemidbrain.

Thecerebralpedunclescontaincorticospinal(formtractsofthespinalcord)andcorticobulbarfibers(relatedtosomeofcranialnervesnuclei).

Theinterpeduncularfossaisthespacebetweenthecerebralpeduncles.

MIDBRAIN

Thesubstantianigraisthelargestnucleusofthemidbrainthatextendsthroughthewholelengthofthemidbrain.Thesubstantianigraislocatedbetweenthecruscerebriventrolaterallyandascendinglemniscalfibresdorsomedially.

ThesubstantianigrahasmajorconnectionswiththebasalgangliaandiscentraltothepathologyofParkinson’sdisease.

CRANIALNERVENUCLEIThetrochlearnucleusislocatedjustbeneaththeperiaqueductalgraynearthemidlinebetweenthesuperiorandinferiorcolliculi.TheoculomotornucleusandthenucleusofEdinger-Westphalarefoundjustbeneaththeperiaqueductalgraynearthemidlineatthelevelofthesuperiorcolliculi.Twocranialnervesemergefromthemidbrain:theoculomotor(CNIII)andthetrochlear(CNIV)nerves.Theoculomotornervearisesfromtheoculomotornucleusandexitsventrallyfromthemidbrainintheinterpeduncularfossa.

MIDBRAIN

CRANIALNERVENUCLEITheoculomotornerveCNIIIalsocontainspreganglionicparasympatheticaxonsthatarisefromthenucleusofEdinger-Westphal,whichliesadjacenttotheoculomotornucleus.AxonsofthetrochlearnerveCNIVdecussateinthesuperiormedullaryvelumandexitthebrainstemneartheposteriormidlinejustinferiortotheinferiorcolliculi.Onlythetrochlearnervearisesfromthedorsalaspectofthebrainstem.

MIDBRAIN

TEGMENTUMOFTHEMIDBRAINThemediallemniscusandspinothalamictractanddescendinghypothalamicfiberscoursetogetherventrolateraltotheperiaqueductalgray.TheMLF(mediallongitudinalfasciculus)continuestobelocatednearthemidline,justbeneaththecerebralaqueduct.

MIDBRAIN

Themesencephalicnucleiofthetrigeminalnervearelocatedoneithersideofthecentralgray.

Twocranialnerves,theoculomotorandtrochlear(CNIII,CNIV),arisefromthemidbrain.Onlythetrochlearnervearisesfromthedorsalaspectofthebrainstem.Fourcranialnerves-thetrigeminal,abducens,facial,andvestibulocochlearnerves(CNV,VI,VII,andVIII)-enterorexitfromthepons.Threecranialnerves-theglossopharyngeal,vagus,andhypoglossalnerves(CNIX,X,andXII)-enterorexitfromthemedulla.Fibersoftheaccessorynervearisefromthecervicalspinalcord.

CRANIALNERVES