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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
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