SectiontwothebrainThebrainconsistsoffourparts:brainstem,cerebellum,diencephalonandtelencephalon.I.Thebrainstemincludesmedullaoblongata,pons,midbrain(I)Externalfeatures1.Theventralsurfaceofbrainstem①medullaoblongatapyramid,decussationofpyramid,olive,fourpairsofcranialnervesemergefromthemedullaoblongata(IX-XII,glossopharyngealN,vagusN,accessoryNandhypoglossalN)②ponsbasilarsulcus,

middlecerebellarpeduncles,fourpairsofcranialnervesemergefromthepons(V-VIII,thetrigeminalN,abducentN,facialNandvestibulocochlearN)③midbraincerebralpeduncles,twopairsofcranialnervesemergefromthemidbrain(III–IV,theoculomotorNandtrochlearN)2.Thedorsalsurfaceofbrainstem①themedullaoblongatastriaemedullaresgraciletubercle,cuneatetubercle,hypoglossaltrigonevagaltrigone②ponsmediansulcussulcuslimitansmedialeminence:facialcolliculusvestibulararealocusceruleus③midbrainsuperiorandinferiorcolliculi(quadrigeminalbody).thesuperiorcolliculiareinvolvedinvisualreflexesandtheinferiorinvolvedtheauditoryreflexes.brachiumofsuperiorcolliculus(passtothelateralgeniculatebody)brachiumofinferiorcolliculus(passtothemedialgeniculatebody)trochlearnervesuperiorcerebellarpedunclesandsuperiormedullaryvelum3TherhomboidfossaBoundariesupper:superiorcerebellarpeduncleslower:inferiorcerebellarpeduncles,cuneateandgraciletuberlces

4.Thefourthventricle①Location:amongcerebellum,thedorsalsurfaceofthemedullaoblongataandpons②

Communication:continuousbelowwiththecentralcanalandabovewiththemesencephalic(cerebral)aqueductFourthventricle(II)Internalstructureofbrainstem1.Features①Thefourthventricleappears②Relationbetweenthemotornucleusandsensorynucleusismedialandlateral③Grayandwhitemattersarenotcontinuous④Cranialnervenucleihavesevenfunctionalcomponents⑤Theregionofreticularformationiswidely2.Cranialnervenuclei(seventypes)①Generalsomaticefferent(motor)nuclei:innervatestriatedmusclesthatarederivedfromsomitesand

involvedineyeandtonguemovements.Suchas:hypoglossalnucleusofXII,oculomotornucleusofIII,trochlearnucleusofIV,andabducensnucleusofVI.②Generalvisceralefferent(motor)nuclei:innervatesmoothmusclesandtheglandsinthehead,neckandtorso(trunk).Suchas:accessoryoculomotornucleus(Edinger-Westphal)ofIII,superiorsalivatorynucleusofVII,inferiorsalivatorynucleusofIXanddorsalmotornucleusofX.③Specialvisceralefferent(motor)nuclei:innervatemusclesthatarederivedfromthebrachialarchesandareinvolvedinchewing,makingfacialexpressions,swallowing,producingvocalsounds,andturningthehead.Suchas:trigeminalmotornucleusofV,facialnucleusofVII,ambiguousnucleusofIX,X,andXI,spinalaccessorynucleusofXI.④Generalvisceralafferent(sensory)nuclei:receivethevisceralsensefromthehead,neckandtorso.Suchas:nucleusofsolitarytractofVII,IX,andX.⑤Specialvisceralafferent(sensory)nuclei:receivetastesense.Nucleiarethesameasthe④.⑥Generalsomaticafferent(sensory)nuclei:receivesensefromtheskinandmucosaofthehead.Suchas:mesencephalicnucleusofV,pontinenucleusofV,nucleusofspinaltrigeminaltract.⑦Specialsomaticafferent(sensory)nuclei:

receivethesensesofhearingandequilibrium.Suchas:fourvestibularnucleiandtwocochlearnuclei.3Othernervenuclei①Medulla:gracilenucleusandcuneatenucleus②Pons:pontinenucleus③Midbrain:superiorandinferiorcolliculi;rednucleus;substantianigragracilenucleusandcuneatenucleuspontinenucleiinferiorcolliculusSuperiorcolliculus;rednucleus;substantianigra4Ascendingtracts①Mediallemniscus:gracileandcuneatenucleigiverisetoacrossedfiberbundletoformmediallemniscus.Mediallemniscus②Spinallemniscus:Itiscontinuouswiththespinothalamictractinthespinalcord.③Laterallemniscus:Thefibersarisingfromthecontralateralanteriorandposteriorcochlearnuclei,andsuperiorolivarynucleus,andtheipsilateralolivarynucleusmakeupthelaterallemniscus.④Trigeminallemniscus:ThepontinenucleusofV,nucleusofspinaltrigeminaltractgiverisetoacrossedfiberbundletoformthetrigeminallemniscus.5Descendingtracts①Pyramidalsystem:consistsofthecorticospinaltract(pyramidaltract)andcorticonucleartractPyramidaltractCorticonucleartract6Reticularformation①Definition(term):islocalizedinthetegmentumofthebrainstem(withexceptionoftheconspicuousfiberbundlesandnucleiofthebrainstem),thelateralhypothalamicarea,andthemedial,intralaminar,andreticularnucleiofthethalamus.ReticularformationThisareaischaracterizedbyadiffuse,seeminglyunorganizedstructurewithinterlacingcellsandfibers.Scatteredthroughthereticularformationliesomecellaccumulationswithilldefinedboundaries(reticularnuclei:unpairedraphenuclei,pairedmedialnuclei,pairedlatera