神经学教学课件：Principles of Neuroscience

February2003PrinciplesofNeuroscienceFebruary2003Contents1Introduction

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1BasicDesignandTerminology32DevelopmentoftheNervousSystem93MeningesandCerebrospinalFluid432DissectionoftheBrain57

4DissectionoftheBrain59

FirstDissection:MeningesandsubarachnoidCisterns;SuperficialArteries,Vertebral-BasilarSystem;InternalCarotidSystem;BasalSurfaceandCranialNerves61

SecondDissection:MidsagittalSection;Lobes,Sulci,andGyri73

February2003

ThirdDissection:WhiteMatter;BluntDissectionofMajorFiberSystems83

FourthDissection:Ventricu1arSystem,HippocampusFormation,Thalamus,andBasalGanglia93

FifthDissection:FourthVentricle,Cerebellum,BrainStem1053AtlasoftheBrain1195AtlasoftheBrain:Frontal,Sagittal,andHorizontalBrainSections;AnatomicalCorrelationofMagneticResonanceImaging(MRI)1214FunctionalNeuroamatomy1436Neurohistology145February2003

7NeuroanatomicTechniques1718TheSpinalCord1879AscendingSensoryPathways20110BrainStem,MonoaminergicPathways,andReticularFormation21911CranialNerves241OlfactorySystem269VisualSystem279AuditorySystem301SpinalCordMotorStructuresandtheDescendingSupraspinalPathways315BasalGangliaandRelatedBasalForebrainStructures337February2003Cerebellum363Hypothalamus379TheAutonomicNervousSystem399AmygdaloidBodyandExtendedAmygdala415HippocampalFormation423CerebralCortexandThalamus433Neurotransmitters455CerebrovascularSystem465PeripheralNerves481February2003Theevolutionofmammalianbrainfromrodenttoanthropoid,evenHumanbeing

February2003Hedgehogn.刺猬AnyofseveralsmallinsectivorousmammalsofthefamilyErinaceidaeofEurope,Africa,andAsia,havingthebackcoveredwithdense,erectilespinesandcharacteristicallyrollingintoaballforprotection.Opossum〈动〉负鼠Anyofvariousnocturnal,usuallyarborealmarsupialsofthefamilyDidelphidae,especiallyDidelphismarsupialisoftheWesternHemisphere,havingathickcoatofhair,alongsnout,andalongprehensiletail.Batn.蝙蝠AnyofvariousnocturnalflyingmammalsoftheorderChiroptera,havingmembranouswingsthatextendfromtheforelimbstothehindlimbsortailandanatomicaladaptationsforecholocation,bywhichtheynavigateandhuntprey.February2003Muskrat〈动〉麝鼠Alargeaquaticrodent(Ondatrazibethica)ofNorthAmerica,relatedtothelemmingandthevoleandhavingadensebrowncoatandmuskglandsunderabroad,flattail.Capybara水豚Alargesemiaquaticrodent(Hydrochoerushydrochaeris)oftropicalSouthAmerica,havingshortlimbsandavestigialtailandoftenattaininglengthsofmorethan1.2meters(4feet).Mongoosen.[动]猫鼬AnyofvariousOldWorldcarnivorousmammalsofthegenusHerpestesandrelatedgenera,havingaslenderagilebodyandalongtailandnotedfortheabilitytoseizeandkillvenomoussnakes.February2003Racoonn浣熊Zebran.斑马Anyofseveralswift,wild,horselikeAfricanmammalsofthegenusEquus,havingdistinctiveoverallmarkingsofalternatingwhiteandblackorbrownstripes.Lemurn.狐猴Anyofseveralsmallarboreal,mostlynocturnalprimateschieflyofthefamilyLemuridaeofMadagascarandadjacentislands,havinglargeeyes,alongslimmuzzle,andalongtail.Baboon〈动〉狒狒Anyofseverallarge,terrestrialAfricanandAsianmonkeysofthefamilyCercopithecidae,especiallyofthegenusPapioorChaeropithecusandrelatedgenera,characterizedbyanelongated,doglikemuzzle,ashorttail,andbarecallusesonthebuttocks.February2003Gorillan.大猩猩Thelargestoftheanthropoidapes(Gorillagorilla)nativetotheforestsofequatorialAfrica,havingastockybodyandcoarse,darkbrownorblackhair.Chimpanzeen.黑猩猩Agregariousanthropoidape(Pantroglodytes)oftropicalAfrica,havinglongdarkhairandsomewhatarborealhabitsandexhibitinghumanlikebehaviorandahighdegreeofintelligence.Itisnowconsideredvulnerabletoextinctioninthewild.February2003Thebraindevelopmentduringembryofromat25daysto9monthsFebruary2003Averagebrainweightsofhumanmalesandfemalesatdifferentages.Noticehowthebraingrowsrapidlyafterbirth,doublinginthefirstyearoflife,beforereachingitsfullsizeatabouttheageof11.Atallages,malebrainshaveagreateraverageweightthanfemalebrains.February2003February2003However,asindicatedinB,adultfemalebrainsactuallyaccountforagreaterpercentageofbodyweightthandoadultmalebrains.Braingrowthissubstantialinutero,andwearebornwithbrainsthatareverylargerelativetoourbodies.Afterthebraingrowthspurtofthefirst1to3yearsoflife,bodygrowthtakesoverandthebrainweight/bodyweightratiodeclinesprogressivelyuntilaboutage17.February20031Introduction

BasicDesignandTerminologyDevelopmentoftheNervousSystemMeningesandCerebrospinalFluidFebruary20031BasicDesignandTerminology

TheCentralandthePeripheralNervousSystemsTheNeuronGrayandWhiteMatter

CollectionsofNerveCellBodies

CollectionsofNerveFibers

NervousPathwaysMajorSubdivisionsoftheBrain

Te1encephalon

Diencephalon

Mesencephalon

Metencephalon

MyelencephalonFebruary2003lnfreshlycutsectionsofthebrainandspinalcordsomeregionsdisplayawhiteglisteningcolor,othersagrayishcolor.Thewhitematterrepresentscollectionsofbundlescontaininglongnervefibers,whicharesurroundedbywhiteglisteningmyelinsheaths.Nervecellbodies,ontheotherhand,tendtoaggregateinasuperficialcorticalsheathorinsubcorticalnuclei,referredtoasgraymatter.February2003Ifthedifferentiationbetweengrayandwhitematterisaccentuatedbyappropriatestainingmethods,itiseasilyappreciatedthatthecentralnervoussystemiscomposedofalargenumberofmoreorlessdistinctregions.Althoughitisimportanttohaveanunderstandingofthesevarioussubdivisionsandtheirtopography,suchknowledgeisnotveryusefulbyitself.Amoremeaningfulpicturewillemergefromtheknowledgeofhowthevarioussubdivisionsrelatetoeachotherandhowgroupsofneuronsareinterconnectedtoformanatomicpathwaysandfunctionalsystems.February2003TheCentralandthePeripheralNervousSystemOnthebasisofgrossanatomicfeatures,thenervoussystemcanbedividedintotwoparts:the

centralnervoussystem(CNS),whichconsistsofthebrainandspinalcord(Fig.Left),andthe

peripheralnervoussystem

(PNS),whichconsistsofthecranialandspinalnervesandtheirganglia.February2003TheperipheralnervesconnecttheCNSwiththesenseorgansandtheeffectororgans(musclesandglands).Althoughitisusefultosubdividethenervoussysteminthismanner,oneshouldrememberthatfunctional-anatomiccircuitsandsystemspaylittleattentiontotheboundariesbetweenthecentralandperipheralnervoussystems.Forinstance,thecranialandspinalnervesaredescribedaspartofthePNS,buttheircellsoforigin(motorneurons)ortheirterminalbranches(sensoryneurons)aresituatedwithintheCNS.February2003Boththecentralandtheperipheralnervoussystemscontainsomaticaswellasautonomicparts.Thesomaticpartsofthenervoussystemcontrolmovementsandinnervatesensoryorgans,whereastheautonomicpartsareconcernedwiththeinnervationofthevisceralorgans.Althoughtheautonomicnervoussystemwasoriginallyconceivedofasaperipheralsystem,albeitwithcellsoforiginlocatedinthespinalcordandbrainstem,manyotherareasintheCNS,includingthehypothalamus,amygdaloidbody,andcerebralcortex,arealsoinvolvedinthecontrolofthevisceralorgans.February2003TheNeuronThenervoussystemcontainsabout100billionnervecellsorneurons,and,inaddition,anevenlargernumberofneuroglialcellsthatsubservevarioussupportive,metabolic,andphagocyticfunctions.Theneuronsarespecializedtoreceive,conduct,andtransmitnervousimpulses,andtheyhavecertaincharacteristicsincommon,eventhoughtheyvarywidelyinshapeandsize.Twotypesofprocessesextendfromthecellbody.February2003Thereceivingprocesses,thedendrites,haveabroadbaseandtaperawayfromthecellbody;theybranchintheimmediateneighborhoodofthecellbody.Thereusuallyaremanydendritesperneuron.Theaxon,ofwhichthereisonlyonepercell,conductstheimpulsesawayfromthecellbody.Axonsvarygreatlyinlengthanddiameter.Thethickeraxons,whichconductimpulsesmorerapidlythanthethinnerones,areinsulatedfromtheenvironmentbyalipoproteinsheath,themyelinsheath.February2003Thinaxonsareeitherunmyelinatedorthinlymyelinated.Theaxonlosesitsmyelinsheathatitsdestination,anddividesintoseveralsmallbranches,whichtypicallyendasswellingsreferredtoasaxonterminalsorboutons.Theboutonsestablishcontactswithotherneuronsorwithaneffectororgan.Thesiteofinterneuronalcontact,i.e.,wheretheimpulsesaretransmittedfromonecelltoanother,iscalledasynapseorsynapticjunction.TheneuronanditsprocessesarediscussedinmoredetailinChapter6.February2003Thedistributionofgrayandwhitemattercanbeeasilyappreciatedinmacroscopicsectionsthroughthebrainandspinalcord.Thebasisforthisdistinctionrelatestothestructureofthenervecell.Asalreadyindicated,manyaxonsaresurroundedbyamyelinsheath,whichhasawhiteglisteningcolor;consequently,thosepartsthatcontainmanymyelinatedaxonsarewhite,i.e.,

whitematter.Grayand

WhiteMatterFebruary2003Thepartsthatcontainaggregationsofnervecellbodiesembeddedinanetworkofdelicatenerveprocesseshaveagraycolor,hencethetermgraymatter.Therearetwomajorterritoriesofgraysubstanceinthebrain,thecerebralcortexonthesurfaceandlargesubcorticalnuclei,e.g.,caudatenucleus,putamen,globuspallidus,andthalamus,intheinterior.ThegraysubstanceinthespinalcordislocatedinthecenterandhasatypicalH-shapedappearanceintransversesectionsitissurroundedbywhitematter,whichcontainslongascendinganddescendingpathways.February2003CollectionsofNerveCellBodiesThecortexisthesuperficialcoatofgaymatterinthecerebralhemispheresandincerebellum.Thenervecellbodiesinthecortexarearrangedinmoreorlesswell-definedlaminaeorlayers.Groupsofnervecellbodiesinotherpartsofthebrainandspinalcordarereferredtoasnuclei,orcolumnsiftheyoccurinlongrowsastheyoftendointhespinalcord.Notethatthetermcolumnisalsousedforthewhitematterinthedorsalpartofthespinalcord.AccumulationsofnervecellbodiesoutsidetheCNSarecalledganglia.February2003CollectionsofNerveFibersAcollectionofnervefiberswithacommonoriginanddestinationconstitutesatracte.g.,thecorticospinal(orpyramidal)tract.Atractdoesnotnecessarilyfromawell-definedbundle,sincethefibersofatractofteninterminglewithfibersofneighboringpathways.Adistinctcollectionofnervefibersisreferredtoasafasciculus,peduncle,orbrachium.Suchcollectionsoffibersoftencontainmorethanonetract.Thetermlemniscusisspecificallyusedforascendingsensoryfiberbundlesinthebrainstem.February2003Whitematterofthespinalcord,whichcontainsthelongascendinganddescendingtracts,issubdividedinto

ventral

(anterior),lateralanddorsal(posterior)funiculus.Thedorsalfuniculus,however,isoftenreferredtoasthedorsalcolumn,especiallyinthephysiologicliterature.Fibersconnectingsimilarareasonthetwosidesofthebrainformwell-definedbundlesknownascommissures.Manyfiberbundlescrossthemidlineintheircoursefromonelevelofthenervoussystemtoanother.Suchcrossingfibersformadecussation.Nerves,nerveroots,nervetrunks,andramiareexamplesofbundlesofnervefibersinthePNSFebruary2003NervouspathwaysEfferentpathwaysprojectawayfromtheregionunderconsideration.Afferentpathwaysprojecttotheregionunderconsideration.Extrinsicpathwaysprojectintooroutoftheregionunderconsideration.Intrinsicpathwaysareconfinedtotheregionunderconsideration.February2003MajorSubdivisionsoftheBrainThebrainissubdividedintofivemajordivisions:telencephalon,diencephalon,mesencephalon,metencephalon,myelencephalon.February2003February2003TelencephalonThetelencephalon(endbrain)consistsofthetwocerebralhemispheres,whichformthelargestpartofthehumanbrain.Thehemispheresareconnectedbyamassivebundleofcommissuralfibers,thecorpuscallosum.Eachhemisphereiscoveredbyathinlayerofgraysubstance,thecerebralcortex,whichisheavilyfolded.Thefoldsarecalledconvolutions,orgyri,andthegroovesarereferredtoassulciorfissures,iftheyareverydeep.Intheinteriorofeachhemisphereisacentrallyplacedcavity,thelateralventricle,andalargemassofwhitesubstanceaswellasseverallargenucleireferredtoFebruary2003asthebasalganglia.Althoughthethalamusinthediencephalonissometimesincludedinthebasalganglia,thetermismostcommonlyusedforthetwotelencephalicnuclei,thecaudateandthelentiformnuclei,andsomecloselyrelatedbrainstemnuclei,knowntobeofspecialimportanceforcontrolofmovements.Theclaustrum,whosefunctionsareunknown,issometimesincludedinthetermbasalganglia.February2003DiencephalonFebruary2003Thediencephalonisdividedintotwohalvesbyaslit-formedmediancavity,thethirdventricle,whichiscontinuousviathesmallinterventriclarforamen(ofMonro)withthelateralventricleineachhemisphere.Thethalamusandhypothalamusarethetwomajordivisionofthediencephalon.Thethalamusisacollectionofseveralnucleiwithvariousfunctionsandanatomicrelations.Thehypothalamus,likewise,containsavarietyofdifferentcellgroupsandpathways.ItiswellknownforitsregulationofautonomicandendocrinefunctionsFebruary2003MesencephalonThemesencephalon(midbrain)isashortsegmentofthebrainbetweenthediencephalonandthemetencephalon.Itsinternalstructureiscomplicated,butitsgrossanatomicfeaturesaresimple.Itistraversedbyanarrowcanal,thecerebralaqueduct,whichconnectsthethirdventricleinthediencephalonwiththefourthventricleinthemetencephalon.February2003MetencephalonThemetencephalonconsistsofthecerebellumandthepons(bridge).Thecerebellum,whichisconcernedwithcoordinationofmovements,islocatedunderneaththetentoriumcerebelliintheposteriorcranialfossa.Itconsistsofthetwocerebellarhemispheresjoinedinthemidlinebyanarrowwormlikeportion,thevermis.Likethecerebralhemispheres,thecerebellumiscoveredbyalayerofgraysubstance,thecerebellarcortex,whichischaracterizedbyalargenumberofparallelfolds,folia,separatedbydeepfissures.February2003Althoughtheponsisusuallysaidtobelocatedventraltothecerebellum,itisactuallysituatedinfrontofthecerebellumwhentheheadisinthenormaluprightposition.Whenviewedfromitsventralside,theponsappearsasabridgeoveracanal(thebrainstem).February2003MyelencephalonThemyelencephalon,ormedullaoblongata,iscontinuouswiththespinalcordattheleveloftheforamenmagnum.Thecavitythatislocatedbetweenthecerebellumontheonehandandtheponsandmedullaoblongataontheotherisreferredtoasthefourthventricle.February2003Themedullaoblongata,pons,andmesencephalon,whichtogetherformthebrainstem,containmanyimportantnucleargroupsincludingthecranialnervenuclei.Theyalsocontainimportantpathwaysconnectingthespinalcordandcerebellumwiththediencephalonandthetelencephalon.February20032DevelopmentoftheNervousSystemEarlyDevelopmentoftheNervousSystem

NeuralTubeNeuralCrestSpinalCordVentricular,Intermediate,andMarginalZonesBasalandAlarPlatesNeuronalMigrationandMaturationDorsalRootGangliaandSpinalNervesCaudalEquinaFebruary2003EarlyDevelopmentoftheBrainMylencephalonMetencephalon

TheCerebellarCortexMesencephalonDiencephalon TheEyeTheHypophysis(Pituitary)Telencephalon

TheCerebralHemispheresTheCerebralCortexTheVentricleSystemTheChoroidPlexus

February2003

CorpusCallosumandFornixStriatumandGlobasPallidusTopographyofCorpusStriatum,Thalamus,andInternalCorpsuleClinicalNotesAgingandtheBrainChromosomalAbnormalitiesandEnvironmentalFactorsNeuralTubeDefectsHydrocephalusAgenesisofCorpusCallosumMicrocephalyCraniopharyngiomaDisordersoftheEyeClinicalExampleArnold-ChiariMalformationwithMyelomeningocele(ChiariII)February2003KnowledgeoftheimportantdevelopmentaleventswillfacilitatetheunderstandingoftheanatomyoftheCNS.Itwillalsomakeiteasiertounderstandmalformationsofthenervoussystemthataremetinclinicalpractice.Proliferationandmigrationofcellinthenervoussystemstartearlyinembryoniclifeandcontinueinsomeareasuntilafterbirth.Inaddition,neuronsmatureandneuronalprocessesdeveloptheirelaboratebranchingpatternandsynapticconnectionsinboththepre-andpostnatalperiods.EarlyDevelopmentoftheNervousSystemFebruary2003Theprecisionwithwhichsynapticconnectionsareformedisasremarkableasthemultitudeofmoleculesandcellularmechanismsinvolved.Althoughtheexternalshapeofthebrainandspinalcordiswelldevelopedatbirth,0thesizeoftheCNSincreasesdramaticallyalsointheneonatalperiodandduringinfancy.Theprotractedmaturationofthenervoussystemmakesitespeciallyvulnerabletoavarietyofharmfulenvironmentalfactors,includinginfections,toxicsubstances,malnutrition,etc.February2003NeuralTubeThefirstsignofanervoussystemappearsinthe3rdweek,whentheectodermonthedorsalsideoftheembryothickenstoformtheneuralplate.Themechanismwherebysomeoftheectodermalcellsaretransformedintospecializednervoustissuecellsisknownasinduction.Peptidesreleasedfromtheunderlyingmesodermapparentlyserveasinducersinthisprocess.February2003February2003Theneuralplatesoonstartstofoldandagrooveisformed,theneuralgroove.Thetwosidewalls,theneuralfolds,approacheachotherinthemidlineandthegrooveisgraduallytransformedintotheneuraltube,whichbecomesseparatedfromtherestoftheectoderm.February2003Theprocessbywhichtheplateistransformedintoatubeiscalledneurulation.Theclosureoftheneuralgroovebeginsatthemidcervicallevelandproceedsinbothcranialandcaudaldirection.February2003Thelastportionsoftheneuraltubetoclose,locatedrostrallyandcaudally,arecalledtheanteriorandposteriorneuropores.Theyarenormallyclosedbytheendoftheembryonicweek,atwhichtimetheneuraltubesurroundsatfluid-filledcentralcavity.Embryoat24-25daysFebruary2003Thesiteoftheclosureoftherostralneuroporecorrespondstotheregionofthelaminaterminalisofthematurebrain.Itshouldbenotedthatthemajordivisionsofthebrain,i.e.,theforebrain,midbrain,andhindbrain(seebelow),canbeidentifiedbeforetheclosureoftheneuraltube.Adefectiveclosureoftheneuraltubeisacommoncauseofneurologicmalformation(seeClinicalNotes).Theneuroepithelialcellsthatformthewallsoftheneuraltuberapidlymultiplyandeventuallygiverisetoalltheneuronsandmacroglialcells(astrocytes,oligodendrocytes,andependymalcells)ofthebrainandthespinalcord.Microgliaarebelievedtobeofmesodermalorigin,andtheyapparentlyinvadetheCNSatthetimethevasculaturedevelops.February2003NeuralCrestAstheneuralplatefoldstoformtheneuraltube,themostlateralcellsarepinchedoff.Thesecells,theneuralcrestcells(blackdotsincross-sections)developinrostrocaudalsequencealongthedorsolateralpartofthetube.February2003Theybecomesegmentedintocellgroupsthatgiverisetothesensoryganglioncellsofthespinalandcranialnerves.Manyoftheneuralcrestcellsmigratetootherpartsofthebodywheretheyappearinavarietyofcelltypes,includingpost-ganglionicautonomicneurons,chromaffincellsoftheadrenalmedulla,andsupportingcellsinthePNSsuchassatellitecells,Schwanncells,andneurolemmacells.Allmelanocytes,likewise,comefromtheneuralcrest.February2003SpinalCordFebruary2003Ventricular,Intermediate,andMarginalZonesWhentheneuraltubehasclosed,theneuroepithelialcellsformathickpseudostratifiedcolumnarepithelium(Fig.A).Thecellsmultiplyatarapidrateinthiszone,whichisreferredtoastheventricularzone(neuroepithelium).Duringtheproliferativeperiodthenucleimigrateinacharacteristicfashiontoandfromthelumenateachdivision.

February2003Intheinterphase,whentheundifferentiatedneuroepithelialcellssynthesizeDNA,theirformiswedge-shaped;thenucleilieintheouterzoneoftheprimitiveependymalzoneandslendercytoplasmicprocessesextendtowardtheventricularsurface.Duringthemitoticcyclethenucleimigratetowardthelumen,wheretheydividewithinafewhoursfollowingtheDNAsynthesis.February2003Afterthedivision,thenucleipassawayfromthelumenandanygivencellmaycontinueinalateraldirectionandparticipateintheformationoftheintermediate(mantle)zone(Fig.A).Intheintermediatezonetheyoungneuronsthathavejustbeenproducedbytheirneuroepithelialprecursorsaccumulateanddifferentiate.February2003Theaxonsofmanyoftheneuronscollectalongtheperipheryoftheintermediatezone,wheretheyformthemariginalzone.Thisdescriptionreferstotheeventsinthecaudalpartoftheneuraltube,whichdevelopsintothespinalcord.Theventricular,intermediate,andmarginalzonesarerepresentedbytheependymallayer,thegraysubstance,andthewhitesubstanceinthematurespinalcord(Fig.B).Theeventsintherostralpartoftheneuraltube,whichgivesrisetothebrain,aremorecomplicated.

February2003BasalandAlarPlatesAlongitudinalgroove,thesulcuslimitans,appearsonbothsidesofthecentalcaivityandrunsalongthelengthofthespinalcord.Thissulcus,whichcanheclearlyrecognizedasfarrostrallyasthemidbrain,dividestherapidlyexpandingsidewallsoftheneuraltubeintoaventralthickening,thebasalplate,andadorsalthickening,thealarplate(Fig.A).ThecellbodiesinthebasalplateformFebruary2003theventralandlateralgraycolumns,whichcontainsomaticandautonomicmotorneurons,respectively.Thecellbodiesinthealarplateformthedorsalgraycolumn.Thesecolumnsareoftenreferredtoastheventral,lateral,anddorsalhornswhenviewedincross-section(Fig.B).Themarginalzone,whichincreasesinthicknessasmoreandmoreneuronssendaxonsintoitformsthewhitematterofthespinalcord.Asaresultofcontinuousenlargementofthewallsoftheneuraltube,thewildcentralcavitygraduallynarrowandeventuallybecomesthecentralcanalofthespinalcord.February2003NeuronalMigrationandMaturationThedevelopmentofneuronsisanelaborateprocess,whichbeginswiththeearlyformationoftheneuroepitheliumandthesubsequentproliferationanddifferentiationofcellsintopostmitoticimmatureneurons.Theseyoungneuronsmigratefromtheventricularzonetotheintermediatezonewheretheymaturebyacquiringtheiruniquepatternofneuronalprocesses.Whereasthedistanceofmigrationisingeneralrathershortinthespinalcord,themigrationroutesinthedevelopingbraincanbelongandsometimesrathercomplicated.February2003Duringmigration,manyoftheyoungneuronsareguidedbyradiallyarrangedglialfibersthatspanthewalloftheneuraltubefromtheventricularsurfacetothepia.Theradialglialcellsdisappearlaterindevelopment;somedegenerateandothersdevelopintoastrocytes.Howtheneuronsfindtheircorrectpositionorlocaladdressinthenervoussystemisnotfullyunderstood.Nonetheless,havingsettledintheirfinallocation,theyoungneuronsmaturebydevelopingaxonalanddendriticprocessesandestablishingsynapticcontacts.Howthiscanbeaccomplishedwithsuchprecisionisacentralquestion,whichisattractinganever-increasingnumberofyoungandcuriousscientistsintothefieldofdevelopmentalandmolecularneurobiology.February2003Howcanaxonsandtheirdistanttarget,stopattheappropriateplace,andestablishthecorrectsynap