Nerve. 周?chē)窠?jīng)解剖

NervousTissueZHUYONGHONG(朱永紅〕Deparementofhistologyandembryology1.Introduction2.Neurons

(1).Classifications (2).Structures

A.CellBody B.Processes:(Dendrites,Axon)3.Synapses

(1).Chemicalsynapses (2).Electricalsynapses4.GlialCells5.NerveFiberOrganization

(1).Nervefibers:myelinatedorunmyelinatedfibers (2).Nerves6.NerveEndings (1).Sensorynerveendings (2).MotorEndings(Myoneuraljunctions)NervousTissueCellularElements Neuron(NerveCell) NeuroglialCells

centralneurglia:

astrocyte,oligodendrocyte,microgliaand ependymalcell

peripheralneuroglia:

Schwanncellinnerveandgaglion,satellite (capsular)cellinganglionIntercellularSubstance:extremlysparse2.NeuronTwospecialproperties:Irritability(theabilitytorespondtoastimulus).Propagationofimpulses(theabilitytoconductimpulses).

Cellbody/perikaryon(5-150

mdia)Axon(myelinatedorunmyelinated)withterminalsandsynapticjunctionsDendrites-receiveinputVarietyofneuronshapes:multipolar,bipolar,pseudounipolarMorphologycategoriesFunctionalCategories(1).Classifications99%ofneuronsareinterneuronsNeurites:Neuronscanbeclassifiedbythenumberofneurites(axonsanddendrites)thatextendfromthecellbody.Cellscanbeunipolar,bipolar,ormultipolar.

Dendrites:Dendritictreescanvarywidelybetweenneurons.Inthecerebralcortex,therearetwotypes:pyramidalcellsandstellatecells.Cellscanalsobeclassifiedbywhetherthedendriteshavespinesornot.Thosethatdoarecalledspinyandthosethatdon'tarecalledaspinous.Pyramidalcellsarespiny,whilestellatecellscanbespinyoraspinous.Connections:Neuronscanbeclassifiedbasedontheirconnectionstodifferenttissuetypesinthebody.Neuronsthathaveconnectionstosensorysurfaces,suchastheskin,areprimarysensoryneurons.Neuronsthatformsynapseswithmusclesaremotorneurons.However,mostneuronsformconnectionswithotherneurons-thesearetheinterneurons.AxonLength:Neuronscanbedifferentiatedbasedonhowlongofanaxontheyhave.NeuronswithverylongaxonsthatextendfromonareaofthebraintoanotheraretheGolgiTypeI,whilethosewithshort,localcircuitaxonsaretheGolgiTypeII.Neurotransmitter:Asidefromneuronmorphology,neuronscanalsobeclassifiedbasedontheirchemistry-the

neurotransmitter(s)

theyrelease.Forexample,neuronsthatreleaseacetylcholinearecalled

cholinergicneuron.Nucleus:large,euchromatic,prominentnucleolus,usuallycenterofsomaNisslbodies:clumpsofRERGolgicomplexandmitochondriaNeurofilamentsandmicrotubulesInclusions:melanin(rare)Cellmembranehasspecializedreceptiveareas,thesubsynapticmembranes

ofsynapses.

Perikaryon(CellBody)Ingraymatterandganglia(2)StructuresNisslbodies:basophil,concentrationofgranularER.IncellbodyanddendritesNeurofilaments:avarietyofintermediatefilament-arebundleintoneurofibrilsvisibleaftersilverimpregnationsinthecytoplasm.SkeletonandtransportationDendrites樹(shù)突MaybehighlybranchedandtaperedContainmitochondria,microtubules,andgranularER.ReceptivesurfaceforsynapticjunctionsTensofthousandsofsynapsesonlargedendritesDendriticspines〔樹(shù)突棘〕integratetheexcitatoryinfluencesandmodifytheirresponsesandmorphologyinlearninginputsfromover200,000othercells

PurkinjecellofcerebellumAxons軸突1axonprojectsfromcellbody,longandthinner⑤Membraneistheaxolemma〔軸膜〕,atitsterminal/synapsewhichholdsvesicles.⑥Mayormaynotbemyelinatedaroundtheaxolemma,sometimesrecurrentbacktonearthesoma.④Hasmitochondria,neurofilaments,microtubules,travellingvesicles,and,insomeneurons,secretiondroplets,intheaxoplasm.

①Canbemeterlong②Containsaxoplasmflowingfromtheaxonhillock〔軸丘〕.③Theinitialsegments,thetriggeringoftheactionpotential

retrogradetransportationanterogradetransportationTargettissueAxontransportation3.Synapses(Chemical)突觸Synapsesarespecializedaxonterminalcontactstodendritesandcellbodies,andfunctionallypolarizedtotransferexcitationoneway.Inthisimage,usedtoquantifysynapses,theproteinsassociatedwithsynapsesappearasblueandreddots.①Axodendriticsynapse②Axosomaticsynapse③Axoaxonicsynapse①②③StructureofsynapseTypesofsynapseLM:theaxonterminalformbulbousswellingorterminalbouton.cleftreceptors1.presynapticelement①

presynapticmembrane②synapticvesicleswithneurotransmitter,e.g.,serotonin,acetylcholine,dopamine,etc.

③denseprojection④synapsinI⑤mitochondria,SER,microtubule,microfilament.2.synapticcleft15-30nm3.postsynapticelementreceptor①Actionpotentialstraveldowntheaxonoftheneuron toitsend,theaxonterminal(synapticknob).②ThesynapticknobopensCa2+channelsintheplasmamembrane.③TheinfluxofCa2+triggerstheexocytosisofsomeofthevesicles.④Theirneurotransmitterisreleasedintothesynapticcleft.⑤Theneurotransmittermoleculesbindtoreceptors(ionchannels)onthepostsynapticmembrane.TheSynapseNervecellscoounicatewithelectricalimpulsescallednerveimpulses.Anerveimpulsetravelsoutfromthenervecellalongalongextensioncalledanaxon.Eventuallytheimpulsereachesthetipendoftheaxon,usuallypositionedveryclosetoanotherneuronortoamusclecellorgland.Axons,however,donotactuallymakedirectcontactwithotherneuronsorwithlargetissue.Instead,anarrowgap,10to20nanometersacross,separatestheaxontipandthetargetneuronortissue.Thisjunctionofanaxonwithanothercelliscalledasynapse.Themembraneonthenear(axon)sideofthesynapseiscalledthepresynapticmembrane;themembraneonthefar(receiving)sideofthesynapseiscalledthepostsynapticmembraneFigure1Asynapsebetweentwoneurons.Thismicrographclearlyshowsthespacebetweenthepresynapticandpostsynapticmembranes,whichiscalledthesynapticcleft.Eventsatthesynapse.(a)Whenanerveimpulsereachestheendofanaxon,itreleasesaneurotransmitterintothesynapticspace.Theneurotransmittermoleculesdiffuseacrossthesynapseandbindtoreceptorsonthepostsynapticcell,aneuroninthiscase,passingthesignaltothatcell.Enzymesdestroytheneurotransmittermoleculestopreventcontinuousstimulationofthepostsynapticcell.(b)Atransmissionelectronmicrographofthetipofanaxonfilledwithsynapticvesicles.(a)Manydifferentaxonssynapsewiththecellbodyofthepostsynapticneuronillustratedhere.Thesummedinfluenceoftheirinputdetermineswhetherornotanactionpotentialwillberealizedandanerveimpulsewillbesentdowntheaxonextendingbelow.(b)Micrographofaneuronalcellbodywithnumeroussynapses.Gliaorneuroglia:namedfromtheGreekwordfor"glue".

CNS:astroglia,oligodendroglia,microgliaandependyma PNS:Schwanncell,satelliteOnlynucleivisiblewithoutspecialstain4.NeurogliaFUNCTIONSOFNEUROGLIA(1)Structuralsupport(especiallytheastrocytesintheCNS)(2)Participationintheblood-brain-barrier(astrocytes)(3)Formationofthemyelinsheathofaxons(4)Isolationofjunctionalsurfacesofsynapses(5)Repairprocessesfollowingdamageorinjurytonerves.AstrocytesProtoplasmicastrocytes:large,star-shapedwithmanyprocesses,mayattachtobloodvessels;havefilamentsandmicrotubules;commoningreymatter.Fibrousastrocytes:havemorefilamentsandglycogen,andlieinthewhitematter.GlialscaratinjurypediclesOligodendrocytesSmallerthanastrocytes;plumpcellbodywithfairlydensecytoplasmandadarkerroundnucleusandfewer,shorterprocessesthananastrocyte;commoninwhitematter,butsomeareperineuronal.MyelinformingcellofCNSDamagedinmultiplesclerosisLanePhalen,amultiplesclerosissufferer,withherservicedog.ThedoghelpsherwithdailyactivitiesthathavebecomedifficultbecauseofMS.(ReproducedbypermissionofAP/WideWorldPhotos)MicroglialCellsSmallest,nucleusofheterochromatinShortprocesses;phagocyticOriginatefrommonocytesEpendymalCellsLineventriclesofCNSCuboidalshapeSomeareciliated5.NerveFibersandNerves#Smalleraxonsunmyelinated.#LargeraxonsMyelinatedbySchwanncellsoroligodendrocytesThelongprocessofaxonordendriteenvelopedbygliacellTheSchwanncell’splasmalemmawindsandwrapsaroundtheaxon.Thelayersofmembransuniteandformmylin.Themembranepropagatestheactionpotential.insulatornodesofRanvier.internodeMyelinsheatharoundtheaxolemmaisinterruptedatregularintervalstoleavetheaxolemma'bare'atnodesofRanvier.EMrevealsmyelintohavelamellaewithalternatedarkandlightlines,apparentlyconcentricaroundtheaxon.Schmidt-Lantermanclefts:Ineachinternodeofmyelinesheath,Schwanncellcytoplasmpenetrates,separationsofthemyelinlamellae,constitutingtheincisuresofSchmidt-Lantermann,perhapsgivingthemyelinsomeflexibilityandaidingmolecularturnover.NervefibersNervebundlesnervePeripheralNerveEpineuriumPerineuriumEndoneuriumPeripheralNerveEpineurium:severallayersoffibroblastswithcollageninbetweenNervebundleEndoneurium:reticularfibers,fibroblastsandcollagenPerineurium:2-3layersoffibroblast-likecellswithsomecollagenFreenerveendingsMeissner'scorpuscles:tactilemechanoreceptorsfoundindermalpapilla.Paciniancorpuscles:mechanoreceptorscommoninthedermisofthickskin.

Neuromuscularspindle6.NerveEndingsEfferentnerveendings(motor)Afferentnerveendings(sensory)MotorunitUnencapsulatedendingsEncapsulatedendingsFreenerveendings:sensoryendingsindeeplayersofepidermisandpapillarylayerofdermis;responsivetopain,temperature,movement(associatedwithhairfollicles).Tactile(Meissner)corpuscle:ovoid,astackofflattenedcells(modifiedSchwanncells)intertwinedwithnerveendingsandsurroundedbyaCTcapsule;papillarylayeroftheskin(e.g.,fingertips);responsivetotouch;Lamellar(Pacinian)corpuscle:big,roundoroval,thenerveendingandconcentriclayersofflatcells;hypodermis,mucousmembranes;responsivetopressure;Musclespindle:adoubletaperedobject,lyingbetweenandamongmyofibers,isanimportantproprioceptor,thedeviceforbodyposition,tactileinput,jointmovement,etc.,todeterminehowmuchtensionisbeinggeneratedbythemyofibers;tothenreportthatbacktotheCNS.NeuromuscularjunctionMotorendplatesorneuromuscularjunctionsarespecializedstructuresattheendsofmotoraxonsandarethesitesofinnervationofskeletalmusclefibers.

Motorunit:Asinglemotornerveandallthemusclefibersitinnervates.Thesarcolemmaisdepressedintoatroughandinvaginatesintomanysmallsecondarysynapticclefts.IthasAChreceptors.Theswollenendingoftheaxonliesinthetrough.(c)TheaxonterminalhasMt,andvesicles(d)Schwanncellscovertheaxonanditsterminalbag.Foralongtimeitwasbelievedthatonlytheyoungbraincanundergosuchstructuralchanges.However,recentfindingshaveshownthattheadultbraincanalsogrow,reorganizeandformnewconnectionbetweenneurons.Thus,theresearchonneuroplasticitypointstothebrainasagrowingorganthatrespondsstructurallynotonlytothedemandsoftheexternalenvironment,butalsotointernallygeneratedstates,includingaspectsofconsciousness.Neuronalplasticity

theabilityofneuronsinthebraintochangeinresponsetoexperience,hasbeenoneofthemostexcitingdiscoveriesinneuroscience.Neuralstemcell(NSC)StepOne-NeuralStemCellsNeuralstemcells(NSC)arecellsthatcanmatureintoanytypeofcellandcancontinuallyrenewthemselves.StepTwo-ProgenitorCells

Progenitorcellsdonotthemselvesperformactiveneurologicalfunctionsbutare“reserve〞cellsthatbecomeactivatedwhenmaturenervetissueisdamaged.Theprogenitorcellscandevelopintomatureactivecellsinordertorepairorreplacethedamagedtissue.Progenitorcellsinclude:GRP:Glial-RestrictedPrecursors

NRP:NeuronRestrictedPrecursors

O-2A/OPC:Oligodendrocyte-Type-2AstrocytesStepThree-MatureNerveCellsAstrocytesNeuronsaretheprimarynervecellsinthenervoussystemthatprocessandtransmitinformationviaelectricalsignals.OligodendrocytesAlz