Nerve tissue

Introduction

Nervoustissueisoneofthebasictissuesofhumansandhigheranimals,andisthemaincomponentofthenervoussystem.Nervetissueiscomposedofneurons(thatis,nervecells)andglial.Neuronisthemaincomponentofnervetissue,hasthefunctionofreceivingstimulationandconductingexcitement,andisalsothebasicfunctionalunitofnerveactivity.Gliaplaysasupporting,protectiveandnourishingroleinnervetissue.Humannervetissueismainlycomposedofnervecells.Nervecellsarealsocalledneurons,andincludetwoparts:cellbodyandprotrusion.Generally,eachneuronhasalongaxonwithfewbranchesandafewshortdendriteswithtree-likebranches.Theprocessesofneuronsarealsocallednervefibers.Thetinybranchesattheendsofnervefibersarecallednerveendings,whicharedistributedtotheinnervatedtissues.Neuronscanproduceexcitementafterbeingstimulated,andcanconductexcitementalongnervefibers.

Nerveregeneration

Thebodyofnervecellsisthemetabolismandnutritioncenterofneurons.Whenneuronprocessesorbrainbodiesareinjuredoraxonsaredisconnected,iftheinjuredsiteisfarfromthecellbody,thecellbodymayundergoretrogradechanges,thecellbodyswells,thenucleusisdeviated,Nisslbodydissolves,andthenucleusdisappearsinseverecases.Iftheinjuryismild,thecellbodybeginstorecoverafter3weeks.Theaxonandmyelinsheathatthedistalendoftheinjurednervefibercangraduallyappeardisintegrationandlipiddropletswithin12to24hours.Thisprocessiscalledevolutionaryresponse.

Attheproximalendoftheinjurysite,theremainingSchwanncellsdivideandproliferate,formingacellcordtowardYuanrui.Theinjuredproximalaxongrowsinabuddingmanner.ExtendintothenewlybornSchwanncellcord.UndertheinductionofSchwanncells,theaxongrowsalongthecorduntilitreachestheoriginalaxonterminalposition.Thenewlybornaxonterminalcanbranchandestablishcontactwiththecorrespondingcelltissueandrestore.Inordertofunction,thisprocessiscallednerveregeneration.Generally,nerveaxonshavetheabilitytoregenerateandcanrestoretheiroriginalfunctions.Thetimerequiredisgenerallyabout3to6months.Goodconnectionwitheachotherwillaffectregeneration.Schwanncellscaninduce,nourishandpromotethegrowthandmaturationofaxonsduringtheregenerationandrepairofperipheralnerves.Althoughthecentralnervefibersalsohavetheabilitytoregenerate,theglialcellsintheinjuredpartproliferatefasterandformglialtraces,whichblockthenervedockingandaffecttheregeneration.

Severedamagetotheneuroncellbodyornearthecellbodycanleadtothedisintegrationanddeathofnervecells,anditisgenerallydifficulttorepairandregenerate.Mitosisofnervecellscanbeseenaroundtheinjuredsite,indicatingthatnervecellsstillhaveacertainabilitytodivideundercertainconditionsafterinjury,buttheconditionsforregenerationandtherecoveryoffunctionsarestillaffectedbymanyfactors.Studieshaveshownthatneurotrophicfactors(neurotrophicfactors))Isasolublechemicalsubstancethatcansupportthesurvivalofneuronsandpromotethegrowthofneurites.Suchsubstancesplayanimportantroleinthedevelopmentofthenervoussystemandnerveregeneration.SuchasnervegrowthfactorNGF(nervegrowthfactor),fibroblastgrowthfactorFGF(fibroblastgrowthfactor),epidermalgrowthfactorEGF(epidermalgrowthfactor)andsoon.Regardingnerveregenerationisstillanimportantsubjectofcurrentresearch.

Organs

Nervetissueisahighlydifferentiatedtissuethatconstitutesthemaincomponentofthehumannervoussystem.Itiswidelydistributedinvarioustissuesandorgansofthehumanbody,andhasthefunctionofconnecting,regulatingandcontrollingvariousorgans,makingthebodyacoordinatedandunifiedwhole.Nervetissueiscomposedofnervecellsandglialcells.

Nervecell

Nervecell(nervecell)isthemaincomponentofnervetissue.Itisahighlydifferentiatedcell.Ithasalargenumber,diversemorphology,andcomplexstructure.Ithasphysiologicalfunctions.Feelthecharacteristicsofresponsetostimulationandconductionimpulse(analyzeandsynthesize).Itisthestructuralandfunctionalunitofnervetissue,sonervecellsarealsocalledneurons.

Nervetissue

Neurolialcellsareauxiliarycomponentsofnervetissue,andmostcellsalsohaveprotrusions.Thecellbodyofglialcellsisgenerallysmallerthanthatofnervecells;butthenumberisabout10timesthatofnervecells,whichsupportsnervecells.Functionssuchasnutrition,insulation,protectionandrepair.Neuronshavecellbodiesandprotrusions,andprotrusionsaredividedintoaxonsanddendrites.

Structure

1.Soma:Thesomaofaneuronislocatedinthegraymatterandgangliaofthebrainandspinalcord.Itsshapesaredifferent.Thecommonshapesarestar,pyramid,pear,andsphericalshapes.Thecellbodiesvaryinsize,rangingfrom5to150μmindiameter.Thecellbodyisthemetabolismandnutritioncenterofneurons.Thestructureofthecellbodyissimilartothatofanormalcell,withnucleolus,cellmembrane,cytoplasmandnucleus.

(l)Cellmembrane:Thecellmembraneofthecellbodyandthemembraneonthesurfaceoftheprotrusionarecontinuousandcompletecellmembranes.Exceptfortheuniquestructureofthecellmembraneatthesynapse,mostofthecellmembraneisaunitmembranestructure.Thecharacteristicofnervecellmembraneisasensitiveandexcitablemembrane.Therearevariousreceptorsandionicchannelsonthemembrane,eachofwhichiscomposedofdifferentmembraneproteins.Thecellmembranethatformspartofthesynapseisthickened.Receptorsonthemembranecanbindtothecorrespondingchemicalsubstanceneurotransmitter.Whenthereceptoriscombinedwiththeacetylcholinetransmitterortheγ-aminobutyricacidtransmitter,theionpermeabilityofthemembraneandthepotentialdifferencebetweentheinsideandoutsideofthemembranechange,andthecellmembraneproducescorrespondingphysiologicalactivities:excitementorinhibition.

(2)Thenucleus:mostlylocatedinthecenterofthenervecellbody,largeandround,withlessheterochromatin,mostlylocatedontheinnersideofthenuclearmembrane,andmoreeuchromatinscatteredinthemiddleofthenucleus,sothecolorationislightandthenucleusRen1-2,largeandobvious.Whencellsdegenerate,thenucleusmostlymovestotheperipheryanddeviates.

2.Synapse

Aspecializedcellconnectionbetweenneuronsandneurons,orbetweenneuronsandnon-nervecells(musclecells,glandcells,etc.),Calledsynapse(synapse).Itisthekeystructurethatconnectsneuronsandcarriesoutphysiologicalactivities.Synapsescanbedividedintotwocategories,namelychemicalsynapseandelectricalsynapse.Generallyspeaking,synapsesrefertotheformer.

ChemicalSynapse

Underlightmicroscope,mostsynapsesareintheformofaxonsthatarenotsphericalorring-shaped,attachedtothecellbodyordendriticsurfaceofanotherneuron,Itsbulkiscalledsynapticcorpuscleorsynapticbouton.

Accordingtothesynapseformedbetweentwoneurons,therearedifferenttypes.Themostcommonareaxo-somaticsynapseandaxo-dendriticsynapse.Therearealsoaxonalsynapse(axo-spinous),axo-axonalsynapse(axo-axonalsynapse)andtree-dendriticsynapse(dendroden-driticsynapse)andsoon.Usually,aneuronhasmanysynapsesandcanreceiveinformationfrommultipleneurons.Forexample,amotorneuronintheanteriorhornofthespinalcordhasmorethan2000synapses.Thereareapproximately30,000synapsesinthepyramidalcellsofthecerebralcortex.ThePurkinjecellsofthecerebellumcanhaveasmanyas200000synapses.Thesynapsesaremostdenselydistributedinthecellbodiesanddendriticbasesofneurons,andthedendritictipsandaxonstartingsegmentsaretheleast.

Undertheelectronmicroscope,thesynapseiscomposedofthreeparts:thepresynapticpart,thesynapticcleft,andthepostsynapticpart.Thecellmembranescorrespondingtothepresynapticandpostsynapticpartsareslightlythickerthantherest,andarecalledpresynapticmembraneandpostsynapticmembrane,respectively.Thenarrowgapbetweenthetwomembranesiscalledthesynapticcleft.

1.Theneuronaxonterminalofthepresynapticelementissphericallyenlarged,andtheaxonmembraneisthickenedtoformapresynapticmembrane,whichisabout6-7nmthick.Inthecytoplasmofthepresynapticmembrane,therearemanysynapticvesicles,somemicrofilamentsandmicrotubules,mitochondria,andslipperyendoplasmicreticulum.Synapticvesiclesareacharacteristicstructureofthepresynapse.Thevesiclescontainchemicalsubstancescalledneurotransmitter.Theshapeandsizeofsynapticvesiclesinvarioussynapsesarequiteinconsistentbecauseofthedifferentneurotransmitterscontainedinthem.Commontypesofsynapticvesiclesare:①sphericalvesicles,about20-60nmindiameter,clearvesicles,whichcontainexcitatoryneurotransmitters,suchasacetylcholine;②granularvesicles,insidethevesiclesContainsdenseparticleswithhighelectrondensity,whichcanbedividedintotwotypesaccordingtotheirparticlesize:smallparticlevesicleswithadiameterofabout30-60nm,usuallycontainingamineneurotransmitterssuchasepinephrine,norepinephrine,etc.;largeparticlevesiclesThediametercanreach80-200nm,andtheneurotransmitterscontainedinitarepeptidessuchasserotoninorenkephalin;③Flatvesicles,thelongdiameterofthevesiclesisabout50nm,inaflatroundshape,whichcontainsinhibitorypropertiesNeurotransmitters,suchasγ-aminobutyricacid,etc.

Neurogliacell(neurogliacell)isabbreviatedasneuroglia(neuroglia),whichiswidelydistributedinthecentralandperipheralnervoussystem.Ordinarystainingcanonlyshowthenucleus,andthespecialsilverstainingmethodcanshowtheoverallmorphologyofglialcells.Glialcellsaregenerallysmallerthannervecells,withmanyirregularprocesses,andthenumberisabouttentimesthatofnervecells.Theyaremostlydistributedaroundneuroncellbodies,protrusionsandcentralnervecapillaries.Theglialcellshavesupport.1.Nutrition,protection,myelinformationandinsulation,andhasmultiplefunctionssuchasdivision,proliferation,regenerationandrepair.

Glialcells

1.Astrocytesarethelargesttypeofglialcells.Thecellbodyisstar-shaped,thenucleusislarge,roundoroval,andthestainingislighter.Thereareintertwinedneuroglialfilamentsinthecytoplasm.Therearemanyradialprotrusionsextendingfromthecellbody.Theendsofsomeprotrusionsswelltoformanendfoot,whichattachestothecapillarybasementmembrane,orextendstothesurfaceofthebrainandspinalcordtoformagliolimitan.Astrocytesaccountforabout20%ofallglialcells.Astrocytescanbedividedintotwotypesaccordingtotheirdistributionandstructure.

(1)Protoplasmieastrocyte:Distributedinthegraymatterofthecentralnervoussystem,locatedaroundthenervecellbodyanditsprotrusions.Protoplasmicastrocyteshaveirregularprotrusions,manybranchesandshortcurves,andtheirsurfaceisnotsmooth.Therearefewglialfilamentsinthecytoplasm.

(2)Fibrousastrocyte:Distributedinwhitematterandlocatedbetweennervefibers.Theprotrusionsareradial,slenderandstraight,withfewbranchesandsmoothsurface.Therearemanyinterwovenfibrilsinthecytoplasm.Itsultrastructureisanintermediatefilament,calledglialfilament,whichcontainsglialfibrillaryacidicprotein(GFAP),whichisstainedbyimmunocytochemicalstainingtechnology.Thistypeofcellcanbedisplayedspecifically.

AstrocytescontainhighconcentrationsofK+andcantakeupcertainneurotransmitters(suchasγ-aminobutyricacid).ItaffectsthefunctionalactivitiesofneuronsbyadjustingtheK+andneurotransmitterconcentrationsintheintercellularspace.Therefore,astrocytesplayanimportantroleinmaintainingthestabilityoftheneuronalmicroenvironmentandregulatingmetabolicprocesses.Whenthecentralnervoussystemisinjured,astrocytesdivideandproliferaterapidlytoformglialscarsforrepair.

Nervefiber

Nervefiber(nervefiber)istheprocessofnervecells(includingaxonsanddendrites)asthecentralaxis,outsourcingglialcells(SchwanncellsorOligodendrocytes).Accordingtothepresenceorabsenceofmyelinsheath,nervefiberscanbedividedintotwotypes:myelinatedandunmyelinated.

Nervefiber

Myelinatednervefiber

Myelinatednervefiberconsistsofaxons(ordendrites),myelin,andnervemembraneconstitute.Themyelinsheathandneurolemmaaresheathedaroundtheaxons.Thereisnomyelinsheathatthebeginningoftheaxon,whichiscalledthepartoftheaxondistaltotheinitialsegmentoftheinitialsegment.Theaxonalmembraneisexposed,andmembranepotentialchangescanoccur.Thispartiscalledthenodeofnervefiber,alsoknownastheRanviernode.Thesectionbetweentwoadjacentnodes,calledtheinternode,isabout0.5to1mmlong.ItiscomposedofamyelinsheathformedbyaSchwanncellandthesurroundingnervemembrane.Schwanncellnucleusisoblonginshape,locatedinasmallamountofcytoplasmattheedgeofmyelinsheath.Myelinsheathismainlycomposedoflipidsandproteins,calledmyelin(myelin).Onconventionalstainedspecimens,becausethelipidsinthemyelinsheatharedissolved,onlytheremainingproteinisseenasanetwork,calledtheneuralanglenetwork(neurokeratinnetwork).Onosmiumacid-dippedspecimens,themyelinsheathwasblack,andseveralobliquefissuresintheshapeofafunnelwerealsoseen,calledincisureofmyelinorSchmidt-Lantermanincisure.Undertheelectronmicroscope,themyelinsheathisarrangedinlightanddarkconcentriccircularslabs.Myelinsheathhasaprotectiveandinsulatingeffect,whichcanpreventthespreadofnerveimpulses.ThenerveimpulseconductionofmyelinatednervefibersjumpsfromoneLangfeiknottotheadjacentLangfeiknot.Longnervefibershavethickaxonsandthickmyelinsheath.Theinternodalbodyisalsolongandtheconductionspeedisfast.Onthecontrary,theconductionspeedisslow.Mostofthebrainandspinalnervesaremyelinatednervefibers.Themyelinsheathofmyelinatednervefibersinthecentralnervoussystemisformedbyoligodendrocytes.Severalprotrusionsofanoligodendrocytecanrespectivelyenvelopseveralaxonstoformmyelinsheath.ItsLangfeiknotiswider,withoutmyelinsheathnotch,anditscellbodyislocatedbetweennervefibers.

UnmyelinatedNerveFiber

NonmyelinatedNerveFiber(nonmyelinatednervefiber)iscomposedofthinaxonsandSchwanncells,withoutmyelinsheathandno-flyknots.Undertheelectronmicroscope,aSchwanncellwith5to15axonsofdifferentthicknessescanbeseen.Thenerveimpulseconductionofunmyelinatednervefibersiscontinuousconductionalongaxons,anditsconductionspeedismuchslowerthanthatofmyelinatednervefibers.Thepostganglionicfibersandsomesensorynervefibersoftheautonomicnerveareunmyelinatednervefibers.