IntroductiontoNerveCells
Althoughneuronshavevariousshapesandfunctions,theycanberoughlydividedintotwoparts:somaandneuriteinstructure.Therearetwotypesofprotrusions:dendriteandaxon.Axonsareoftenverylong,separatedbytheaxonhillock(axonhillock)ofthecell,andtheirdiameterisuniform.Thefirstsectioniscalledtheinitialsection.Afteracertaindistancefromthecellbody,itbeginstoobtainmyelinsheathandbecomesanervefiber.ItiscustomarytodividenervefibersintoTherearetwotypesofmyelinatedfibersandunmyelinatedfibers.Infact,theso-calledunmyelinatedfibersalsohaveathinlayerofmyelin,whichisnotcompletelyunmyelinated.
Neuron
Neuronisahighlydifferentiatedcell,oneofthebasicstructureandfunctionalunitsofthenervoussystem,ithasthefunctionofsensingstimulationandconductingexcitement.
Neuronsarethestructuralandfunctionalunitsofthenervoussystemofhigheranimals.Thenervoussystemcontainsalargenumberofneurons.Itisestimatedthatthereareabout100billionneuronsinthehumancentralnervoussystem,andthereareabout14billioninthecerebralcortexalone.
Neurondescription:Nervecellsaretriangularorpolygonalandcanbedividedintothreeregions:dendrites,axonsandcellbodies.
Thesizeofthecellbodyisverydifferent,thesmalldiameterisonly5-6μm,andthelargeonecanreachmorethan100μm.Theshape,numberandlengthoftheprotrusionsarealsoverydifferent.Dendritesaremostlytree-likebranches,whichcanreceivestimulationandtransmitimpulsestothecellbody;axonsarefilamentousandoftenhavebranchesattheends,whicharecalledaxonterminals.Axonstransmitimpulsesfromthecellbodytotheend.end.Usuallyaneuronhasoneormoredendrites,butonlyoneaxon.Thelargerthecellbodyofaneuron,thelongeritsaxon.
Neuronsaredividedintothreetypesaccordingtotheirpurpose:inputnerve,efferentnerve,andconjoinednerve.
Functionsofneurons
Functionsofneurons:Thebasicfunctionofneuronsistoexchangeinformationbyreceiving,integrating,conductingandoutputtinginformation
Itisthemaincomponentofthebrain.Theneurongrouprealizestheanalysisfunctionofthebrainthroughtheinformationexchangeofeachneuron,andthenrealizestheexchangeoutputofsamples.TheoutputsampleilluminatesQiuJuetoproduceconsciousnessthroughtheconnectionpath.
Receptionandtransmissionofinformation
Therearephotoreceptorcellsontheretinaoftheeyethatcanreceivelightstimulation,andthereareolfactorycellsonthenasalmucosathatcanreceiveodorsInthetastebuds,therearetastecellsthatcanbestimulatedbychemicalsubstances.Thesecellsarenervecells.Thecellstructureofaneuronincludestwoparts:thecellbodyandtheprotrusion.Theprotrusioncanbedividedintodendritesandaxons.Neuronsarethebasicunitstructureandfunctionalunitofthenervoussystem.Allkindsofinformationaroundusisobtainedandtransmittedthroughtheseneurons.
Thefunctionaldivisionofneurons,whetheritisamotorneuron,asensoryneuronoraninterneuron,canbedividedinto:
1)Theinput(reception)areaisjustamotornerveElementallyspeaking,thereceptoronthecellbodyordendriticmembraneistheinputareaforreceivingincominginformation,andthisareacangeneratepostsynapticpotentials(localpotentials).
2)Integration(triggerimpulse)zoneTheinitialsegmentbelongstotheintegrationzoneortriggerimpulsezone,wherenumerouspostsynapticpotentialsaresummed,andwhenthethresholdpotentialisreached,theactionpotentialisfirstgeneratedhere.
3)ImpulseconductionareaAxonsbelongtotheimpulseconductionarea,andtheactionpotentialistransmittedtothetargetorgansitdominatesinanundecayedmanner.
4)Output(secretion)areaThesynaptosomeattheendoftheaxonistheinformationoutputarea,whereneurotransmittersarereleasedthroughexocytosis.
Nervefibers
Nervefiberscanplaytworolesinthetissuestheyinnervate.Ontheonehand,theyuseexcitatoryimpulseconductiontoreleasespecialpresynapticmembraneswhentheyreachtheterminals.Neurotransmittersthenactonthepostsynapticmembranetochangethefunctionalactivitiesoftheinnervatedtissues.Thiseffectiscalledfunctionaleffects.Ontheotherhand,nervescanoftenreleasecertainsubstancesthroughtheirterminalstocontinuouslyadjusttheinnervatedtissues.Theinternalmetabolicactivityofthehumanbodyaffectsitspersistentstructural,biochemicalandphysiologicalchanges.Thiseffecthasnothingtodowithnerveimpulseandiscallednutritionaleffect.Onlythetrophiceffectsofnervesarediscussedhere.
Theresearchonneurotrophiceffectsismainlycarriedoutonthemotornerve.Experimentshaveshownthataftercuttingthemotornerve,glycogensynthesisinthemuscleslowsdown,proteindecompositionaccelerates,andthemusclegraduallyshrinks;ifthenerveissuturedtoregenerate,themusclechangescanberestored.Atpresent,itisbelievedthatthetrophiceffectiscompletedbytheregularreleaseofcertainnutritivesubstancesfromtheterminals,whichactonthetissuestheydominate.Nutrientsubstancesaresynthesizedbyneuroncellbodies,andaftersynthesis,theyaretransportedtonerveendingsbyaxoplasmicflowforrelease.Axoplasmicflowhasnothingtodowithnerveimpulseconduction,becausecontinuousblockingofnerveimpulseconductionwithlocalanestheticswillnotstoptheaxoplasmicflow,andthemusclesitinnervateswillnotundergometabolicchangesandatrophy.Theaxoplasmisalwaysflowing,andtheflowisbidirectional:ontheonehand,partoftheaxoplasmflowsfromthecellbodytotheaxonterminal,ontheotherhand,partoftheaxoplasmflowsfromtheterminaltothecellbodyintheoppositedirection.
Typesofneuronalsynapses
Chemicalsynapses
Thenervoussystemiscomposedofalargenumberofneurons.Thereisnoprotoplasmicconnectionbetweentheseneuronsinstructure.Theyonlycontacteachother.Thecontactpartiscalledsynapse.Duetothedifferentcontactpositions,synapsescanbemainlydividedintocategories:(1)axon-cellbodytypesynapse;(2)axon-dendritictypesynapse;(3)axon-effectortypesynapse(4))Synapse-synapticsynapse.Theaxonterminalofaneuronbranchesrepeatedly,andtheendexpandsintoacuporball,calledasynaptosome,whichisincontactwiththecellbodyorprotrusionofthepost-synapticneuron.Onepresynapticneuroncanformsynapseswithmanypostsynapticneurons,andonepostsynapticneuroncanalsoformsynapseswiththeaxonterminalsofmanypresynapticneurons.Thecellbodyanddendritesofaspinalanteriorhornmotorneuronarecoveredwithabout1,800synaptosomes.
Thesynapseisdividedintothreeparts:thepresynapticpart,thesynapticcleft,andthepostsynapticcomponent.Observedunderanelectronmicroscope,therearetwolayersofmembranesatthesynapse,calledthepresynapticmembraneandthepostsynapticmembrane,withasynapticgapbetweenthetwomembranes.Thethicknessofthefrontfilmandthebackfilmisgenerallyonlyabout7nm,andthegapisabout20nm.Theaxoplasmneartheanteriormembranecontainsmitochondriaandsynapticvesicles,thediameterofthevesiclesis30-60nm,whichcontainschemicaltransmitters.Thereisavesiclefenceformedbydenseprotrusionsandgridsontheinnersideoftheanteriormembrane.Thegapcontainsasynapticvesicle,whichmayguidethesynapticvesicletocontacttheanteriormembraneandpromotethesynapticvesicle.Thereleaseoftransmitters.Whentheimpulsefromthepresynapticneuronreachesthesynaptosome,thetransmitterinthevesicleisreleasedfromtheanteriormembrane,entersthesynapticcleft,andactsonthereceptoronthepostsynapticmembrane.Ifthiseffectislargeenough,itcancauseexcitationorinhibitionofpostsynapticneurons.
Atpresent,ithasbeenobservedthatthereisanotherwayofsynaptictransmissionofmonoaminetransmittersinneurons.Thistypeofneuronhasmanybranchesattheendoftheaxon,andtherearealargenumberofnodularvaricosebodiesonthebranches.Varicosecontainsalargenumberofvesiclesinthebody(Figure11-3),whichiswherethetransmitterisreleased.However,varicosebodiesarenotindirectcontactwithpostsynapticneuronsoreffectorcells,butareintheirvicinity.Whenthenerveimpulsereachesthevaricosebody,thetransmitterisreleasedfromthevaricosebodyanddiffusestothereceptorsofthepost-synapticcellmembranetoproduceatransmissioneffect.Thismodeoftransmissionexistsbothinthecentralnervoussystemandonthesympatheticpostganglionicfibers.
Electricalsynapses
Theinformationconnectionbetweenneuronsinhigheranimalscanalsobecompletedthroughgapjunctions.Forexample,stellatecellsinthecerebralcortexandbasketcellsinthecerebellarcortexhavegapjunctions.Thelocalcurrentcanbeconnectedthroughthegap.Whenonesideofthemembraneisdepolarized,theothersideofthemembranecanalsobedepolarizedduetotheeffectofelectrotonicity.Therefore,gapjunctionsarealsocalledelectricalsynapses.
Neuronregeneration
Thedegreeofneurondifferentiationishigh,sooncetheneuronisinjured,itisveryslowtorepair.Iftheinjuryisserious,itmaycauseirreparabledamageandrepairTheeffectofneuronaldrugsisnotverysatisfactory.Therefore,oncethereisdamage,theconsequencesareveryserious.
Neuronexperiment
Neurons:Section40,Section4,etc.
Underlowmagnification,somelargeblue-stainedcellswithprotrusionscanbeseen-Motornervecellsintheventralhornofthespinalcord.Thiskindofnervecellhasmanyprotrusions,butduetotheslicerelationship,onlyafewofthemcanbeseen.Inthecytoplasm,thereisadarkblue-purpleblockorgranularsubstancecalledNisslbody,whichisaroughendoplasmicreticulumundertheelectronmicroscope.Thenucleusislightlycolored,mostlylocatedinthecenterofthecell,containingasmallamountofchromatin,thenuclearmembraneisobvious,andthereisalarge,roundnucleolus.
Drawingunderhighmagnification:showsthestructureofnervecells.
Micrographsofnervecells(3photos)
Notes:cellbody,cellprocess,nucleus,Nisslbody,nucleolus.Thebasicstructureofaneuron
Thebasicstructureofaneuron:Itcanbedividedintotwoparts:thecellbodyandtheprotrusion.Thecellbodyincludesthecellmembrane,cytoplasmandnucleus;theprotrusionsareemittedfromthecellbodyandaredividedintodendritesandaxons.Therearemanydendrites,thickandshort,branchingrepeatedly,andgraduallybecomingthinner;axonsaregenerallyonlyone,slenderanduniform,withfewerbranchesinthemiddle,andmanybranchesattheend.Thedistalpartofeachbranchisenlargedandspherical,calledaprocess.Touchthesmallbody.Atthesitewheretheaxonemerges,thecellbodyoftenhasacone-shapedbulgecalledtheaxonhill.Aftertheaxonisemittedfromtheaxon,thefirstsegmentisnotcoveredbymyelinsheath,whichiscalledtheinitialsegment.Sincethevoltage-gatedsodiumchanneldensityoftheinitialcellmembraneisthehighest,thethresholdforgeneratingactionpotentialsisthelowest,thatis,theexcitabilityisthehighest,soactionpotentialsareoftengeneratedfirst.Axonsgetmyelinsheathafterleavingthecellbodyforacertaindistanceandbecomenervefibers.
Glialcells
Therearealsomanyneurogliacellsinthenervoussystem(severaldozensoftimesmorethanneurons),suchastheastrocytesinthecentralnervoussystem.Glialcells,oligodendrocytes,microgliaandSchwanncellsintheperipheralnervoussystem,etc.DuetothelackofNa+channels,variousglialcellscannotgenerateactionpotentials.
Glialcells
Themainfunctionsofglialcellsare:
①Supportingrole
StarTheprotuberancesoftheoligodendrocytesareintertwinedintoanetwork,supportingthecellbodiesandfibersoftheneurons;
②Insulationeffect
Tenthousandcellsrespectivelyconstitutethemyelinsheathofthecentralandperipheralnervefibers,sothattheactivitiesbetweenthenervefibersbasicallydonotinterferewitheachother;
③Barrierfunction
Someoftheprotrudingendsofastrocytesswellandterminateonthecapillarysurface(perivascularfoot),covering85%ofthecapillarysurfacearea,whichisanimportantpartoftheblood-brainbarrier;
④Nutritionaleffect
Astrocytescanproduceneurotrophicfactors(NTFs)tomaintainthegrowth,developmentandsurvivalofneurons;
⑤Repairandregenerationeffects
Microgliacanbetransformedintomacrophages,andtheneuronsandtheircelldebristhathavebeendegeneratedduetoaginganddiseaseareeliminatedthroughphagocytosis;Glialcellsproliferateandmultiplytofillthedefectsleftbythedeathofneurons,butiftheyproliferateexcessively,theycanbecomethecauseofbraintumors;
⑥MaintainK+balancearoundneurons
Whenneuronsareexcited,theycauseK+toefflux,andastrocytespumpK+intothecellthroughtheNa+-K+pumponthecellmembrane,andtransferK+intothecellthroughtheintercellularchannel(gapjunction).K+quicklydispersesintootherglialcells,sothatK+aroundneuronswillnotincreaseexcessivelyandinterferewithneuronalactivity;
⑦Uptakeneurotransmitter
Theglialcellsofmammaliandorsalrootganglia,spinalcordandautonomicgangliacantakeupneurotransmitters,soitisrelatedtothemaintenanceofneurotransmitterconcentrationandsynaptictransmission.
Classificationofneurons
Thereareseveralclassificationsofneurons.
Accordingtothenumberofprotrusions
Accordingtothenumberofprotrusions,neuronscanbedividedintothreetypes:
①Multipolarneurons(Multipolarneuron),hasoneaxonandmultipledendrites;
②Bipolarneuron,hastwoprotrusions,oneisdendritesandtheotherisaxons;
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③Pseudounipolarneuron(pseudounipolarneuron),whichemitsaprotrusionfromthecellbody,isnotfarfromthecellbodyandisdividedintotwobranchesina"T"shape.Oneisanorgandistributedtootherperipheraltissues,calledthesurrounding突(peripheralprocess);Theotherbranchentersthecentralnervoussystemandiscalledthecentralprocess.Thesetwobranchesofpseudo-monopolarneurons,accordingtothedirectionofnerveimpulseconduction,thecentralprocessisaxons,andthesurroundingprocessesaredendrites;butthesurroundingprocessesarethinandlong,similarinshapetoaxons,sotheyareoftenreferredtoasaxons.
Accordingtothelengthoftheaxon
Accordingtothelengthoftheaxon,neuronscanbedividedinto:①Largeneuronswithlongaxons,calledGolgitypeINeurons,thelongestaxonsaremorethan1m;
②Smallneuronswithshortaxons,calledGolgitypeIIneurons,areonlyafewmicronsshort.
Accordingtothefunctionoftheneuron
Accordingtothefunctionoftheneuron,itcanbedividedinto:
①Sensoryneuron(sensoryneuron),Orafferentneuron(Afferentneuron)aremostlypseudo-monopolarneurons.Thecellbodyismainlylocatedinthecerebrospinalganglia,andtheperipheralprocessterminalsaredistributedintheskinandmuscles.Theyreceivestimulationandtransmitthestimulationtothecenter.
②Motorneuron,orefferentneuron,ismostlymultipolarneurons.Thecellbodyismainlylocatedinthebrain,spinalcordandautonomicganglia.Ittransmitsnerveimpulses.Giveeffecttomusclesorglands.
③Interneuron(interneuron)isbetweenthefirsttwotypesofneurons,andmostofthemaremultipolarneurons.Themoreanimalsevolve,themoreinterneurons.Interneuronsinthehumannervoussystemaccountforabout99%ofthetotalnumberofneurons,formingacomplexnetworkinthecentralnervoussystem.
Accordingtotheneurotransmitterreleasedbytheneuron
Accordingtotheneurotransmitterreleasedbytheneuron(neurotransmitter),orneuromodulator,alsoCanbedividedinto:
①cholinergicneuron(cholinergicneuron);
②aminergicneuron(aminergicneuron);
③peptidergicneuron(Peptidergicneuron);
④Aminoacidneuron.