Morphologicalfeatures
Observedundertheelectronmicroscope,therearetwolayersofmembranesatthesynapse,calledthepresynapticmembraneandthepostsynapticmembrane,withasynapticcleftbetweenthetwomembranes.Therefore,asynapseiscomposedofthreeparts:thepresynapticmembrane,thesynapticcleft,andthepostsynapticmembrane.Thethicknessofthefrontfilmandthebackfilmisgenerallyonlyabout7nm,andthegapisabout20nm.Theaxoplasmneartheanteriormembranecontainsmitochondriaandsynapticvesicles,thediameterofthevesiclesis30-60nm,whichcontainschemicaltransmitters.
Transmissionprocess
Ontheinnersideoftheanteriormembrane,thereisavesiclefenceformedbydenseprotrusionsandgrids.Thegapcontainsasynapticvesicle,whichmayhaveaguidingsynapse.Thecontactofthecontactvesicleswiththeanteriormembranepromotesthereleaseoftransmittersinthesynapticvesicles.Whentheimpulsefromthepresynapticneuronreachesthesynaptosome,thetransmitterinthevesicleisreleasedfromtheanteriormembrane,entersthesynapticcleft,andactsonthepostsynapticmembrane;ifthiseffectislargeenough,Itcancauseexcitatoryorinhibitoryresponsesofpostsynapticneurons.
Thereisanotherwayofsynaptictransmissionofmonoaminetransmittersinneurons.Thistypeofneuronhasmanybranchesattheendoftheaxon,andtherearealargenumberofnodularvaricosebodiesonthebranches.Varicosecontainsalargenumberofvesiclesinthebody,whichiswherethetransmitterisreleased.However,varicosebodiesarenotindirectcontactwithpostsynapticneuronsoreffectorcells,butareintheirvicinity.Whenthenerveimpulsereachesthevaricosebody,thetransmitterisreleasedfromthevaricosebodyanddiffusestothereceptorsofthepost-synapticcellmembranetoproduceatransmissioneffect.Thismodeoftransmissionexistsbothinthecentralnervoussystemandonthesympatheticpostganglionicfibers.
Transmissioncharacteristics
1.One-waytransmission
SynaptictransmissioncanonlybetransmittedbypresynapticneuronsalongaxonsTopost-synapticneurons,irreversibletransmission.Becauseonlythepresynapticmembranecanreleasethetransmitter.Therefore,theexcitementcanonlybetransmittedfromtheafferentneuron,throughtheinterneuron,andthenfromtheefferentneuron,sothattheentirenervoussystemcanproceedregularly.
2.Sumeffect
AnimpulsesentbyapresynapticneuronandtheamountoftransmitterreleasecausedbyitisgenerallynotenoughtomakethepostsynapticMembraneneuronsgenerateactionpotentials.Onlywhenaseriesofimpulsesaretransmittedcontinuouslyfromtheterminalofapresynapticneuron,orarowofimpulsesaretransmittedfromtheterminalofmanypresynapticneuronsatthesametime,andthereleasedchemicaltransmittersaccumulatetoacertainamount,canthepost-synapticneuronbestimulatedtoproduceActionpotential.Thisphenomenoniscalledsummation.Inhibitorypostsynapticpotentialscanalsobesummed.
3.Synapticdelay
Nerveimpulsesaretransmittedfrompresynapticterminalstopostsynapticneurons,whichmustundergo:thereleaseofchemicaltransmitters,DiffusionanditseffectontheposteriormembranecauseEPSP,andonlyafterthesummationcanpost-synapticneuronsgenerateactionpotentials.Thecharacteristicofthistransmissionthattakesalongtimeissynapticdelay.Accordingtomeasurement,thetimeforanimpulsetopassthroughasynapseisabout0.3～0.5ms.
4.Changesinexcitementrhythm
Inareflexactivity,ifAtthesametime,theimpulsefrequenciesofthedorsalrootafferentnerveandtheabdominalrootefferentnervearerecordedseparately,anditcanbefoundthatthefrequenciesofthetwoarenotthesame.Becausetheexcitementoftheefferentnervenotonlydependsontherhythmoftheafferentimpulse,italsodependsonthefunctionalstateoftheefferentneuronitself.Inthepolysynapticreflex,thesituationismorecomplicated.Aftertheimpulseentersthecenterfromtheafferentnerve,itmustbetransmittedbytheinterneuron.Therefore,thefrequencyoftheefferentneuron'sfiringalsodependsonthefunctionalstateandcontactmethodoftheinterneuron.
5.Sensitivitytointernalandexternalenvironmentalchanges
Thesynapsesbetweenneuronsaremostsusceptibletointernalenvironmentalchanges.Hypoxia,riseandfallofpH,changesinionconcentration,etc.canchangethetransmissioncapacityofsynapses.Hypoxiacanmakeneuronsandsynapsesloseexcitability,conductiondisturbancesandevenneuronaldeath.Inalkalosis,theexcitabilityofneuronsisabnormallyincreased,andevenconvulsionsoccur;inacidosis,theexcitabilityisdecreased,andinseverecases,itcausescoma.
6.Sensitivitytocertainchemicalsandeasyfatigue
Manycentraldrugsaremostlylocatedatsynapses.Somedrugscanblockorenhancesynaptictransmission,suchascaffeine,theobromine,andtheophyllinecanincreasethesensitivityofpostsynapticmembranestoexcitatorytransmitters,especiallyforsynapsesinthebrain.Strychninecanreducethesensitivityofthepostsynapticmembranetoinhibitorytransmitters,leadingtoexcessiveneuronalexcitement,andhasaparticularlyobviouseffectonthespinalcord.Itisclinicallyusedasaspinalcordstimulant.Variousreceptoragonistsorblockerscandirectlyactonpostsynapticmembranereceptorstoexertphysiologicaleffects.
Thesynapseisthemostfatigue-pronelinkinthereflexarc.Thecauseoffatigueinsynaptictransmissionmayberelatedtothedepletionofthetransmitter.Theappearanceoffatigueisaprotectiveinhibitiontopreventthecentralnervoussystemfromoverexciting.
Transmissionmechanism
Synaptictransmission:Theprocessofimpulsetransmissionfromoneneurontoanotherneuronthroughasynapseiscalledsynaptictransmission.
1.Thetransmissionmechanismofchemicalsynapses
Whenthenerveimpulseistransmittedtotheaxonterminal,thepresynapticmembraneisexcited,andtheactionpotentialandIontransfer.Atthistime,thepermeabilityofthepresynapticmembranetoCa2+increases.Ca2+flowsintothesynaptosomefromthesynapticcleftalongtheconcentrationgradient,andthenthechemicaltransmittercontainedinthevesicleisreleasedintheformofquantumrelease,reachingthesynapseTouchthegap.Afterthechemicaltransmitterisreleased,itdiffusestothepostsynapticmembranethroughthesynapticcleft,andbindstothespecialreceptorsontheposteriormembrane,changingthepermeabilityoftheposteriormembranetoionsandchangingtheposteriormembranepotential.Thischangeinthepotentialoftheposteriormembraneiscalledpostsynapticpotential.Duetothedifferenceintransmittersandtheireffectsonthepermeabilityofpostsynapticmembranes,therearetwotypesofpostsynapticpotentials,namelyexcitatorypostsynapticpotentialsandinhibitorypostsynapticpotentials.
①Excitatorypostsynapticpotential(EPSP)
Whentheactionpotentialistransmittedtotheaxonterminal,itexcitesthepresynapticmembraneandreleasesexcitatorychemicaltransmitters(excitatorytransmitter),thetransmitterdiffusestothepostsynapticmembranethroughthesynapticcleftandbindstothereceptorsoftheposteriormembranetoincreasethepermeabilityoftheposteriormembranetoNa+,K+,Cl-,especiallytoNa+,andNa+inflows.Localdepolarizationoftheposteriormembraneoccurs.Thislocalpotentialchangeiscalledexcitatorypostsynapticpotential(EPSP).Itcanbespreadintheformofelectrictensionandcanbesummed.Ifmultipleactionpotentialsaretransmittedcontinuouslyfromthesamepresynapticterminal,orarowofactionpotentialsaretransmittedfrommultiplepresynapticterminalsatthesametime,theexcitatorypostsynapticpotentialscanbesuperimposedtoincreasethepotentialamplitude.Atthethresholdpotential,thatis,whenthemembranepotentialisdepolarizedfrom-70mVtoabout-52mV,itwillcausetheinitialsegmentoftheaxonofthepost-synapticneurontofirstbursttheactionpotential,generateaspreadingactionpotential,andconductitalongtheaxon,Spreadtotheentirepost-synapticneuron,showingtheexcitementofthepost-synapticneuron.Thisprocessiscalledexcitatorysynaptictransmission.
②Inhibitorypostsynapticpotential(IPSP)
Wheninhibitoryinterneuronsareexcited,theirterminalsreleaseinhibitorytransmitters.Thetransmitterdiffusestotherearmembraneandbindstothereceptorsontherearmembrane,whichincreasesthepermeabilityoftherearmembranetoK+,Cl-,especiallytoCl-.Increasedpolarizationmeanshyperpolarization.Thishyperpolarizationpotentialiscalledinhibitorypostsynapticpotential(IPSP),andthisprocessiscalledinhibitorysynaptictransmission.
2.Themechanismofelectricalsynaptictransmission
Thetransmissionofelectricalsynapsesisthroughtheactionofelectricity.Thatis,whentheactionpotentialofthepresynapticneuronreachesthenerveterminal,theactionpotentialofthepostsynapticcomponentiscausedbytheactionofthelocalcurrent.Whentheimpulsedoesnotreachthepresynapticterminal,ithasananodicelectrotoniceffectonthepostsynapticmembrane,whichincreasesthemembranepotentialofthepostsynapticmembraneandreducesitsexcitability.Whentheactionpotentialistransmittedtothepresynapticterminal,thenerveterminalisnegative,justlikeacathode,whichactsasacathodicelectrotension,whichreducesthemembranepotentialofthepostsynapticmembraneandincreasestheexcitability.Whentheexcitabilityisincreasedtoacertainlevel,anerveimpulseisgeneratedandspreadswithalocalelectriccurrent.
Function:Promotethefunctionofdifferentneuronstoproducesynchronizedfiring.
Transmissiondifference
Synaptictransmission
Thereisnoprotoplasmicconnectionbetweenneurons.Theconnectionbetweenthemonlydependsoncontactwitheachother,thatis,throughaneuronTheaxonterminalmakescontactwithotherneuronsandtransmitsexcitementorinhibition.Thesecontactsitesarecalledsynapses.
1.Classificationofsynapses
①Classificationofsynapticcontactsites
Canbedividedinto:(1)Axo-dendriticsynapses(axo-dendriticsynapse)”referstothecontactbetweentheaxonendofaneuronandthedendritesofthenextneuron.(2)Axo-somaticsynapse(axo-somaticsynapse)referstothecontactbetweentheaxonterminalofaneuronandthecellbodyofthenextneuron.(3)Axo-axonicsynapse(axo-axonicsynapse)"referstothecontactbetweentheaxonendofoneneuronandtheaxon(startingsegmentofaxon)oraxonendofthenextneuron.
Inaddition,inthecentralnervoussystem,therearetree-tree,body-body,body-treeandtree-bodyandotherformsofsynapticconnections.Inrecentyears,ithasalsobeendiscoveredthatanaxis-treeortree-treetypeautoapse(autoapse)canbeformedbetweentheprotrusionsofthesameneuron.②Classificationaccordingtothewaysynapsestransmitinformation
Canbedividedintochemicalsynapsesandelectricalsynapses.Chemicalsynapsesrelyonpre-synapticneuronterminalstoreleasespecialchemicalsubstancesasmediafortransmittinginformationtoaffectpost-synapticneurons.Electricalsynapsesrelyonthebioelectricityandionexchangeofpre-synapticneuronstodirectlytransmitinformationtoaffectpost-synapticneurons.
③Accordingtothefunctionalclassificationofsynapses
Canbedividedintoexcitatorysynapse(excitatorysynapse)andinhibitorysynapse(inhibitorysynapse).Theinformationtransmissionofthesynapsedepolarizesthepostsynapticmembraneandgeneratesexcitatorypostsynapticpotentialscalledexcitatorysynapses.Theinformationtransmissionofthesynapsecausesthepostsynapticmembranetohyperpolarizeandgeneratesinhibitorypostsynapticpotentialscalledinhibitorysynapses.
2.Basicstructureofsynapse
①Chemicalsynapse
Theaxonterminalofaneuronfirstdividesintomanysmallbranches,Theendofeachsmallbranchexpandsintoasphericalshape,calledsynaptosome.Thecorpuscleformsasynapticconnectionwiththecellbodyordendritesofanothernerve.Undertheelectronmicroscope,itwasobservedthatthecellmembranesofthetwoneuronsatthesynapsedidnotfuse.Thereisagapbetweenthetwoneurons,whichisabout200-500wide,calledthesynapticcleft.Themembranethatformsthesynapticcleftbythesynaptosomesiscalledthepresynapticmembrane,andtheothermembranethatformsthesynapticcleftiscalledthepostsynapticmembrane.Therefore,asynapseiscomposedofthreeparts:thepresynapticmembrane,thesynapticcleftandthepostsynapticmembrane.Synapticvesiclescontainmoremitochondriaandalargenumberofvesicles,whicharecalledsynapticvesicles.Thevesiclescontainexcitatoryorinhibitorytransmitters.Mitochondriacontainenzymesthatsynthesizetransmitters.Therearespecialreceptorsonthepostsynapticmembrane,whichcanspecificallybindtospecifictransmitters.
②Electricalsynapse
Thestructuralbasisofelectricalsynapseisgapjunction,whichistheplacewheretwoneuronmembranesareinclosecontact,andthegapbetweenthetwomembranesOnly20-30.Therearenosynapticvesiclesorneurotransmittersintheaxonendsofpresynapticneurons.Therearechannelsthatcommunicatethecytoplasmofthetwocellsattheconnectionsite.Chargedionscantransmitelectricalsignalsthroughthesechannels.Thissignaltransmissionisgenerallybidirectional.Therefore,theinformationtransmissionconnectedtothecollectionandsortingpartsofthemedicaleducationnetworkisakindofelectricaltransmission,whichiscompletelydifferentfromthechemicaltransmittertransmissionoftheclassicalsynapse.Thefunctionofelectricalsynapsesmaybetopromotesynchronousdischargeofdifferentneurons.Thespeedofelectricaltransmissionisfast,andthereisalmostnoincubationperiod.Electricalsynapsescanexistbetweendendritesanddendrites,cellbodiesandcellbodies,axonsandcellbodies,axonsanddendrites.
Non-synaptictransmission
Inadditiontosynapticchemicaltransmission,thereisalsonon-synapticchemicaltransmission.Theresearchinthisareawasfirstcarriedoutonthesympatheticganglionadrenergicneurons.Experimentshaveobservedthattheaxonendsofadrenergicneuronshavemanybranches,andtherearealargenumberofnodularvaricosebodiesonthebranches.Thevaricosebodycontainsalargenumberofsmallvesicles,whicharethesiteswherethetransmitterisreleased.However,thevaricosebodydoesnotformasynapticconnectionwiththeeffectorcells,butislocatedneartheeffector.Whenthenerveimpulsereachesthevaricosebody,thetransmitterisreleasedfromthevaricosebody,andreachesthereceptorscollectedandsortedbytheeffectorcellmedicaleducationnetworkthroughdiffusion,sothattheeffectorcellreacts.Sincethischemicaltransmissionisnotcarriedoutthroughsynapses,itiscallednon-synapticchemicaltransmission.Inthecentralnervoussystem,thereareknowntransmissionmethods.Forexample,thereareunmyelinatedfiberswithveryfinediametersinthecerebralcortex,whicharenoradrenergicfibers.Therearemanyvaricosebodiesonthefiberbranchesthatcanbereleased.Norepinephrine,mostofthisvaricosebodydoesnotformsynapseswithinnervatedneurons,sothetransmissionisanon-synapticchemicaltransmissionmethod.Inaddition,serotonergicfibersinthecentercanalsocarryoutnon-synapticchemicaltransmission.
Comparedwithsynapticchemicaltransmission,non-synapticchemicaltransmissionhasthefollowingcharacteristics:①Thereisnospecialstructureofpresynapticmembraneandpostsynapticmembrane.②Thereisnoone-to-onedominancerelationship,thatis,onevaricosebodycandominatemoreeffectorcells.③Thedistancebetweenthevaricosebodyandtheeffectorcellisatleast200?Above,thedistancecanbeuptoseveralμm.④Thediffusiondistanceofthetransmitterislarge,sothetimeittakestotransmitcanbelongerthan1s.⑤Whenthetransmitterdiffusestotheeffectorcell,whetherthetransmissioneffectcanoccurdependsonwhetherthereisacorrespondingreceptorontheeffectorcellmembrane.
Features:Thereisnotypicalsynapticstructure;thedistancebetweenthevaricosebodyandthetargetcellisrelativelylong;thereisnoone-to-onedominancerelationship.