Introduction
Theconductionspeedoftheactionpotentialvarieswiththetypeofanimal,thetypeofnervefiber,thethicknessandtemperatureandotherfactors,generallyabout0.5to200meterspersecond.Undernormalcircumstances,nerveimpulsesaregenerallytransmittedforward,thatis,fromthecellbodytothedistalendoftheaxon.Ifartificialstimulationisused,theimpulsecanbetransmittedinthereversedirection.Thespeedofforwardandreverseconductionisthesame.Ifelectricalstimulationisusedtosimultaneouslycausetwonerveimpulsesconductedinoppositedirections,thecollisionwilldisappearwhentheymeet.Actionpotentialsareanimportantwayforthenervoussystemtotransmitvariousinformation;nerveimpulsessentbyreceptors(suchaseyes,ears,etc.)transmitinformationaboutchangesintheorganism'sinternalandexternalenvironmenttothecentralnervoussystem(brainandspinalcord),alongtheincoming(orSensory)nervefiberconduction;nerveimpulsesfromthecentralnervoussystemtransmit"commands"totheeffectororgans(suchasmuscles,glands,etc.),whichareconductedalongefferentormotornervefibers.
Inthenaturalstate,nerveimpulsesareconductedwithinasingleneuron,whetherintheperipheryorinthecentralnervoussystem.Atthenerveendings(synapsesorjunctionswithmuscles),nerveimpulsescausethenextcelltoexciteorinhibitthroughchemicalorelectricaltransmission.Placeapairofelectrodesonthenervefiber,andthenerveimpulsecanbedisplayedontheoscilloscopescreenthroughtheamplifier.Thisisashortnegativewave.Ifthemicroelectrodeisinsertedintothenervefiber,thesignalrecordedismuchlarger.Thereisanegativepotentialinthefiberatrest,andwhentheactionpotentialpasses,itbrieflybecomesapositivepotential(seeexcitement).Thehighestfrequencyofnerveimpulsesisrelatedtothelengthoftheabsoluterefractoryperiodofnervefibers.
Conduction
Thelocalcurrenttheoryassumesthatwhenasmallsegmentofanunmyelinatedfibergeneratesanactionpotentialduetoastrongenoughstimulus,themembraneatthatlocationwillchangefromresting.Theinner-negativeandouter-positivechangetemporarilybecomesinner-positiveandouter-negative,butthenervesegmentadjacenttothenerveisstillinthestateofinner-negative-outer-positivepolarizationatrest.Thesolutiononbothsidesofthemembranehasconductivity.Betweentheexcitednervesegmentandtheunexcitedsegmentadjacenttoit,therewillbechargemovementduetotheexistenceofthepotentialdifference,whichisthelocalcurrent.Thedirectionofitsflowis:thepositivechargeoutsidethemembraneflowsfromtheunexcitedsectiontotheexcitedsection,andthepositivechargeinsidethecellflowsfromtheexcitedsectiontotheunexcitedsection.Thiscurrentdirectiondepolarizestheunexcitedsectionofthefibermembrane.Whenthiscurrentisstrongenoughtodepolarizethemembranetothethreshold,anewnerveimpulse──actionpotentialwillbegenerated.Inthisway,theactionpotentialreliesonthelocalcurrenttoconductforwardalongthenervefibersectionbysection.Sincethereisalongrefractoryperiodafterthefibermembraneisexcited,theconductionofnerveimpulsesisalwaysone-wayconductionalongtheexcitedsegmentofthenervefibertotheunexcitedsegment.
Whentheactionpotentialisgenerated,theslopeandamplitudeofthepotentialchangeareverylarge,andthesolutionsonbothsidesofthemembranehavegoodconductivity,sothestrengthofthelocalcurrentcanoftenexceedthatoftheadjacentparts.Thethresholdintensityofexcitementisseveraltimes;thatis,onceexcitementoccurs,itsconductioninthesamecellhasagreat"safetyfactor"andisnoteasytobeinterrupted.
Thereisalayerofmyelinsheathoutsidethemyelinatednervefiberthatisalmostnon-conductive.ThemyelinsheathisonlyinterruptedattheLangfeijunction.Theaxonmembraneandtheextracellularfluidareindirectcontact,allowingionstomoveacrossthemembrane.Therefore,whenthemyelinsheathfiberisstimulated,theactionpotentialonlyoccursattheLangfeijunction.Duringnerveimpulseconduction,localcurrentscanonlyflowintooroutofthefiberattheLangfeijunction.Inthefiber,thepositivechargeflowsfromtheexcitedLangfeinodethroughtheinternodestotheadjacentunexcitedLangfeinode,whileoutsidethecell.Intheliquid,thepositivechargeflowsfromtheunexcitedLangfeiknotalongtheinternodefiberstotheexcitedLangfeiknot.ThisdirectionofcurrentdepolarizestheunexcitedLangfei'sconjunctiva,justlikeunmyelinatedfibers.Whenthiscurrentislargeenough,itcausestheunexcitedLangfei'sjunctiontoproduceactionpotentials.SincenerveimpulsesareonlygeneratedonadjacentLangfeiknots,theconductionofnerveimpulseswithmyelinfibersisskipped,calledskipconduction.Underothersimilarconditions,theconductionvelocityofmyelinfibersisobviouslyItisfasterthanunmyelinatedfibers.Theconductionspeedofmyelinatednervefibersinfrogswithathicknessofafewmicronsisequivalenttotheconductionspeedofunmyelinatedfibersinasquidwithadiameterofnearly1mm.Theappearanceofnervemyelinacceleratesnerveconductionspeedandsavesenergy.Itisanadaptationfororganismstoprocessgreatlyincreasedamountofinformationwiththesamevolumeandmaterial.
Transmission
Theconductionprocessofnerveimpulsesisanelectrochemicalprocess,whichisanelectrochemicalchangethatoccurssequentiallyonnervefibers.Whenthenerveisstimulated,thepermeabilityofthecellmembranechangesdrastically.
Theelectricalconductionofnerveimpulses
Experimentswithisotope-labeledionsprovedthatwhennervefibersarestimulated(suchaselectricalstimulation),Na
Weknowthattherearechannelsforsubstancesmadeofhydrophilicproteinmoleculestoenterandexitthecellonthecellmembrane.Somepipesareopenedfrequently.However,manypipelinesareoftenclosedandonlyopenedwhentheyreceiveacertainstimulus.Thiskindofpipelinecanbesaidtobeapipelinewithagate.Fornerveconduction,themostimportantionchannelsareNa+,K+,Cl-,Ca2+andotherchannels.Whenthenervefiberisresting,thatis,whenthenervefiberisinapolarizedstate(potentialdifferenceof-70mV),theNa+channelismostlyclosed.TheNa+gradientinsideandoutsidethemembraneismaintainedbytheNa+-K+pump.Whenthenervefiberisstimulated,thestimulatedlocationonthemembranelosesitspolarity,thepermeabilitychanges,someNa+pipesareopened,andalargeamountofNa+outsidethemembraneflowsintothemembranefromtheNa+pipealongtheconcentrationgradient.Thisfurtherdepolarizesthemembrane,opensmoreNa+pipes,andresultsinmoreNa+influx.Thisisapositivefeedbackmultiplicationprocess.ThisprocessbalancestheNa+insideandoutsidethemembrane,andthemembranepotentialchangesfrom-70mVatrestto0,andcontinuestotransitionto+35mV(actionpotential).Inotherwords,theinsideofthemembranethatwasoriginallynegativeistemporarilyconvertedtopositive,andtheoutsideoftheoriginallypositivemembranebecomesnegative.Atthistime,therearemorecationsinthemembrane,andtheNa+pipelineisgraduallyclosed.Sincethepolarityofthemembranehasnotreturnedtotheoriginalrestingpotentialatthistime,theNa+pipelinecannotbereopenedwhenitencountersastimulus,sotheNa+pipelineisinaninactivestateatthistime.Onlywhenthemembranereturnstoitsoriginalrestingpotential,theclosedNa+pipelinecanbereopenedwhenitencountersastimulustoallowNa+toflowinfromtheoutside.ThisshortperiodofinactivationoftheNa+channelisequivalenttotherefractoryperiod(ofnerveconduction).AfterNa+flowsintothenervefiber,therearemorepositiveionsinthemembrane.Atthistime,thedooroftheK+pipeopens,andthepermeabilityofthemembranetoK+increases,soK+flowsoutofthemembranealongtheconcentrationgradient.DuetotheoutflowofK+,theoriginalnegativechargeinsidethemembraneisrestored,andtheoriginalpositivechargeoutsidethemembraneisrestored,sothatrepolarizationofthemembraneoccurs,thatis,themembranerestoresitsoriginalrestingpotential.Thiscycleofpotentialchange,thatis,themembranechangesitspolarityfromthepenetrationofNa+,fromtheoriginalouterpositiveandinnernegativetoouternegativeandinnerpositive,andtheexudationofK+restoresthemembranetoitsoriginalouterpositiveandinnernegative.,Calledtheactionpotential(ActionPotential).
Theso-callednerveconductionisthesequenceofactionpotentialsalongnervefibers.Acertainpointofanervefiberisstimulated.Iftheintensityofthestimulusissufficient,theresponseofthispointtothestimulusisachangeinpolarity:Na+flowsinandK+flowsout.Itwasoriginallyapositivelychargedmembranesurface,butnowithasbecomenegativelycharged.Thiscausesapotentialdifferencebetweenitanditsleftandrightneighbors(positiveelectricity).Asaresult,thepermeabilityoftheleftandrightadjacentmembranesalsochanges,andactionpotentialsaregeneratedinthesamewayastheaboveprocess.Suchachainreactionstepbystepleadstothesequentialpropagationofactionpotentials,whichistheconductionofnerveimpulses.
Theappearanceofactionpotentialsisveryfast,eachactionpotentialisonlyabout1msintime,anditis"allornothing".Inotherwords,whenthestimulationisnotstrongenough,noactionpotentialwilloccur,andtherewillbenonerveimpulse;oncethestimulationreachestheminimumeffectiveintensity,theactionpotentialwilloccurandspreadfromthestimulationpointtobothsides,whichisnerveimpulse;andincreasingthestimulationintensitydoesnotItwillincreasetheintensityandconductionspeedofnerveimpulses.Nerveimpulsesareconductedbidirectionallyonnervefibers,butbecauseinanimals,theplacewherenervesreceivestimulationistheendofthenerve,thenerveimpulsecanonlytravelinonedirection;and,moreimportantly,wherethenervefibersconnectwitheachother(Synapse),nerveimpulsesareconductedinonedirection,impulsesfromtheoppositedirectioncannotpassthrough,sonerveimpulsescanonlymoveinonedirection.
Aftertheactionpotentialoccurs,thenervefibercannotimmediatelygenerateanewactionpotential,thatis,afterthenerveimpulseistransmitted,thenervehasashortrefractoryperiod.Duringtherefractoryperiod,theNa+pipelineisclosedandtheactionpotentialcannotoccur.Therefore,nerveimpulsescanonlymoveinonedirection,butcannottravelintheoppositedirection.
Theconductionprocessofnerveimpulsecanbesummarizedasfollows:
①Stimulationcausesthepermeabilityofnervefibermembranetochange,andalargeamountofNa+flowsinfromoutsidethemembrane,whichcausesthereversalofmembranepotential.Theouterpositiveandinnernegativechangetotheouternegativeandinnerpositive,whichistheactionpotential.Thesequentialpropagationoftheactionpotentialistheconductionofnerveimpulses;
②K+inthefibercontinuestoleakout,sothatthemembraneThepolarizationstateisrestored;
③TheactivetransportationoftheNa+-K+pumpcausestheNa+inthemembranetoflowout,andtheK+outsidethemembranetoflowin,duetoNa+:TheactivetransportvolumeofK+is3:2,thatis,theoutflowofNa+ismore,andtheinflowofK+isless,becausethereareorganicnegativeionsinthemembranethatcannotbeexuded,Sothatthemembrane’spositiveandnegativerestingpotentialandthenormaldistributionofNa+andK+arerestored.
Chemicaltransmissionofnerveimpulses
Thetransmissionofnerveimpulsesbetweensynapsesisaccomplishedbymeansofneurotransmitters.Whenthenerveimpulsereachestheendoftheaxon,somesynapticvesiclessuddenlyrupture,andthestoredneurotransmitterisreleasedthroughtheopeningatthefrontofthesynapse.Whenthisneurotransmitterpassesthroughthesynapticcleft,itquicklyactsonthepostsynapticmembraneandstimulatesthemolecularreceptorsinthepostsynapticneurons,therebyopeningorclosingcertainionchannelsinthemembraneandchangingthemembrane’sPermeability,andcausethepotentialchangesofpostsynapticneuronstorealizethetransmissionofnerveexcitement.Thiskindofsynaptictransmission,mediatedbychemicals,isthemainwayforneuronsinthebraintosignal.
Theneurotransmitterhasnotbeendestroyedafteruse.Itisexpelledfromthereceptorbymeansofanionpump,returnstotheaxonend,repackagedintosynapticvesicles,andthenreused.
Influencingfactors
Theconductionvelocityofactionpotentialsvarieswithanimaltypes,nervefibertypesanddiameters,andtemperaturechanges.
NerveTheinfluenceoffibercategory
Itcanbeseenthatthenervefiberconductionvelocityiscloselyrelatedtothepresenceorabsenceofmyelinandthethicknessofthefiber.
Theinfluenceoftemperature
Temperaturehasacertaininfluenceonnervefiberconductionvelocity.Anincreaseintemperatureisconducivetoconduction.Ifitisbelow10°C,thenervefibersofwarm-bloodedanimalsoftenlosetheirconductionfunction.Temperaturehaslittleeffectontheconductionofunmyelinatedfibers.
Theinfluenceofnervefiberdiameter
Thenerveimpulseconductionvelocityismainlydeterminedbythecablenatureofthenervefiberitself.Thelongitudinalresistanceinthicknervefibersissmall,andthelocalcurrentislarge,whichisconducivetoconduction.Ifthemembranecapacitanceislarge,themembranepotentialchangecausedbythesameamountofchargechangeissmall,whichisnotconducivetoconduction.Thelargemembraneresistanceallowstheintracellularcurrenttospreadfar,whichisgenerallyconducivetoconduction.Thethickeningofmyelinsheathhasmanyinfluencesontheconductionvelocity.Inasense,thethickeningisanincreaseinmembraneresistance.Inaddition,theincreaseintheinter-nodaldistanceoftheLangvie’sjunctionisconducivetoconduction,buttheincreaseinthemyelinsheathThicknessisoftenaccompaniedbyadecreaseintheactualdiameteroftheaxon,whichisnotconducivetoconduction.Boththeoreticalcalculationsandactualmeasurementsshowthatwhentheratioofaxondiameter/fiberouterdiameterisabout0.7,theconductionspeedisthefastest.Whatisinterestingisthatinanimalmyelinfibers,theratiooftheaxondiametertotheouterdiameterofthefiberisexactlyabout0.7.Inaddition,regardingtherelationshipbetweenfiberdiameterandconductionvelocity,cabletheoreticalcalculationsandactualmeasurementresultsarealsoconsistent,thatis,theconductionvelocityofunmyelinatedfibersisproportionaltothesquarerootofthefiberdiameter,whiletheconductionvelocityofmyelinatedfibersisproportionaltothediameter(includingThethicknessofthemyelinsheathisproportionaltotheouterdiameter).
Characteristicsoftheconductionofexcitementonnervefibers:
(1)Physiologicalintegrity:nerveconductionrequiresthestructureandphysiologicalfunctionofnervefibersAllarecomplete.Ifthenervefiberissevered,destroyingtheintegrityofthestructure,theimpulsecannotpassthroughthefracture.
(2)Bidirectionalconduction:stimulateanypointonthenervefiber,theimpulsegeneratedcanbetransmittedalongthenervefibertobothsidesatthesametime.