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Bioelectricity:actionpotentialPrincipleAsatypeofapropagatingdisturbanceofthenerve(ormuscle)membranepotentialtheelectrotonicpropagationhasbeendescribedinBioelectricity:electrotonicpropagation.Ithappensindendrites,axonsandmusclefibers.Thesestructurescanbesomemmlong,butaxonsandmusclefibersaremostlysomecmandaxonsmayreachmanymeters(asinwhales).However,mostlyanelectrotonicpotentialisstronglyreducedalongdistancesofcentimeters,ispropagatingslowand,duetotemporalfiltering,thepeaktimeatthenerveterminalwillnotbeverywelldefined.Thereforeitisnotveryappropriatetotransmitinformationoverlongdistancesinlivingorganisms.Thereexistabetterwayofpropagatingnerveinformation.Thisisviaactionpotentials(spikes),anactivewayofpropagation.Withspikesinformationtransportisfaster,withhighertemporalresolutionandmoreinformationcanbetransmitted.Spikesarisefromthesomaticpotential,thesumofthedendriticpotentials,attheaxonhillockandthenpropagatealongaxons(sometimescertaintypesofdendrites),musclefibersandalsoheartmusclefibers.Fig.1visualizesthispropagation.
Fig.1Principleofpropagation.Fig.2depictsthe3maintypes.Spiesarefoundinvertebratesandinvertebrates,butalsosomeplants(relyingonK+andCa++,withthephloemaschannels).
Fig.2Fromlefttorightactionpotentialofaxon,musclefiberandheartmusclecell.DepolarizationandrepolarizationBelowtheaxonalspike,thecommononeisdescribed,firstforanunmyelinatedaxon.Fig.3givesthevariousphaseswhichcanbedistinguishedduringitstimecoarse.Oftenonedendriticpotentialcangiverisetoacoupleofspikes,dependingonitsamplitude.Apropagatingspikegenerallymaintainsitswaveformandamplitude.Thisiscausedbythefactthatthemembraneconductancegm(=1/rm)oftheaxonisnotconstant.Anexcellentwaytoinvestigatethechangesoftheconductancesisthevoltageclamptechnique(holdingthemembranepotentialataconstantvaluewhateverinjectedcurrentisneeded,seeElectrophysiology:clampingtechniques).Essentialforthistechniqueisthatthereflowsnoaxialcurrentthroughtheaxon.Inathickaxon(squid),thisisachievedbyinsertingafinesilverwirelongitudinallyinanaxon.Byanintracellularelectrodeandanextracellularelectrodecurrentisinjectedintotheaxontocompensateforchangesincurrentthroughthemembrane.Thecurrentneededintheclamptechniquecompensatestheioniccurrents(andtheinitialcapacitivecurrent)andismeasuredasafunctionoftime.AccordingtoOhm'slawthetotalioniccurrentatanytimeisproportionalwiththetotalmembraneconductancesincethemembranepotentialiskeptconstant.TheNa+
andK+conductancescanbemeasuredseparatelybyapplyingcertaindrugswhichmakeeithertheNa+ortheK+
conductancezero.TheNa+andK+havedifferenttypesofpores,i.e.selectivechannels.ThepermeabilityforNa+andK+
appearstobeafunctionofthemembranepotential.Inrest,mostNa+channelsareclosed,buttheK+channelsopen,causingaconstantleakingoutofK+.ThisiswaytherestpotentialismainlydeterminedbyK+withits75timeshigherconductivity.TheoutflowofK+isconstantlycompensatedbyNa+inflow.Ifnowthemembraneisstimulatedandeitherdepolarizedorhyperpolarizedthemembranepotentialwillchangewithtime,resultingintimeandvoltagedependentionicresistances.NowrmiscomposedofaresistancerNaformedbytheNa+channels,aresistancerKformedbytheK+channelsandrLformedbyotherpassiveionicchannels,mainlyforCl,eachofthemconnectedwiththeNernstequilibriumpotential(seeBioelectricity).ThechannelmechanismsinvolvedinthegenerationofaspikearetherapidincreaseinthepermeabilityforNa+andthedelayedandslowerincreaseinthepermeabilityforK+ifthecellisdepolarized(socalledcathodalstimulation).Na+movesinundertheinfluenceofthedrivingforce,thedifferencebetweenthemembranepotentialandtheequilibriumpotentialofNa+.Inthesquidaxonthisoccursinafewms.AssoonastheNa+permeability(orconductance)increases,moreNa+streamsintotheaxonorsoma,diminishingthemembranepotentialstillfurther.ThiscausesafurtherincreaseoftheNa+permeabilityandatacertaincriticalvalue,thefiringthreshold(about15mVmorepositivethanErest)thisbecomessostrongapositivefeedbackthatthecellevenreversesitspotentialtopositivevaluesinthedirectionoftheNa+equilibriumpotential.Actually,thethresholdisreachedwhentheinwardNa+currentexceedstheoutwardK+current.Veryshortlyafterwards
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theNa+permeabilityreturnstoitsoriginalvalueandtheK+permeabilityincreasestemporarily.AswiththeNa+influx,itisnotthemovementofK+thatchangesE.ItisthevalueforgKrisingabovethatforgNa,draggingEbacktowardstheequilibriumconstantforK+.SincethevoltagegatedK+channelshaveadelayedresponse,suchthatK+continuestoflowoutofthecellevenafterthemembranehasfullyrepolarized.Thiscausestheundershoot(shorthyperpolarization).ThereisacommonmisconceptionthattheNa+/K+pumprestorestherestingpotentialduringthespikefallingphasebyactivelypumpingNa+outandK+intotheneuron.This(alongwiththemisconceptionthatsodium'floods'thecelltocausethespike),isnotcorrect.TheNa+/K+ATPase(thepump)doesultimatelymaintaintherestingpotentialbymaintainingtheconcentrationgradientsforNa+andK+,butdoessoonamuchslowertimescale.
Fig.3Basictimecoarseofactionpotential.RefractoryperiodDuringashortperiodaftertheoccurrenceofaspikethecellcannotbestimulated.Thisistherefractory(15ms)periodconsistingofanabsoluteandrelativephase.Intheformer,theNa+channelscannotbeopenedbyastimulusirrespectiveofappliedvoltage.Inthesubsequentrelativephase,spikescanbeinitiated,sinceNa+channelsarereactivated(inastochasticmanner)butthethresholdisgreater.ThisiscausedbytheslightlyhyperpolarizedstateduetostillhigherthanrestingvalueforgK,somorevoltageisrequiredtoreachthreshold,andalsothethresholditselfishigherthanusualbecausesomeoftheNa+channelswillstillbeinactivated.(NotethatNa+channelhasatleastthreestates:closed,openandinactivatedclosedandnotabletoopen).Therefractoryperiodisimportantbecauseitensuresunidirectional(oneway)propagationofthespike.Thebasictheoryofspikepropagation,theHodgkinHuxley(HH)theory,isdescribedinMoreInfo.ApplicationSpikes,mostlyintheformofspiketrains,areusedmostextensivelybythenervoussystemforcommunicationbetweenneuronsandfortransmittinginformationfromneuronstootherbodytissuessuchasmusclesandglands(neurohypophysis).SpikesaremeasuredwiththerecordingtechniquesofelectrophysiologyandmorerecentlywithneurochipscontainingEOSFETs(electrolyteoxidesemiconductorfieldeffecttransistor).Suchchipsareappliedinretinalandcorticalimplantstorecordandstimulateneuronalactivity.(Acochlearimplantisformallynotaneurochipsinceitisonlyusedforstimulationitisaneuroprosthesis).Anoscilloscopeshowingthemembranepotentialrecordingfromasinglepointonanaxonshowseachstageofthespikeasthewavepasses.Aspeakerisveryusefultolistentotheelicitedspike(trains).Spikesingeneralcannotbemeasuredatdistance,since,dueoitsdipolenature,itdiminisheswiththethirdpowerofdistance.Theelectrotonicpotentialchangescausedbysynaptictransmissionwhich,ifstrongenough,giverisetothespike,havealessstrongdecaywithdistance.Theyalsolastlonger.Ifthereisenoughgeometricalcoordinationbetweenagroupofexcitedneurons,socalledsloworgradedpotentialscanberecordedforinstanceontheskullofman.Theyarealwayssignofmassaction.IftheyarespontaneouswespeakoftheEEG,iftheyareexcitedbylight,soundorperipheralnervestimulationwespeakofvisual,auditoryorsomatosensoryevokedpotentials(EPs)respectively.Alsotheelectroretinography(ERG)reflectsgradedpotentialsandnotthespikesoftheopticnerve.Somediseasesreducethespeedofspikeconductance.Themostwellknownofthesediseasesismultiplesclerosis,inwhichthebreakdownofmyelinimpairscoordinatedmovement.MoreInfoTheconductancesforNa+andK+changeaccordingto:gNa+=Na+m3h,gK+=K+n4,(1)whereNa+andK+arethemaximalconductances.Thevariablesn,m,andhhaveavaluebetween0and1.IntheequationfortheK+conductancen4denotesthefractionoftheK+channelswhichareopen.Ifallchannelsareopenthenn4n1.Ifallareclosedn=0.ApparentlyfoureventsofequalnaturehavetocoincidetoopenthefourfoldedlockedK+
channel.FortheNa+channeltwokindsofkeys(events)areused.Threeidenticalkeysareneededtoopenthethreefoldedmlock.Anotherlock(h)isopeninrest,butcloseswhenthemembraneisdepolarized.Thesefractionsm,handn
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arevoltageandconsequentlytimedependent.Theycanbefoundbysolvingthreeexperimentallyfounddifferentialequations.Fornthisequationis:dn/dt=n(1n)nn(2a)wherendenotestheopenconditionandntheclosedcondition.ThisisvisualizedinFig.4a.Aftersolving,nappearstobeapositiveexponentialfunctionofthemembranepotentialEandnisanegativeone(Fig.4b1).SinceduringexcitationEchangeswithtimethetwovariablesalsochangewithtimewhatfinallyresultsintheinitiallyprogressiveincreaseofn4,forastepwisechangeofE(Fig.4c1).Fig.4c2givesthefinalvalueofnandn4measuredduringvoltageclamp.ForNa+wehavetodowithachangeofmandh.Bothcanbecalculated.Thedifferentialequationsare:dm/dt=m(1m)mmand(2b)dh/dt=h(1h)hh.(2c)
Fig.4Thegatingmodelforpotassiumandsodium.b2)ande2)depictthedependencyofnandn4,andm,handhm3,whenavoltagestepisappliedverylong(infinite).Thetimecourseofmlookslikethatofnbutisfaster.However,hbehavesdifferently.Itdecreasesinsteadofincreasesduetothedecreaseofhandincreaseofhwhenthemembraneisdepolarized(Fig.4e2).Therefore,alsoforlonglastingdepolarization(voltageclamp)theNa+conductancerestorestoitsoriginallylowrestvaluewithinabout5ms(Fig.4e2).Theoppositebehaviorofhandmduringlonglastingdepolarizationisclearlyshown.ThecurrentswhichflowthroughthemembranearecomposedofthecapacitivecurrenticandthreeioniccurrentsofNa+
(throughthevariablegNa)ofK+(throughthevariablegK)andtheanioncurrent(mainlyCl,throughthefixedgI),togetherii.Fig.5givesthetimecoarseofaspike,togetherwithic,ii,theresumim,andalsotheunderlyinggKandgNa.Forthe4composingcurrentstogethertherelationis(seeequation(2)and(3)ofBioelectricity:electrotonicpropagation):im=(1/ra)2E/x2=cmE/t+n4K(EEK)+m3hNa(EENa)+L(EEL).(3)
Fig.5Timecoarseofconductancesandcurrentsduringanactionpotential.AttimeAandBtheslopesoftheactionpotentialaremaximalandimzero.AtBbothreachtheirextrema.Supposethatbysummationofdendriticandsomaticpotentialsattheaxonhillockaspikearises.Atthatsitethelocalcurrentsbecomesostrongthatalsothenextpartofthe(axonal)membranebecomesenoughdepolarizedtobeexcited
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andapropagatedspikewithoutdecrementrunsalongtheaxon.Therefractoryperiodmakesthatthespikewillnotreverseandoccuronlyonceforashortlastingstimulus.Ifanerveisstimulatedinthemiddleofanaxontheimpulsewillpropagatetobothsides.Longerlastingstimulimaycausetrainsofspikes.Justlikeforthepropagationofadendriticpotentialitcanbeshownthattheconductionvelocityisaparameteroftheequation:2d2E/dt2=cmdE/dt+n4K(EEK)+m3hNa(EENa)+L(EEL).(4)Thenumericalsolutionofthisnonlineardifferentialequationgivesaquitecomplicatedexpressionof.Spikesrecordedclosetothesoma(oraxonhillock)arebiphasic,astheoneofFig.3,butwhenrecordedinthevicinityofanaxontheyaretriphasic.OnecouldthinkthatanerveimpulsewhichreversesthenervepotentialwouldbringaboutanimportantdepletionofK+
whichleavesthecellbecausethereisnopotentialgradientanymorekeepingitintheinterior,andthattheinflowofNa+wouldcauseapermanentdisturbanceofthemembranepotential.However,theamountsofionsdisplacedaresmallcomparedtotheactualnumberpresent.EveninverysmallnervesseveralthousandsofspikescanbegeneratedwithoutasignificantlyincreasedmetabolismtoexpelNa+.Thebehaviorofthechannelshasextensivelybeenstudiedbyclampingtechniques(seeElectrophysiology:clampingtechniques),inwhichthei/E(soconductance)isinfluencedbyadministratingallkindofdrugs.Adiscussionofthesephenomenaisbeyondthescopeofthischapter.MyelinatedaxonsPropagationspeedcanbeincreasedbyincreasingtheaxondiameter.Takingforsimplicityequation(8)ofBioelectricity:electrotonicpropagation(=2/=0.5Cm1(d/(RmRi))0.5)thisspeedisproportionalwiththesquarerootofdiameter(d).However,formetabolicreasons,thediameterislimited(onlythecoldbloodedsquidreachesavalueof1mm).Unmyelinatedfibers(about2m)aregenerallyfoundintheautonomicnervoussystemofvertebrateswherespeedsofabout1m/saresufficient.Invertebrates,sensoryandmotoronesaregenerallymyelinated.Thisismoreeffectivetoincrease.Theeffectofmyelincanalsobeevaluatedsinceallaboveconsiderationscanbeappliedinprincipletothemyelinatednerve.Myelincanbeconsideredasthedielectricumbetweentwocondenserplates.Itdecreasesmembranecapacitance,sincemyelinhasalowerrelativedielectricconstantm(about718)thaninterstitialfluid(mclosetomofwater,beingabout80Cm~m/d).Now,Cmisonlyabout4nF/cm2.andRmisabout105cm2.Increasingtheeffectivemembranethicknessbyusingmyelin(leavingtheinnerfiberdiameterconstant)alsodecreasesCm,so.Whenmyelinthicknessandinnerdiameterincreasewiththesamefactor,thenincreaseslinearwiththisfactorasfollowsfrom(5).Thisshowstheefficiencyofdiameterincrease.Experimentallythishasbeenfoundindeedforvertebrateperipheralfibers.However,amyelinatedfiberlongerthansome100mdoesnotworkproperly.Myelinallowstherapid(essentiallyinstantaneous)conductionofions,butpreventstheregenerationofspikes.Therefore,thecylindricalshapeofthemyelinsheathisinterruptedevery0.010.1mmbyanodeofRanvier,anakedpieceofca.0.5mofaxon.TheirCmisabout4F/cm2andRmisonlyabout15cm2.AnabundanceofvoltagegatedNa+channelsonthesebaresegments(upto104morethantheirdensityinunmyelinatedaxons)allowsspikestobeefficientlyregeneratedatthenodesofRanvier.Theexcitationjumpsfromonenodetotheother,whichisapassive,soelectrotonictransmission(seeBioelectricity:electrotonicpropagation)implyingsomedecrement.Basically,thiscangoineitherdirections,butthespiketravelsunidirectionalbecausethenodebehindthepropagatingspikeisrefractory.Thiswayofpropagationofthespikeiscalledsaltatoryconduction:atthemyelinatedsegmentsthepropagationisveryfast(duetotheinsulation),whereasatthenodesthereisasmalldelayof0.01to0.1ms.Thelengthoftheinternodalsegmentsaresuchthatone,orsometimeseventwonodescanbepassedandthattheamplitudeisstillsufficienttoreachthethresholdforrestoringtheamplitudeofthespike.Thus,thesafetyfactorofsaltatoryconductionishigh,allowingtransmissiontobypassnodesincaseofinjury.Mammalianmyelinatedmotorneuronscanreach100m/s.Saltatoryconductionincreasesnerveconductionvelocitywithouthavingtodramaticallyincreaseaxondiameter.Withoutsaltatoryconduction,conductionvelocitywouldneedlargeincreasesinaxondiameter,resultinginorganismswithnervoussystemstoolargefortheirbodies.AlternativemodelsAfewobservationsarenoteasilyreconciledwiththemodel.Asignaltravelingalonganeuronisaccompaniedbyaslightlocalthickeningofthemembraneandaforceactingoutwards.Also,aspiketravelingalonganeuronresultsinaslightincreaseintemperaturefollowedbyadecreaseintemperature,whereaselectricalchargestravelingthrougharesistoralwaysproduceheat.TherecentsolitonmodelexplainstheaboveobservationsandpossiblyallpropertiesoftheHHmodel.Asolitonisaselfreinforcingsolitarywave(awavepacketorpulse)thatmaintainsitsshapewhileittravelsatconstantspeedsolitonsarecausedbyacancelationofnonlinearanddispersiveeffectsinthemedium.Thistheoryattemptstoexplainsignalsinneuronsaspressure(orsound)solitonstravelingalongthemembrane,accompaniedbyelectricalfieldchangesresultingfromPiezoelectricity.LiteratureHHmodel:Noble,Physiol.Review,46,150,1966.Solitonmodel:HeimburgT,JacksonAD.Onsolitonpropagationinbiomembranesandnerves.PNAS,102,122005.HeimburgT,JacksonAD.Thethermodynamicsofgeneralanesthesia.BiophysicalJournal,9,February2007.
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