All rights reserved Overview Questions Most neurons have elaborately branching dendritic trees that receive thousands of synaptic inputs Figure 1 A majority of dendritic synapses are excit atory but inhibitory synapses also contact dendrites A numb ID: 75594
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Howdoesthespatialrelationshipbetweenexcit-atoryandinhibitorysynapsesaffectdendriticinte-gration?.Howdoesthepresenceofvoltage-gatedchannelsindendritesaffectdendriticintegration?.Canspikesbeinitiatedindendritesand/orpropa-gatealongthem?.Howdodendriticspinesaffectsynapticintegration?Theanswerstothesequestionsdependonseveralimportantfactors,suchasdendriticstructureandexcit-ability,aswellasthepatternofsynapticactivationand restingmembranepropertiesaffectsynapticintegra-tionasmuchasdothepropertiesofthesynapses.Incerebellargranulecells,theEPSCdecayswithatimeconstantofabout1ms(apropertyofthesynapse), precisionatacerebellarsynapse.JournalofNeuroscience allypermeabletoNa#andK#,thesynapticreversalpotential(Erev)is0mV,sothedrivingforce(VmÐErev)is70mV.WhenoneEPSPhasdepolarizedthemembrane,thedrivingforceisaccordinglyreduced,thusreducingthechargethatentersatthesynapse.Forexample,a7mVEPSPwouldreducethedrivingforceby10%.ThisisthebasisforsublinearEPSPsummationina ).2msVmIsynVmVmabcFigure2IntegrationofEPSPs.(a)Activationofasinglesynap-ticinput(asshownschematicallyforasynapseonacerebellargranulecell)resultsinafastinwardcurrent(Isyn)thatproducesachangeinmembranepotential(Vm insightsprovidedbytheanalysisofcurrentflowintransatlantictelephonecables.RallÕscabletheorynowservesasthefoundationformoderncomputermodelingofneuronswithdendritictrees.Suchmod-elsarenowbeingusedtoprovideinsightintohowchargespreadsinelaboratelybranchingdendritictreescontainingmanydifferenttypesofionchannels.AnotherwaythatdendritesinfluenceEPSPsisviatheireffectonspatialsummation.Iftwosynapsesonthesamedendriteareactivatedtogether,theywillsumsublinearlyforthesamereasonsasdescribedforthesimplecaseofacerebellargranulecell.Ifthetwosynapsesareondifferentdendrites,however,thesublinearityofspatialsummationwillbereduced,becauseeachsynapsehasareducedinfluenceontheother,duetotheattenuationfromonelocationtotheother.Forsynapseslocatedondifferentdendritesthiseffectcanbequitelarge.DendritesalsoinfluenceEPSPsbyaffectingtempo-ralsummation.Becauseoftheresistanceandcapaci-tanceofdendrites,EPSPtimecourseisfilteredbetweenthedendritesandthesoma.Atthesiteof IntegrationSomesubtypesofinhibitoryinterneuronsspecificallytargetdendrites.Althoughrelativelylittleisknown+++++++ axon,anunderstandingofcabletheoryinformsspec-ulationaboutthepossiblyspecializedroleofden-driticinhibition.JustasEPSPsareattenuatedandfilteredbetweenthedendritesandthesoma,inhibitorypostsynapticpotentials(IPSPs)generatedinthedendriteswillbesmallerandslowerinthesoma.However,themecha-nismofactionofmostinhibitorysynapsesisactiva-tionofCl"channels(e.g.,GABAAreceptors).Inmostcells,thereversalpotentialofsynapsesusingClchannelsisveryclosetotherestingpotential,sothedrivingforceatrestisverysmall.Therefore,activa- synapseslocatedonthesomacanshuntexcitatorycurrentoriginatinganywhereinthedendritictree.Bycontrast,dendriticinhibitionwillshuntexcitatorysynapsesonthesame(ornearby)dendritesmoreeffectivelythanitwillthoseonother(ormoredistal)dendrites.Shuntinginhibitionalsohasagreatereffectonsomaticmembranepotential(andthereforeonactionpotentialinitiationintheaxon)ifitislocatedonthepathbetweentheexcitatorysynapsesandthesoma(Figure4).Inhibitorysynapseslocatedoffthispatharelesseffectiveatshuntingcurrentasitflowsfromthesynapsetothesoma.EffectsofVoltage-GatedChannelsonDendriticIntegrationMostoftheforegoinganalysisofdendriticintegra-tionhasbeenbasedonpassivedendrites(i.e.,lackingvoltage-gatedchannels).However,itiswellknownthatdendritescontainvoltage-gatedconductances,whichdramaticallyinfluencedendriticfunction.Dendriticpatch-clamprecordingshaveprovideddirectevidenceforthepresenceofNa#,Ca2#,K#,andothervoltage-gatedchannelsinthedendritesof fromthedendritesofcerebellarPurkinjecells.Voltage-gatedchannelsexpressedinthedendrites b++++++++++++++++++ bythespinehasbeendemonstratedbynumer-ousstudies.Electricalcompartmentalizationismorecomplicatedandhasalsobeenmoredifficulttodem-onstrate.Membranepotentialchangesinthedendritearenotlikelytoattenuatesignificantlyastheyspreadintospines.Thisisbecausethesmallsize(highimped-ance)ofspinesmakesthemveryeasytodepolarize.Ontheotherhand,EPSPsinspinesmayattenuatesignificantlyastheyspreadintothelarger(lowerimpedance)parentdendrite.Suchattenuation,ifsig-nificant,couldmeanthatEPSPsinspinesareverylarge,possiblyresultinginactivationofvoltage-gatedchannelsinspines,suchasCa2#channelsknowntobepresentinthespinehead.Quantitatively,themagnitudeoftheelectricalcompartmentalizationimposedbyspinesdependsontheresistanceofthespineneck.Thisparameterisnotwellknown,butimagingstudiesarebeginningtoyieldimportantdatathatwilladdresstheseandotherissues,leadingtoanimprovedunderstandingofthefunctionofdendritic Thiswillrequireanunderstandingnotonlyofthestructuralandfunctionalpropertiesofdendrites,butalsoknowledgeofthepatternsofsynapticactivationandneuromodulatorybackgroundthatoccursduringvariousbehaviors.Ultimately,thiswillrequirerefine-ments,improvements,andextensionsofthemethodsusedtostudydendritesinvitroandinvivoSeealso:ActionPotentialInitiationandConductioninAxons;Activity-DependentRemodelingofPresynapticBoutons;AxonalandDendriticIdentityandStructure:Controlof;DendriteDevelopment,SynapseFormationandElimination;DendriticSpineHistory;EphReceptorSignalingandSpineMorphology;IonChannelLocalizationinCellBodiesandDendrites;LIMKinaseandActinRegulationofSpines;RhoGTPasesandSpines;TranslationalRegulationattheSynapse.FurtherReadingHa¬usserM,SprustonN,andStuartG(2000)Diversityanddynam-