ActionPotentialLesson12

LearningGoals

1. Whatisanactionpotential

2. Howdoeselectricalimpulsemoveacrossaneuron

Actionpotential

Actionpotential• Atemporaryshiftinthemembranepotential• Resultsfromthevoltagedifference acrossamembranewhenthenerveisexcited• Theelectricalcurrent(impulse)travelsthelengthofanaxon

Membranepotential(voltage)• Differences inelectricalpotentialacrossamembrane(chargeseparation)• Unequalsodium(Na+)andpotassium(K+)ionsinsideandoutsidethecell

MembraneRestingpotential(MRP)

•Membranepotentialofaneuronatrest (-70mV)

Sodium-potassiumpump• Activetransportthatmoves3Na+outofthecelland2K+intothecellacrosstheirconcentrationgradients

RestingMembranePotential

• Atrest,themembraneinside aneuronhasanegative charge

• Atrest,outside oftheneuronmembraneispositively charged

Axonhillock

AxonHillock• Specializedpartofthecellbodythatconnectsittotheaxon• Thesitewheretheaxonbegins

Thresholdlevel

• IncomingNa+fromastimulusraisesmembranepotentialuntilitreachesthethresholdpotentialof-50 to-55mV

Thresholdlevel• Minimumlevelofastimulus

requiredtoproducearesponse(depolarization)

• Causeallvoltage-gatedNa+channelstoopen

ActionPotential

• Themagnitudeofapotentialstaysthesameasittravelsalonganaxon• Theall-or-nothingcharacteristicoftheAPmeansthattheintensityorstimulusisreflectedinthefrequencyofAPratherthanintheirmagnitude

• Thegreaterthestimulus,thefastertheAP

Depolarization

Depolarization• Diffusionofsodiumionsintothenervecellresultinginachargereversal• Insidethecellbrieflybecomespositive

Repolarization

• Whenthemembranepotentialreaches+30mV,theNa+voltage-gatedionchannelsclose

Repolarization• Processofrestoringtheoriginalpolarityofthenervemembrane

• K+voltage-gatedchannelsopenandK+ionsexitalongitsconcentrationgradient

• Depolarizationandrepolarizationonlytakesabout5milliseconds!

Hyperpolarization

Hyperpolarization• Processofrestoringtheoriginalpolarityofthenervemembrane• K+channelscloseslowly =K+outflow=-80mV

RefractoryPeriod• Recoverytime requiredbeforeaneuroncanproduceanotheractionpotential• Thresholdtodepolarizetheneuronishardertoreach

RestingMembranePotential

• Na+/K+pump returnstheneurontorestingpotential

• Neuronisreadyforanewactionpotential

“All-or-Nothing”response

• Anactionpotentialisonlyproducedifthestimulusisstrongenoughtocausedepolarizationatthethresholdlevel(-55mV)

• Neuronswillfiremaximallyornotatall

• Intensityofimpulseandspeedoftransmissionisindependent ofstimulusstrengthoncethresholdisreached

ActionPotentialPropagation

• Depolarizationofonesitecausesnearbyvoltage-gatedNa+channelstoopenà resultingindepolarizationofanadjacentsite

• Actionpotential“travels”byrepeatedregenerationalongaxon

• Refractoryperiodensurestheactionpotentialtravelsinonedirectiononly =unidirectional

SpeedofTransmission?

Axondiameter

• Larger=fasterconduction• Lessresistance

Myelination(myelinsheath)

• Voltage-gatedionchannelsonlyinnodesofRanvier• Axonsonlyexposedtoionsinextracellularfluid(ECF)nodes• Noactionpotentialinregionsbetweennodes

UnmyelinatedConduction

MyelinatedConduction

• Currentgeneratedbyactionpotentialatanode“leaps”tonextnode

• Na+andK+exchangecanonlyoccurwheretheaxonsareexposedtotheECF

• Allowsforfastersignalconductionalongtheaxon

Synapse

• Asmallregionthatcontrolscommunicationbetweentwoneuronsorbetweenneuronsandeffectors

• Actionpotentialistransmittedthroughthesynapsebyneurotransmitters thatarereceivedbyreceptors ontheothersideofthesynapse

• Neurotransmitters=chemicalmessengers containedinvesicles• Releasedfromthepresynapticneuron

anddepolarizesthepostsynapticneuron

• Moresynapses=slowertransmission• Veryfewsynapsesinareflexarc,butMANYinvolvedinproblemsolving!

ActionPotential

• Themagnitudeofapotentialstaysthesameasittravelsalonganaxon• Theall-or-nothingcharacteristicoftheAPmeansthattheintensityorstimulusisreflectedinthefrequencyofAPratherthanintheirmagnitude

• Thegreaterthestimulus,thefastertheAP

ActionPotentialSummary

Homework

1. Worksheetquestions

2. UnitTaskResearch:DueTuesday• ComputerlabwillbeopenonMondayatlunch

3. Reproductive(female)assignment:DueMonday