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Unformatted text preview: Lecture4 OnlineHandouts: Lec4Notes Review NernstPotenEal MembranePotenEal Announcements: None Reading(Recommended): Reading:Ch4(pp85100) AcEonPotenEal ConducEonofAP REV:EquilibriumPotenEal MembranepotenEalatwhichelectricalandchemical concentraEonforcesareinequilibrium EquilibriumPotenEalforanion(Ei)
NernstPotenEalforion"i" Ei=61xlog(Co/Ci)[unitsinmV] C=[i]=ConcentraEon oisECF,iisICF NernstequaEon EK=61xlog(5mmol)/(150mM)=61x1.5=90mV ENa=61xlog(150mmol)/(15mM)=61x1=60mV REV:MembranePotenEal MembranepotenEal(Em)determinedby:
[Na+]and[K+]inICFandECF(definesEi) Na+andK+Ionmembranepermeability(Pi) TwotypesofmembraneproteinsdeterminePi
IonselecEvechannelsforNa+andK+(Passive) Na/KATPasePump(AcEve) ResEngmembranepotenEal(RMP;Em~70mV)
PK>Pna(~75:1) Na/KATPasePump AnAcEonPotenEalMayOccurWhenEm DepolarizesPastThreshold
= Action potential = After hyperpolarization Na+ equilibrium potential Threshold potential Resting potential K+ equilibrium potential Triggering event Fig.46,pg.90 AcEonPotenEals
1. RapidstereotypedchangeinEm,whichoccurs inresponsetoasEmulus. 2. Thereare6primarycharacterisEcsofanAP:
a) b) c) d) e) f) LargeregeneraEveincreaseinPNa RapidrepolarizaEonofEm EveryAPfollowedby2refractoryperiods EveryAPisAllorNone APconductedovertheaxon APareconductedrapidly AcEonPotenEals WhenmembranereachesthresholdpotenEal
Voltagegatedchannelsinthemembraneundergo conformaEonalchanges FirstincreasePNa FlowofsodiumionsintotheICFreversesthe membranepotenEalfrom70mVto+30mV SecondincreasePK FlowofpotassiumionsintotheECFrestoresthe membranepotenEaltotheresEngstate AcEonPotenEals AddiEonalcharacterisEcs
SodiumchannelsopenduringdepolarizaEonby posiEvefeedback. WhenthesodiumchannelsbecomeinacEve,the channelsforpotassiumopen.Thisrepolarizesthe membrane. AstheacEonpotenEaldevelopsatonepointinthe plasmamembrane,itregeneratesanidenEcalacEon potenEalatthenextpointinthemembrane. Therefore,ittravelsalongtheplasmamembrane undiminished. TheAcEonPotenEalResultsfrom PosiEveFeedback
Triggering event Depolarization (decreased membrane potential) Positive-feedback cycle Influx of Na+ (which further decreases membrane potential) Opening of some voltage-gated Na+ channels Fig.48,pg.92 AcEonPotenEalsareStereotypical What'sHappeningDuringanAcEon PotenEal? IonPermeabilityCHANGES
x VoltageGatedNa+andK+Channels Caused a + influ Rising pha se d by N TheChangeOccursina TEMPORALMANNER
Threshold potential Resting potential Falling phas + lux by K eff Cause Fig.49,pg.92 e SubThresholdResponses
Graded potential (change in membrane potential relative to resting potential) Threshold! Resting potential Time Magnitude of stimulus Fig.42,pg.87 Stimuli applied GradedPotenEal Occursinsmall, specializedregion ofexcitablecell membranes Magnitudeof gradedpotenEal variesdirectly withthe magnitudeofthe triggeringevent
Fig.44,5,pg.89 CurrentFlowDuringaGradedPotenEal Fig.43,pg.88 OpeningofVoltageGatedNa+Channels DepolarizestheCell
Voltage-Gated Sodium Channel Extracellular fluid (ECF) Plasma membrane Inactivation gate Activation gate Intracellular fluid (ICF) Rapid opening triggered at threshold Open (activated) From threshold to peak potential (50 mV to +30 mV) Slow closing triggered at threshold Closed and not capable of opening (inactivated) From peak to resting potential (+30 mV to 70 mV) Closed but capable of opening At resting potential (70 mV) Fig.47,pg.91 Na+InfluxDepolarizestheCellandOpens MoreVoltageGatedChannels Na+ activation gate opens At resting potential Threshold reached Action potential begins Depolarizing triggering event ImagesfromPhysioEdgeCD ThePeakoftheAcEonPotenEal
Na+ inactivation gate begins to close Na+ inactivation gate begins to close K+ gate opens K+ gate opens Peak of action potential; potential reversed Peak of action potential; potential reversed ImagesfromPhysioEdgeCD +Channel TheVoltageGatedK Extracellular fluid (ECF) Plasma membrane Intracellular fluid (ICF) Thischannelalso openswith depolarizaEon
Delayed opening triggered at threshold Open Closed At resting potential; delayed From peak potential through opening triggered at threshold; after hyperpolarization remains closed to peak potential (+30 mV to 80 mV) (70 mV to +30 mV) Abitslower Hasasingle acEvaEongate Fig.47,pg.91 RepolarizaEon Na+ inactivation gate opens; Na+ activation gate closes Repolarization begins Action potential complete; after hyperpolarization begins ImagesfromPhysioEdgeCD AmerhyperpolarizaEon a + influ x Caused se Falling phas d by N Rising pha + flux by K ef After hyperpolarization is complete; return to resting potential Cause e Threshold potential Resting potential ImagesfromPhysioEdgeCD Fig.49,pg.91 HowdoWeStopaPosiEveFeedback Process?
Voltage-Gated Sodium Channel Extracellular fluid (ECF) Plasma membrane Inactivation gate Intracellular fluid (ICF) Rapid opening triggered at threshold Open (activated) From threshold to peak potential (50 mV to +30 mV) Slow closing triggered at threshold Closed and not capable of opening (inactivated) From peak to resting potential (+30 mV to 70 mV) Activation gate Closed but capable of opening At resting potential (70 mV) Fig.47,pg.91 ...
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- Spring '08