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1、Ionic Equilibrium Potential Membrane Potential (potential difference across the plasma membrane) at which the net flow of an ion type = zeroThe number of ions moving into the cell = the number of ions moving out of the cell for a particular species of ionNernst Equation VariablesAssumes that membran

2、e is permeable to that ionAs temperature increases the diffusion increasesAs charge on the molecule increases, it decreases the potential differences needed to balance diffusion forces.Simplified Eion (at 37C) Ena = 61.54mV log Nao/NaI = 62 mVEK = 61.54mV log Ko/KI = -80 mVECa = 30.77mV log Cao/CaI

3、= 123 mVCCl = -61.54mV log Clo/ClI = - 65 mVEion = 2.303 RT/zF log iono/ioninGoldman EquationVr= RT/F ln PkKo+PnaNao+PClCli PkKI+PnaNaI+PClCloAlso known as the constant field equation because it assumes that electrical field of the membrane potential is equal across the span of the membraneMembrane

4、PermeabilityMembrane is 50 more permeable to K than to NaPk/Pna = 50PCl/Pk = 0The membrane is so impermeable to Chloride that you drop it from the equationGoldman EquationEion = 2.303 RT/zF log PkKo+PnaNao PkKI+PnaNaIVr= 61.54 mV log505o +1150o 50100i+115I = - 65mV Vr= RT/F ln PkKo+ PnaNao+ PClCli P

5、kKI+ PnaNaI+ PClCloNot to studyDonnans equilibriumOsmolarity considerationsAction PotentialChanges in Ion Permeability allows inward Na flux and triggers an increased outward K flux through voltage gated ion channelsCauses transient change in Membrane PotentialThe change in ion permeability is trigg

6、ered by transient depolarization of the membraneConductance = gHow many charges (ions) enters or leaves cell (inverse of resistance)due to:number of channels/membrane areaHighest density at axon hillocknumber of open channels ion concentration on either side of membrane Measured in Siemens (S), in c

7、ells pS (pico; -12)Historical FiguresHodgkin and Huxley won Nobel Prize for Voltage clamp in 1961 used to identify the ion species that flowed during action potentialClamped Vm at 0mv to remove electric driving force than varied external ion concentration and observed ion efflux during a voltage ste

8、p Sakman and Nehr won Nobel Prize for Patch Clamp in 1991 measured ion flow through individual channels shows that each channel is either in open or closed configuration with no intermediate. The sum of many recordings gives you the shape of sodium conductance. Information CodingIs NOT in shape of a

9、ction potentialIs in the action potential frequency of firing how many are triggeredIn the action potentials pattern or timing of propagationConductance = gHow many charges (ions) enters or leaves cell (inverse of resistance)due to:number of channels/membrane areaHighest density at axon hillocknumbe

10、r of open channels ion concentration on either side of membrane Measured in Siemens (S), in cells pS (pico; -12)Generation of Resting Membrane Potential (-70mV)Plasma membrane Selective permeability, permeable to K, not NaUnequal distribution of ions across membraneDue to open potassium channels and

11、 closed sodium and chloride channelsAction of ion pumps 3Na/2K ATPaseIon Inside Outside Cross PMK+1255yesNA+12120noCl-5125yesH2O55,00055,000yesAnion-1080noIonic Equilibrium Potential The membrane potential that balances the ions concentration gradient so that there is no net current for that ion.No

12、permeability factor.Equilibrium Potential of An IonThe membrane potential at which the net driving force propelling the ion in = the net driving force propelling the ion out.Written Eion; ENa, ECl, EKNernst EquationEion = 2.303 RT/zF log iono/ioninEion = ionic equilibrium potentialZ= charge of ionF=

13、 Faradays constantT= absolute temperature (0Kelvin/-273C) R= gas constantAction PotentialsCan travel up to100 meters/secondUsually 10-20 m/s0.1sec delay between muscle and sensory neuron action potentialAction Potential: a transient and rapid sequence of changes in the membrane potentialMembrane Per

14、meabilityMembrane is 50 more permeable to K than to NaPk/Pna = 50PCl/Pk = 0The membrane is so impermeable to Chloride that you drop it from the equationGoldman EquationEion = 2.303 RT/zF log PkKo+PnaNao PkKI+PnaNaIVr= 61.54 mV log505o +1150o 50100i+115I = - 65mV Vr= RT/F ln PkKo+ PnaNao+ PClCli PkKI

15、+ PnaNaI+ PClCloIon PermeabilityChanges during action potentialThe plasma membrane becomes permeable to sodium ionsPermeability increases from 0.02 to 20=1000 fold increaseCauses Em aka Vr to approach Ena at positive voltages = +20mVrisingovershootFalling undershoot6 Characteristics of an Action Pot

16、ential#1 Triggered by depolarization a less negative membrane potential that occurs transientlyUnderstand depolarization, repolarization and hyperpolarization #2 ThresholdThreshold depolarization needed to trigger the action potential10-20 mV depolarization must occur to trigger action potential #3

17、All or NoneAre all-or- none eventAmplitude of AP is the same regardless of whether the depolarizing event was weak (+20mV) or strong (+40mV).#4 No Change in SizePropagates without decrement along axonThe shape (amplitude & time) of the action potential does not change as it travels along the axon#5

18、Reverses Polarity At peak of action potential the membrane potential reverses polarityBecomes positive inside as predicted by the Ena Called OVERSHOOTReturn to membrane potential to a more negative potential than at rest Called UNDERSHOOT#6 Refractory PeriodAbsolute refractory period follows an acti

19、on potential. Lasts 1 msecDuring this time another action potential CANNOT be fired even if there is a transient depolarization.Limits firing rate to 1000AP/secStimulating electrode:Introduces current that candepolarize or hyper-polarizeRecording electrode:Records change in Potential of the membrane

20、At a distance awayTime (msec)Voltage (mVolts) along Y axis At Threshold Na influx equals K effluxVoltage Sensitive Ion ChannelsSodiumPotassiumIonic Equilibrium Potential Membrane Potential (potential difference across the plasma membrane) at which the net flow of an ion type = zeroThe number of ions

21、 moving into the cell = the number of ions moving out of the cell for a particular species of ionRegenerative Process:Once one Na channelOpens, Na enters,Depolarizes membrane, More and more NaChannels open leading toMore sodium influx & causes upward & depolarizing (more +) phase of the APWhat does

22、a sodium Channel look like?It is one large proteinWith 4 domains thatEach loop through the Plasma membrane 7Times.Property of Voltage Dependent Sodium ChannelSodium channel opens for 1-2 millisecond following threshold depolarizationthen inactivates and does not open even if Vm is depolarized. This

23、is called sodium channel inactivation and contributes to the repolarization of VmM gate= activation gate on Na channel; opens quickly when membrane is depolarizedH gate- inactivation gate on Na channel; Closes slowly after membrane is depolarized causes the absolute refractory period for AP propagat

24、ionNa Channel GatesPotassium Channel PropertyK channels open with a delay and stay open for length of depolarization Repolarize the Vm to Ek= -75mV which is why you have hyperpolarization.Also called a delayed rectifier channel K channels have a single gate (n) that stays open as long as Vm is depol

25、arized. n gate on K channels opens very slowly this allows the Vm to depolarize due to Na influx; Na and K currents do not offset each other right awayGate on the Delayed Rectifier Potassium ChannelRefractory PeriodRefractory period due to Na channel inactivation and the high gk Subsequent Action po

26、tential cannot be generated2 ways to increase AP propagation speedIncrease internal diameter of axon which decreases the internal resistance to ion flowIncrease the resistance of the plasma membrane to charge flow by insulating it with myelin.See and understandwhat happens to the formOf the action p

27、otentialWhen you add a voltageSensitive calcium channelAnd a calcium gatedPotassium channelTest question : think aboutThis and the next 2 slidesChannel DensityDensity is how many channels are in a unit area of plasma membrane, ie how closely they are packed together. Determines the length of the mem

28、brane that will be depolarized at a given time UnderstandRegenerative nature of action potentialOrthodromic and antidromic Voltage gates in sodium channelThreshold potential sodium and potassium fluxes are balancedInitial segment of axon = axon hillockTwo mechanisms for increasing speed of action potential propagationSaltatory conductionUnderstandAction potential occurs because sodium and potassium fluxes change the charge on the cell membrane not because the fluxes change ion concentra