1、Ionic Equilibrium Potential Membrane Potential(potential difference across the plasma membrane)at which the net flow of an ion type=zero The number of ions moving into the cell=the number of ions moving out of the cell for a particular species of ionNernst Equation Variables Assumes that membrane is
2、 permeable to that ion As temperature increases the diffusion increases As 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 mV EK=61.54mV log Ko/KI=-80 mV ECa=30.77mV log Cao/CaI=123 mV CCl=-6
3、1.54mV log Clo/ClI=-65 mVEion=2.303 RT/zF log iono/ioninGoldman Equation Vr=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 Permeability Membrane
4、 is 50 more permeable to K than to Na Pk/Pna=50 PCl/Pk=0 The membrane is so impermeable to Chloride that you drop it from the equationGoldman Equation Eion=2.303 RT/zF log PkKo+PnaNao PkKI+PnaNaI Vr=61.54 mV log505o+1150o 50100i+115I =-65mV Vr=RT/F ln PkKo+PnaNao+PClCli PkKI+PnaNaI+PClCloNot to stud
5、y Donnans equilibrium Osmolarity 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 triggered by transient depolariza
6、tion of the membraneConductance=g How many charges(ions)enters or leaves cell (inverse of resistance)due to:number of channels/membrane area Highest density at axon hillock number of open channels ion concentration on either side of membrane Measured in Siemens(S),in cells pS(pico;-12)Historical Fig
7、ures Hodgkin and Huxley won Nobel Prize for Voltage clamp in 1961 used to identify the ion species that flowed during action potential Clamped Vm at 0mv to remove electric driving force than varied external ion concentration and observed ion efflux during a voltage step Sakman and Nehr won Nobel Pri
8、ze for Patch Clamp in 1991 measured ion flow through individual channels s h o w s t h a t e a c h channel is either in o p e n o r c l o s e d configuration with no intermediate.The sum of many recordings gives you the shape of sodium conductance.Information Coding Is NOT in shape of action potenti
9、al Is in the action potential frequency of firing how many are triggered In the action potentials pattern or timing of propagationConductance=g How many charges(ions)enters or leaves cell (inverse of resistance)due to:number of channels/membrane area Highest density at axon hillock number of open ch
10、annels 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 Na Unequal distribution of ions across membrane Due to open potassium channels and closed sodium an
11、d chloride channels Action 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 permeability fac
12、tor.Equilibrium Potential of An Ion The 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 Equation Eion=2.303 RT/zF log iono/ionin Eion=ionic equilibrium potential Z=charge of ion F=Faradays constant T=a
13、bsolute 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 Permeability Membrane is 50 m
14、ore permeable to K than to Na Pk/Pna=50 PCl/Pk=0 The membrane is so impermeable to Chloride that you drop it from the equationGoldman Equation Eion=2.303 RT/zF log PkKo+PnaNao PkKI+PnaNaI Vr=61.54 mV log505o+1150o 50100i+115I =-65mV Vr=RT/F ln PkKo+PnaNao+PClCli PkKI+PnaNaI+PClCloIon Permeability Ch
15、anges during action potential The plasma membrane becomes permeable to sodium ions Permeability increases from 0.02 to 20=1000 fold increase Causes Em aka Vr to approach Ena at positive voltages=+20mVrisingovershootFalling undershoot6 Characteristics of an Action Potential#1 Triggered by depolarizat
16、ion a less negative membrane potential that occurs transiently Understand depolarization,repolarization and hyperpolarization#2 Threshold Threshold depolarization needed to trigger the action potential 10-20 mV depolarization must occur to trigger action potential#3 All or None Are all-or-none event
17、 Amplitude of AP is the same regardless of whether the depolarizing event was weak(+20mV)or strong(+40mV).#4 No Change in Size Propagates without decrement along axonThe shape(amplitude&time)of the action potential does not change as it travels along the axon#5 Reverses Polarity At peak of action po
18、tential the membrane potential reverses polarity Becomes positive inside as predicted by the Ena Called OVERSHOOT Return to membrane potential to a more negative potential than at rest Called UNDERSHOOT#6 Refractory Period Absolute refractory period follows an action potential.Lasts 1 msec During th
19、is 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 membraneAt a distance awayTime(msec)Voltage
20、(mVolts)along Y axis At Threshold Na influx equals K effluxVoltage Sensitive Ion Channels Sodium PotassiumIonic Equilibrium Potential Membrane Potential(potential difference across the plasma membrane)at which the net flow of an ion type=zero The number of ions moving into the cell=the number of ion
21、s 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 a sodium Channel look like?It is one large protein
22、With 4 domains thatEach loop through the Plasma membrane 7Times.Property of Voltage Dependent Sodium Channel Sodium channel opens for 1-2 millisecond following threshold depolarization then inactivates and does not open even if Vm is depolarized.This is called sodium channel inactivation and contrib
23、utes 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 propagationNa Channel GatesPotassium Channel Property K chann
24、els 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 depolarized.n gate on K channels opens very slowly this allo
25、ws 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 Period Refractory period due to Na channel inactivation and the high gk Subsequent Action potential cannot be generated2 ways to increase AP propaga
26、tion speed Increase internal diameter of axon which decreases the internal resistance to ion flow Increase the resistance of the plasma membrane to charge flow by insulating it with myelin.See and understandwhat happens to the formOf the action potentialWhen you add a voltageSensitive calcium channe
27、lAnd a calcium gatedPotassium channelTest question:think aboutThis and the next 2 slidesChannel Density Density is how many channels are in a unit area of plasma membrane,ie how closely they are packed together.Determines the length of the membrane that will be depolarized at a given time Understand
28、 Regenerative nature of action potential Orthodromic and antidromic Voltage gates in sodium channel Threshold potential sodium and potassium fluxes are balanced Initial segment of axon=axon hillock Two mechanisms for increasing speed of action potential propagation Saltatory conductionUnderstand Act
29、ion potential occurs because sodium and potassium fluxes change the charge on the cell membrane not because the fluxes change ion concentrations.DefinitionV=IRV=voltage,I=current,R=resistance g=1/Rg=conductance Vm=membrane voltage Vr=voltage of membrane at rest Permeability and Conductance gn a is l
30、ow at Vr because sodium channels are closed gk is higher than gna at Vr because some potassium channels are open.V=I/R Ohms Law G=conductance=1/RDefinitions Current=net flow of ions per unit time 1 ampere of current represents movement of 1 coulomb of charge per second Resistance-frictional forces t
31、hat resists movement of ions or charges Measured in ohm Current(A)=V/RDefinitions Conductance is the reciprocal of resistance and measures the ease with which current flows in an object.Measured in siemens(S)Capacitance refers to the ability of plasma membrane to store or separate charges of opposite signs.Myelin has high capacitance so stores charges and ions do not move across the membrane Measured in Farads谢谢你的阅读v知识就是财富v丰富你的人生谢谢
