Lecture12-2009W - Two types of voltage-dependent ionic...

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Action Potential barb2right large and brief change of V m that is propagated along the memb. General properties - threshold potential - regenerative - overshoot - after hyperpolarization (= undershoot) - refractory period Four key elements for AP (1) Active transport of ions by specific protein (2) Unequal distribution of ions barb2right electrochemical gradient (3) Ion-selective channels barb2right (1), (2) & (3) are key elements for V rest (4) Gating (open & closing) of specific ion channels) + Absolute RP Relative RP Ionic basis of action potentials checkbld Na + conductance (gNa) – fast checkbld K + conductance (gK) – delayed checkbld Equilibrium potentials (E Na , E K ) checkbld Voltage-gated ion channels - Na channels - K channels Ohm’s law barb2right I = V/R Then, I x = gX • emf x ( R = 1/gX) I x = gX • (V m – E x )
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Ionic currents underlying action potentials Hodgkin & Huxley voltage-clamping exp.
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Unformatted text preview: Two types of voltage-dependent ionic currents (1) Inward Na current (2) Outward K current (3) Currents underlying AP b sum of I Na + I K History: AP in the Squid giant axon b J. Young (1936), b A. Hodgkin & A. Huxley (1939) NaCl b CholineCl The Voltage-clamping Method- measuring ionic currents generated by a population of ion channels c [Na + ]out > [Na + ]in by a factor of 10. (thus E Na = +55mV & huge emf ) c Entry of positively charged Na + into the cell could produce the depolarization of memb. potential (V m ). c Peak potential (overshoot) of AP, close to calculated E Na . c Size of AP changes as function of extracellular [Na + ]. How do we know Na + is the major ionic species responsible for the AP?...
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