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Lect20 - Feb2 - Cardio III

Lect20 - Feb2 - Cardio III - Phases of the AP Phase 4 =...

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January 20, 2011 11 Ionic Current Flow and the Purkinje Fiber Action Potential Questions for you to consider : 1. During both Phases 0 & 2, is there more inward & outward current current channels open than during Phase 4? 2. If there are more outward current channels open during phase 0 than phase 4, why is the slope of Phase 0 so steep & why is it not rounded? (Revised 7-1-10) 3. Is inward or outward current flow dominating the membrane potential during Phase 0? 4. During Phase 2, is Ca or K current dominating the membrane potential? (Eq Ca = +30 to 50 mv; Eq K = - 95 mv) Phases of the AP - Phase 4 = resting, Phase 0 = upstroke, Phase 1 = initial repolarization, Phase 2 = plateau, Phase 3 = repolarization back to resting. Muscle AP much more complicated than nerve AP. Due to di erent ion channels which behave di erently leading to a very long AP (200-250msec) Phase 4 = Vmr = -70mV, as consequence of open K channels (K current = Ik), small number of Na leak channels (Na current = I(l)Na. Phase 0 = depolarization to threshold --> activation of some fast Na channels (same as in nerves) --> further depolarization activates 2 Ca channels (Latent Ca Channel = I(l) Ca and Transient Ca channel = I(t) Ca) --> activation of additional Ik. There are more open K channels during phase 0 than during phase 4, but we still see pronounced depolarization due to the dominance of the INa and I(t) Ca. Also these K channels are rectified (constrict and allow less ion conductance with further depolarization).
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