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KEY problem set 2 - BIOLOGY 222 Winter 2012 Problem set 2...

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BIOLOGY 222 Winter 2012 Problem set 2 KEY From questions 1 to 11, information in the figures and texts are cumulative. Figure 1: Intracellular current-clamp recording of an action potential in an axon. 1. Considering the plasma membrane at rest is permeable only to Na + and K + , what is the ratio of conductances between Na + and K + (E Na = +55 mV; E K = -80 mV)? a. 1; b. 0.32; c. 0.16; d. 0.08; e. 0.0001.
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2. During the action potential, at the point of maximal depolarization, what is true? If conductances for Na + and K + were the same, the membrane potential would be, instead of +30 mV. At the most depolarized peak of the action potential, there is a maximum of open Na + channels and V m hyperpolarizes as more voltage-gated Na + channels inactivate; the driving force for Na + is , while for K + it is , so the driving force is actually larger for K + ; since in addition to leak channels, there are also both voltage-dependent Na + channels and K + channels open, both conductances are indeed increased when compared to rest; finally, since there is a driving force for K + and both leak and voltage-gated K + channels open, there is a K + current. 3. The fact that the action potential undershoots under the resting potential can be explained by: The intracellular ionic concentrations are maintained by the Na + -K + -ATPase and the ionic fluxes during the action potential are not large enough to change them; besides, reducing the K + concentration gradient would depolarize the cell, not the other way around; during the repolarization, voltage-gated Na + channels are inactivated, while delayed rectifier K + channels open with slow kinetics; since the ratio between conductance to K + and Na + is even higher than at rest, the membrane potential approaches E K even more, i.e. the membrane is hyperpolarized. 4. During the relative refractory period, During the relative refractory period, the threshold is more depolarized than at rest, since a larger depolarization is required for the inwards current to prevail over the increased outward current, since more delayed rectifier K + channels are open than at rest. When Na + channels are almost completely inactivated, another action potential cannot be triggered, i.e., we are in the absolute refractory period. Even in the relative refractory period, the action potential is all-or-
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