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Homework Problems #1 and #2 Homework Problem #1- from Chapter 3: In Figure 3.6c, you can see that the Na channel conductance increases steadily with increasing depolarization. Yet, in the voltage clamp study in Figure 3.2 , you see that the inward sodium current (the early current), gets larger initially, but then declines as the voltage command is set at successively more positive levels. Explain why this is and explain what we mean when we refer to a current’s “reversal potential”. Answer: Figure 3.6c shows that the # of open sodium channels (the conductance ) increases monotonically with increasing depolarization. Note that while, for any given voltage step, this is initially true, there is also a time-dependent rate of closing that occurs even while the cell is depolarized—so Fig. 3.6C shows the peak conductance obtained after each voltage step. So how can we explain the “biphasic” change in sodium current with increasingly larger voltage steps? Initially, few channels are open and there is a small current (although the driving force—the voltage difference between Vm and E Na—is large ). As one steps to 0 mV, one sees a bigger current. At this point the driving force has decreased! So one must conclude that there are many more channels open, to yield a larger current in spite of the smaller driving force or driving potential. From this point on, the current gets smaller and goes to zero. This is because, at 0 mV, the number of Na+ channels open is near its peak, and as we go more positive, and get closer and closer to E Na , the driving force gets smaller and smaller, while the conductances stays constant (and maximal). At E Na , there is zero NET current, because the sodium rushing into the

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