The voltage dependence of Na+ permeability explains the initiation of the action potential but by itself does not explain completely the threshold phenomenon, because the relationship between depolarization and Na+ permeability, although steep, is a continuous function of membrane depolarization (e.g., see Fig. 9A). Threshold can be explained by taking into account the fact that K+ permeability (both resting and voltage-dependent) tends to oppose the effects of increasing Na+ conductance in depolarizing the cell and initiating an action potential. Threshold is the point at which the depolarizing effects of the increased Na+ permeability just exceed the counter (hyperpolarizing) effects of K+ permeability. Once the inward flow of Na+ exceeds the outward flow of K+, threshold is reached, and an action potential occurs through the positive feedback cycle.
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