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Clamped Membrane Potential

FIGURE 11 Changes in Na+ conductance produced by voltage steps to three depolarized levels. The greater the depolarization, the greater the amplitude of Na+ conductance. These data provide experimental support for the relationship depicted in Fig. 9A. (Modified from Hodgkin AL, Huxley AF. J Physiol 1952; 117:500.)

FIGURE 11 Changes in Na+ conductance produced by voltage steps to three depolarized levels. The greater the depolarization, the greater the amplitude of Na+ conductance. These data provide experimental support for the relationship depicted in Fig. 9A. (Modified from Hodgkin AL, Huxley AF. J Physiol 1952; 117:500.)

an all-or-nothing fashion. A single channel has at least two states—open and closed—and once opened it cannot open further in response to depolarization. The gating process is subserved by a membrane-bound protein that is charged, such that when the membrane is depolarized, a conformational change in the protein takes place that results in the channel becoming more permeable to Na+ (see also below).

In response to membrane depolarization (Fig. 13A), individual channels open briefly and then close (Fig. 13B). The opening of single channels is a probabilistic function of time and voltage; however, when the opening of many channels is averaged, the averaged conductance predicts the conductance change of the entire population of channels (Fig. 13C). Thus, the time course of the changes of Na+ permeability (Fig. 11) is a reflection of the average opening and closing times of many individual Na+ channels. At the molecular level, the voltage dependence of the total membrane Na+ permeability (Fig. 9A) can be viewed as the probability that a depolarization will open single Na+ channels; the more the cell is depolarized, the greater the number of individual Na+ channels that will be opened, each in its characteristic all-or-nothing fashion.

level of depolarization, the greater the Na+ permeability. This experiment therefore provides strong experimental support for the proposal that Na+ permeability is voltage dependent and demonstrates the existence of a mechanism that could explain the rising phase (initiation) of the action potential.

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