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FIGURE 1 Schematic diagram of sequence of steps underlying propagation of the action potential. (A) Rest, (B) initiation, (C) propagation.

isolated section of a nerve axon. When an axon is at rest, the potential of the inside is negative with respect to the outside (Fig. 1A). The distribution of charge is simply due to the tendency of K+ to diffuse from its region of high concentration inside the axon to its region of low concentration outside the axon. Consider what will happen if at some point along the axon an action potential is initiated. At the peak of the action potential the inside of the cell will be positive with respect to the outside (Fig. 1B). At this point, there is a new charge distribution at a localized portion of the membrane. Adjacent regions of the axon, however, still have their initial charge distribution (inside negative). Opposite charges attract each other, so the positive charge produced by the action potential will tend to move toward the adjacent region of membrane (still at rest), which has a negative charge (Fig. 1C). As a result of this positive charge movement, the adjacent region of the axon will become depolarized. If sufficient charge moves to depolarize the adjacent portion of the membrane to threshold and elicit voltage-dependent changes in Na+ permeability, a new action potential at this adjacent region will be initiated. So, as a result of an action potential in one portion of an axon and the subsequent charge transfer along the surface of the membrane, a new action potential will be generated. This new action potential then will cause charge transfer to its adjacent region, causing, in a sense, another new action potential to be initiated. It should be clear that this process, once initiated, will propagate all the way to the end of the axon.

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