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During the nerve impulse, current is drawn externally to the relatively negative region and from the "resting," more positive, undisturbed region of the membrane. If the momentarily depolarized zone is at one end of the nerve fiber, then the depolarization will move, as an action potential, away from that end. This "depolarization" has a dramatic effect on the channels. The Na⁺ channels are triggered to open, and Na⁺ ions flood inward (broken line in Fig. 10), making that region more negative outside. Current flows to this newly depolarized zone, and channels open in the area from which the current is drawn. Thus the impulse advances by electrotonic spread of progressive depolarization along the nerve fiber. In this way, the depolarization advances as a propagating wave, the nerve impulse, away from its site of origin.

The increase in Na⁺ conductance is transient, lasting only 1-2 msec. The depolarized region serves as a sink toward which current flows from the recovering zone in its wake as well as from the region ahead that is being depolarized.

Figure 10. Local change in conductance gives rise to longitudinal current.

 
 
At the same time, and in the same place, but more slowly, the K⁺ channels open and K⁺ is driven outward by its concentration gradient as Na⁺ conductance is declining back to its resting level. Thus, following the Na⁺ spike, the membrane moves toward the theoretical K⁺ potential, E_K = –l00 mV inside. With decline of the Na⁺ and K⁺ permeabilities, the membrane potential relaxes back toward the rest level. The recovering zone then serves as a source of current to the depolarized zone, which has now moved forward.

Thus there exist, in opposite directions, longitudinal eddy currents at the leading and at the trailing edge of the impulse center of maximum depolarization.

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