FIGURE 22-12 Unidirectional conduction of an action potential due to transient inactivation of voltage-gated Na⁺ channels. At time 0, an action potential (pink line) is at the 2-mm position on the axon; the Na⁺ channels at this position are open (green shading), and Na⁺ ions are flowing inward. The excess Na⁺ ions diffuse in both directions along the inside of the membrane, passively spreading the depolarization in both directions (curved pink arrows). But because the Na⁺ channels at the 1-mm position are still inactivated (red shading), they cannot yet be reopened by the small depolarization caused by passive spread; the Na⁺ channels at the “downstream” 3-mm position, in contrast, begin to open. Each region of the membrane is refractory (inactive) for a few milliseconds after an action potential has passed. Thus the depolarization at the 2-mm site at time 0 triggers action potentials downstream only; at 1 ms an action potential is passing the 3-mm position, and at 2 ms an action potential is passing the 4-mm position.