The action potential is a wave of transient depolarization that travels along the membrane of a nerve cell (or any other kind of excitable cell, such as a muscle cell) as a result of the movements of ions across the membrane. (Figure 10.31).

At the resting potential of a nerve cell, K⁺ is much closer to its equilibrium distribution than is Na⁺. When the membrane becomes fully permeable to ions, there is a massive influx of sodium ions, with an accompanying shift in the membrane potentials toward Na+.

This influx of sodium ions is what happens when an action potential is transmitted along a nerve (Figure 10.32). The action potential is generated and propagated because a small depolarization of the nerve cell membrane opens gated channels, allowing ions to flow through.