A sudden, large, transient change in the membrane potential of a cell (i.e. a depolarization) that is capable of propagating to other parts of the cell. It therefore can act as means of signalling to adjacent cells. A prime example is the passage of a nerve impulse, when an action potential travels in a wavelike manner along the axon of a nerve; it causes a localized and transient switch in electric potential across the membrane, typically from −70 mV (millivolts; the resting potential) to +40 mV in a mammalian neuron. Essentially the change in electric potential is caused by an influx of sodium ions (Na⁺) via voltage-gated ion channels, which makes the inside of the cell electrically positive with respect to the outside. Then the sodium channels close and voltage-gated potassium channels open, allowing the outflow of potassium ions (K⁺). This has the effect of restoring the resting potential. Nervous stimulation of a muscle fibre also produces an action potential.

Plants too propagate action potentials, although these travel much more slowly than in neurons. They can transmit signals throughout the plant in response to a localized stimulus, such as the progressive folding of the leaflets of a sensitive plant (Mimosa pudica) in response to touching just a single leaflet. Compare graded potential. See also myelin sheath.