Action potential

An action potential, often called a nerve impulse, is the electrical signal that excitable cells such as neurons and muscle fibers use to carry messages. At rest, the inside of a neuron is negative compared with the outside, at about minus 70 millivolts, because the sodium-potassium pump and leak channels keep sodium concentrated outside the cell and potassium inside.

When a stimulus depolarizes the membrane to threshold, around minus 55 millivolts, voltage-gated sodium channels open and sodium rushes in, briefly making the inside positive. The sodium channels then shut and voltage-gated potassium channels open, letting potassium out and restoring the negative charge. The voltage dips slightly below resting level, a step called hyperpolarization, before settling back. The whole event lasts only a couple of milliseconds.

Action potentials are all-or-none: each one reaches the same size, so a stronger stimulus raises how often a neuron fires rather than how big each spike is. A refractory period after every spike stops the signal from running backward. In unmyelinated axons the impulse spreads continuously along the membrane, while in myelinated axons it jumps between nodes of Ranvier, a faster process called saltatory conduction.