Action potential

An action potential is a rapid, temporary change in voltage across a cell membrane. It occurs in excitable cells, including neurons and muscle cells. In a neuron, an action potential can propagate along an axon, transmitting an electrical signal toward its terminals, where it can contribute to communication with other cells.

When depolarizing input brings a neuron's membrane to threshold, voltage-gated sodium channels open and sodium ions enter, producing a rapid rising phase. Sodium channels then inactivate while voltage-gated potassium channels open; potassium ions leave the cell, helping return the membrane voltage toward its resting level. Potassium channels may close more slowly, briefly producing an undershoot below the resting voltage. Ion gradients provide the conditions for these events, and the sodium–potassium pump helps maintain those gradients over time.

Local currents from an active region depolarize adjacent membrane, regenerating the action potential along the axon. In myelinated axons, insulating myelin limits current loss across covered segments, and regeneration occurs at gaps called nodes of Ranvier; this node-to-node mode is called saltatory conduction. After an action potential, inactivated sodium channels create an absolute refractory period when another spike cannot be initiated in that region. A relative refractory period follows, during which a stronger-than-usual stimulus may be needed. These periods help limit backward propagation.

For a given neuron under typical conditions, action-potential amplitude is relatively stereotyped rather than increasing in proportion to stimulus strength. Information about input strength can instead be represented in part by the timing and frequency of action potentials. The detailed waveform and firing patterns vary across cell types and depend on their ion channels and other cellular properties.

Action potentials also occur in muscle and other excitable cells, but their ionic mechanisms can differ. For example, cardiac pacemaker cells rely substantially on calcium currents during depolarization, whereas rapid depolarization in many neurons and working heart muscle cells is associated with sodium-channel opening.