actionpotential

An action potential is a rapid, transient change in the membrane potential of excitable cells, such as neurons and some muscle cells, that travels along the cell membrane. In neurons, it is typically initiated when a depolarizing stimulus raises the membrane potential to a threshold, often around -55 mV, triggering the opening of voltage-gated sodium channels.

The rapid inward Na+ current during depolarization drives the membrane potential toward positive values, producing the upstroke of the action potential. At the peak, sodium channels inactivate, and voltage-gated potassium channels open, allowing K+ efflux that repolarizes the cell. The membrane may briefly dip below the resting potential (hyperpolarization) before the Na+/K+ ATPase restores the resting state.

Action potentials are generally considered all-or-none: once threshold is reached, an action potential of a characteristic amplitude occurs, and varying stimulus strength influences only the probability or rate of firing, not the size of each spike. Propagation along the axon occurs as local currents depolarize adjacent regions to threshold, regenerating the action potential along the membrane. In myelinated axons, saltatory conduction between nodes of Ranvier speeds transmission, while in unmyelinated fibers conduction is continuous.

Two refractory periods regulate firing: an absolute refractory period during which no new action potential can be produced because Na+ channels are inactivated, and a relative refractory period afterward during which a stronger-than-normal stimulus is required. Action potentials enable rapid, long-distance signaling and coordination between neurons and with muscle tissue.