An action potential is a rapid sequence of changes in the voltage across a membrane, which is determined at any time by the relative ratio of ions, extracellular to intracellular, and the permeability of each ion. It is a brief change in the voltage across the membrane due to the flow of certain ions into and out of the neuron. Action potentials occur in neurons, cardiac muscle, and some endocrine cells.

The initiation of a neuronal action potential usually occurs at the axon hillock, the most proximal segment of an axon. During depolarization, voltage-gated sodium ion channels open due to an electrical stimulus. As the sodium ions rush back into the cell, their positive charge changes potential inside the cell from negative to more positive. If a threshold potential is reached, then an action potential is produced. Action potentials will only occur if a threshold is reached. Additionally, if the threshold is reached, then the response of the same magnitude is always elicited, irrespective of the strength of the stimulus. Hence, action potentials can be described as “all-or-nothing” .

Action potentials are propagated along the axons of neurons via local currents. Local currents induce depolarization of the adjacent axonal membrane. Where this reaches a threshold, further action potentials are generated. The areas of the membrane that have recently depolarized will not depolarize again due to the refractory period – meaning that the action potential will only travel in one direction.