Neurophysiology. The all-or-none response of a nerve or muscle cell that is used to communicate information from one cell to another. An action potential is a brief change in membrane potential (depolarization) of neurons or muscle cells caused by the rapid opening and closing of ion channels thus allowing passage of specific ions. Action potentials travel along axons with a speed of 1 to 120 m/s. The cycle of an action potential includes depolarization, hyperpolarization and repolarisation and lasts for about 1to 2 ms.