Action Potential

An action potential is a rapid, temporary change in the electrical potential across the membrane of an excitable cell. Action potentials allow electrical signals to travel along neurons and also occur in other excitable cells, including skeletal and cardiac muscle cells. In neurons, an action potential is typically triggered when membrane depolarization reaches a threshold that activates voltage-gated ion channels .^1,2

In a typical neuronal action potential, voltage-gated sodium channels open rapidly, allowing sodium ions to enter the cell and causing further depolarization. The sodium channels then inactivate while slower voltage-gated potassium channels open. Potassium leaves the cell, driving repolarization toward the resting membrane potential. Because potassium channels can remain open briefly, the membrane may temporarily become more negative than its resting level, producing an after-hyperpolarization .^1,2

Action potentials are described as all-or-none events. Once threshold is reached, the regenerative opening of voltage-gated channels produces a full action potential rather than one whose amplitude increases continuously with stimulus strength. Refractory periods following activation limit how rapidly another action potential can be generated and reduce immediate re-excitation of recently active membrane .^1,2

An action potential should therefore be distinguished from a graded potential, such as a receptor or postsynaptic potential. Graded potentials vary in size with the stimulus and decay as they spread, whereas action potentials regenerate along excitable membrane and can transmit electrical information over long distances without progressively decreasing in amplitude .^2,3