An action potential is a brief electrical signal that is generated by neurons and muscle cells. It is an all-or-nothing event, meaning that once it is initiated, it will continue to completion without any further stimulus.

The action potential is generated when the neuron or muscle cell is depolarized, meaning that the electrical charge inside the cell becomes more positive. This depolarization is typically caused by the influx of positively charged ions, such as sodium, through channels in the cell membrane.

Once the depolarization reaches a certain threshold, it triggers the opening of voltage-gated ion channels, which allow a large influx of sodium ions into the cell. This rapid influx of positively charged ions causes the membrane potential to become even more positive, creating a positive feedback loop that rapidly depolarizes the cell and generates the action potential.

Following the peak of the action potential, the cell repolarizes by allowing positively charged ions to exit the cell and/or negatively charged ions to enter the cell. This repolarization restores the cell to its resting membrane potential and prepares it for another action potential.

Action potentials are critical for communication between neurons and for the contraction of muscle cells. They allow for rapid and precise signaling and are essential for many physiological processes, including movement, sensation, and thought.