action potential (AP)

Action potential propagating along a nerve fibre. A, AP recorded between electrodes inside and outside an axon. B, Voltmeter placed to record the potential difference between external electrodes at two points along an axon. This and the voltage differences across the membrane (+ -) are indicated for three successive stages of propagation (1, 2, 3). C, Record of voltage changes with time, corresponding to stages 1, 2, 3, in digram B.

action potential (AP)

transient voltage change propagating along the membrane of nerve, muscle or other excitable cell; the means whereby information flows rapidly along the cell's length. Triggered by a small depolarization, an AP consists of further depolarization and often an overshoot to an inside-positive membrane potential. The depolarization/positive swing is caused by the inflow of sodium and/or calcium ions, according to the tissue, and the return to normal inside-negative potential by the subsequent outflow of potassium ions. Metabolic energy is used in maintaining the differences between the intracellular and extracellular concentrations of these ions, but not in the AP itself. See also electromyography (EMG), nerve fibre, neuromuscular junction, sarcoplasmic reticulum (SR), t-tubes.
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