aksjonipotentsiaalide

Action potential is a rapid, transient change in the electrical membrane potential that travels along the membrane of excitable cells such as neurons, muscle cells and some endocrine cells. An action potential is generated when the cell membrane depolarizes to reach a threshold, typically around –55 millivolts in neurons, triggering the opening of voltage-gated sodium and sometimes calcium channels. The influx of positively charged ions causes the membrane potential to become positive, reaching a peak of about +30 to +50 millivolts. The membrane then repolarizes as voltage-gated potassium channels open, allowing potassium efflux, and the potential overshoots into a brief hyperpolarized phase before returning to the resting potential.

The all-or-none characteristic of action potentials means that once a threshold stimulus is achieved, the same magnitude of depolarization is produced regardless of strength of the initiating stimulus. This property ensures reliable long-distance signal propagation. Action potentials propagate along axons by local currents generated at the site of the sodium influx, sequentially activating adjacent regions of the membrane. In myelinated axons, saltatory conduction occurs, with the potential jumping between nodes of Ranvier where voltage-gated channels are concentrated.

Reflex arcs often involve action potentials traveling from sensory receptors to motor neurons, which in turn stimulate muscle cells to contract. In endocrine cells, action potentials can trigger the release of hormones. The refractory period, both absolute and relative, restricts the frequency of trains of action potentials and thereby regulates signaling rates. Defects in action potential generation and propagation are implicated in a range of neurological and muscular disorders, including epilepsy, multiple sclerosis, and myasthenia gravis.