Depolarisaation

Depolarisation, or depolarization in American English, is the process by which the resting membrane potential of a cell becomes less negative. It is a key excitatory change in many cell types and is often a prerequisite for the initiation of electrical signaling. The term describes a reduction in the electrical potential difference across the plasma membrane, typically moving from around −70 mV toward zero or positive values.

In excitable cells such as neurons and cardiac or skeletal muscle, depolarisation is usually triggered by the opening of voltage-gated ion channels. In neurons, the initial depolarising stimulus opens fast voltage-gated sodium channels, allowing Na+ to enter and rapidly depolarise the membrane. If the depolarisation reaches a threshold, an action potential is generated. Repolarisation follows as Na+ channels inactivate and voltage-gated K+ channels open, driving the membrane potential back toward the resting value, sometimes with a brief hyperpolarisation.

Propagation occurs as depolarisation moves along the cell membrane. In unmyelinated axons, depolarisation spreads to adjacent segments; in myelinated fibers, saltatory conduction speeds the process. In the heart, a wave of depolarisation coordinates contraction, with the sequence and timing reflected in electrocardiographic signals (for example, atrial and ventricular depolarisation).

Other contexts include plant cells, where depolarisation can occur during slower action potentials involving Ca2+ and ion flux. Measurement and study employ intracellular or patch-clamp electrodes, extracellular recordings, and voltage-sensitive dyes to monitor changes in membrane potential.

See also resting potential, action potential, repolarisation, hyperpolarisation.