depolarisatsioon

Depolarisatsioon is a process in which a cell’s resting membrane potential becomes less negative, often leading to an action potential. In excitable cells such as neurons and muscle fibers, depolarization is the initial step that enables nerve signaling and muscle contraction. Normally, the inside of a neuron is about −70 millivolts relative to the outside. Depolarization occurs when the membrane becomes permeable to positive ions, most commonly sodium, allowing Na+ to enter the cell and raise the membrane potential toward zero and beyond, typically to about +30 to +40 mV during an action potential. The exact values vary by cell type.

In neurons and other electrically active cells, depolarization is typically triggered when a stimulus reaches a threshold, causing a rapid opening of voltage-gated sodium channels. The resulting Na+ influx produces a rapid, all-or-nothing rise in membrane potential. In some cells, especially cardiac pacemaker cells, calcium ions also contribute to depolarization.

Repolarization follows as voltage-gated potassium channels open and Na+ channels inactivate, allowing K+ to leave the cell and restore the negative internal environment. A brief period of hyperpolarization may occur before the resting potential is reestablished.

Depolarisation is distinct from hyperpolarisation, which makes the membrane potential more negative than the resting level. Proper depolarization and repolarization are essential for the propagation of electrical signals in the nervous system and for coordinated muscle and heart activity. Abnormal depolarization patterns can be involved in various disorders, including seizures and certain cardiac arrhythmias, and are studied through electrophysiological techniques.