Depolarization

In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is essential to the function of many cells, communication between cells, and the overall physiology of an organism. It is especially important to electrical signaling in neurons and muscle cells. It also affects many non-excitable cells by changing calcium regulation or gene expression.

Action potential in a neuron, showing depolarization, in which the cell's internal charge becomes less negative (more positive), and repolarization, where the internal charge returns to a more negative value.

Most cells in higher organisms maintain an internal environment that is negatively charged relative to the cell's exterior. This difference in charge is called the cell's membrane potential. In the process of depolarization, the negative internal charge of the cell temporarily becomes more positive (less negative). This shift from a negative to a more positive membrane potential occurs during several processes, including an action potential. During an action potential, the depolarization is so large that the potential difference across the cell membrane briefly reverses polarity, with the inside of the cell becoming positively charged.

The change in charge typically occurs due to an influx of sodium ions into a cell, although it can be mediated by an influx of any kind of cation or efflux of any kind of anion. The opposite of a depolarization is called a hyperpolarization.

Usage of the term "depolarization" in biology differs from its use in physics, where it refers to situations in which any form of electrical polarity ( i.e. the presence of any electrical charge, whether positive or negative) changes to a value of zero.

Depolarization is sometimes referred to as "hypopolarization" (as opposed to hyperpolarization).