Hyperpolarization (biology)

Hyperpolarization is a change in a cell's membrane potential that makes it more negative. Living cells typically have a negative resting potential. Animal excitable cells (neurons, muscle cells or gland cells), as well as cells of other organisms, may have their membrane potential temporarily deviate from the resting value. This is one of many mechanisms of cell signaling.

In excitable cells, activation is typically achieved through depolarization, i.e., the membrane potential deviating towards less negative values. Thus, hyperpolarization, as an opposite process, makes the cell more difficult to activate. When the membrane potential is more negative, a stronger stimulus is needed to surpass the activation threshold.

Neurons naturally become hyperpolarized at the end of an action potential, which is often referred to as the relative refractory period. Relative refractory periods typically last 2 milliseconds, during which a stronger stimulus is needed to trigger another action potential. Cells can also become hyperpolarized depending on channels and receptors present on the membrane, which can have an inhibitory effect.

Hyperpolarization is often caused by efflux of K⁺ (a cation) through K⁺ channels, or influx of Cl^– (an anion) through Cl^– channels. On the other hand, influx of cations, e.g. Na⁺ through Na⁺ channels or Ca²⁺ through Ca²⁺ channels, inhibits hyperpolarization. If a cell has Na⁺ or Ca²⁺ currents at rest, then inhibition of those currents will also result in hyperpolarization. This voltage-gated ion channel response is how the hyperpolarization state is achieved.