Hyperpolarisaatio

Hyperpolarisation, also spelled hyperpolarization, is the process by which a cell’s membrane potential becomes more negative than its resting value. In excitable cells such as neurons and cardiac myocytes, it typically follows an action potential or results from inhibitory synaptic input, reducing the likelihood of immediate subsequent excitation.

The common mechanism in neurons involves an outward flow of positive charge or an inward flow of negative charge. After an action potential, voltage-gated potassium channels remain open, allowing K+ to exit the cell and drive the membrane potential further from threshold. Inhibitory neurotransmitters such as GABA or glycine can open chloride channels, causing Cl− to enter the cell and contribute to hyperpolarisation. The Na+/K+ ATPase and leak currents gradually restore the resting potential.

Functionally, hyperpolarisation increases the distance of the membrane potential from the threshold, suppressing neuronal firing for a short period. It contributes to the afterhyperpolarization (AHP) that follows an action potential and shapes spike timing, refractory periods, and synaptic integration. The magnitude and duration of hyperpolarisation depend on ion gradients, receptor types, and the kinetics of ion channels.

In some cells, hyperpolarisation interacts with hyperpolarisation-activated currents, such as the HCN channel–mediated I_h, which are activated by hyperpolarization and can influence rhythmicity and rebound excitability. Hyperpolarisation is also relevant to cardiac electrophysiology, where it helps regulate pacemaker activity and conduction.

See also: resting potential, action potential, inhibitory postsynaptic potential, afterhyperpolarization.