Repolarisation

Repolarisation, also repolarization, is the process by which excitable cells return to their resting membrane potential after depolarisation. It is essential for resetting the electrical state of neurons and muscle cells and for enabling subsequent action potentials. In neurons and cardiac myocytes, action potentials rise during depolarisation and fall as repolarisation proceeds.

In neurons, after the upstroke, voltage-gated Na+ channels inactivate and voltage-gated K+ channels open, producing an outward K+ current that drives the membrane potential back toward the resting value, typically around -65 to -70 mV. In cardiac myocytes, repolarisation occurs mainly during phase 3 of the ventricular action potential. The plateau phase (phase 2) involves Ca2+ influx balancing K+ efflux; repolarisation ensues when K+ efflux dominates, through delayed rectifier currents I_Kr and I_Ks and inward rectifier currents. The return to the resting potential is maintained by ion pumps such as Na+/K+ ATPase and background currents.

Repolarisation contributes to the refractory period, limiting re-excitation during the same cardiac cycle and helping coordinate heart rhythm. On the electrocardiogram, repolarisation is reflected in the T wave; the duration of repolarisation partly determines the QT interval.

Abnormal repolarisation can indicate pathology or pharmacologic effects. Factors include electrolyte disturbances (K+, Ca2+, Mg2+), ischemia, drugs that modify K+ channels, and genetic channelopathies. Overall, repolarisation is the process that restores the cell to its resting state after depolarisation, enabling periodic, regulated signaling.