Repolarization

In neuroscience, repolarization refers to the change in membrane potential that returns it to a negative value just after the depolarization phase of an action potential which has changed the membrane potential to a positive value. The repolarization phase usually returns the membrane potential back to the resting membrane potential. The efflux of potassium (K⁺) ions results in the falling phase of an action potential. The ions pass through the selectivity filter of the K⁺ channel pore.

Repolarization typically results from the movement of positively charged K⁺ ions out of the cell. The repolarization phase of an action potential initially results in hyperpolarization, attainment of a membrane potential, termed the afterhyperpolarization, that is more negative than the resting potential. Repolarization usually takes several milliseconds.^[1]

Repolarization is a stage of an action potential in which the cell experiences a decrease of voltage due to the efflux of potassium (K⁺) ions along its electrochemical gradient. This phase occurs after the cell reaches its highest voltage from depolarization. After repolarization, the cell hyperpolarizes as it reaches resting membrane potential (−70 mV in neuron). Sodium (Na⁺) and potassium ions inside and outside the cell are moved by a sodium potassium pump, ensuring that electrochemical equilibrium remains unreached to allow the cell to maintain a state of resting membrane potential.^[2] In the graph of an action potential, the hyper-polarization section looks like a downward dip that goes lower than the line of resting membrane potential. In this afterhyperpolarization (the downward dip), the cell sits at more negative potential than rest (about −80 mV) due to the slow inactivation of voltage gated K⁺ delayed rectifier channels, which are the primary K⁺ channels associated with repolarization.^[3] At these low voltages, all of the voltage gated K⁺ channels close, and the cell returns to resting potential within a few milliseconds. A cell which is experiencing repolarization is said to be in its absolute refractory period. Other voltage gated K⁺ channels which contribute to repolarization include A-type channels and Ca²⁺-activated K⁺ channels.^[4] Protein transport molecules are responsible for Na⁺ out of the cell and K⁺ into the cell to restore the original resting ion concentrations.^[5]