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.
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. 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. 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 Ca-activated K channels. Protein transport molecules are responsible for Na out of the cell and K into the cell to restore the original resting ion concentrations.