repolarization

Repolarization is the recovery phase of a nerve cell's electrical signal — the moment when the cell, having briefly flipped its inside from negative to positive, swings back toward its normal negative resting state. Picture a neuron at rest as a tiny battery whose inside is kept slightly negative compared with the outside. When a signal called an action potential fires, that charge suddenly reverses and the inside shoots positive; repolarization is the bounce-back, the part of the spike where the voltage drops back down and the cell returns to its usual negative value, ready to fire again. If depolarization is the upstroke of the signal, repolarization is the downstroke.

What makes it happen is a careful changing of the guard among tiny gates in the cell's outer wall called ion channels, which let electrically charged particles (ions) in or out. During the rise, gates that let positive sodium in snap shut, and a beat later, gates that let positive potassium out swing open. As potassium rushes outward, it carries positive charge away with it, so the inside grows negative again and the membrane voltage falls back toward rest. Potassium tends to overshoot slightly, dipping the cell even more negative than usual for a brief stretch, which helps stop the same signal from doubling back, so messages travel cleanly in one direction.

Repolarization matters because it resets the cell. Without this swift return to a negative baseline, a neuron could not fire a clean, separate signal again, and rapid trains of impulses — the basis of thought, sensation, and movement — would smear into one long jumble. In heart muscle the same recovery sets the rhythm of each beat, which is why drugs or genetic faults that slow repolarization can cause dangerous irregular heartbeats.