hyperpolarization

Hyperpolarization is when the inside of a neuron becomes even more negatively charged than it is at rest — it sinks further below its normal baseline. Picture a neuron's electrical state as the water level in a tank. At rest, the level already sits below the rim by a fixed amount (the inside is about 70 thousandths of a volt more negative than the outside). Hyperpolarization is when the water level drops even lower than that resting mark. So if depolarization is the cell getting more excited and closer to firing, hyperpolarization is the opposite: the cell becoming calmer, more negative, and harder to set off.

This happens when the balance of charged particles flowing across the cell's outer skin shifts. A neuron's membrane is dotted with tiny gates called ion channels that let specific charged atoms in or out. When positively charged potassium atoms rush out, or negatively charged chloride atoms flow in, the inside loses positive charge and tips further negative — that is hyperpolarization. It matters because it pushes the cell away from the threshold needed to fire its signal, so it acts like a brake. The brain uses it both to quiet neurons down (inhibition, the way some messages tell a cell to stay silent) and as the natural overshoot at the tail end of a firing event, briefly making the cell rest before it can fire again.