Neurophysiology. An increase of the membrane potential of a neuron above the resting potential, i.e. the membrane potential moves farther from zero. The inside of the cell becomes more negative than at its resting state. Hyperpolarization occurs after the depolarization of an action potential. No new action potential can be triggered during hyperpolarization. Thus hyperpolarization contributes to the refractory period of the neuron and protects it against overexcitement.