Afterhyperpolarization

Schematic of an electrophysiological recording of an action potential, showing the various phases that occur as the voltage wave passes a point on a cell membrane. The afterhyperpolarisation is one of the processes that contribute to the refractory period.

Afterhyperpolarization, or AHP, is the hyperpolarizing phase of a neuron's action potential where the cell's membrane potential falls below the normal resting potential. This is also commonly referred to as an action potential's undershoot phase. AHPs have been segregated into "fast", "medium", and "slow" components that appear to have distinct ionic mechanisms and durations. While fast and medium AHPs can be generated by single action potentials, slow AHPs generally develop only during trains of multiple action potentials.