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. In this diagram, the afterhyperpolarization is labeled "refractory period", although other processes are also involved in the refractory period.

Afterhyperpolarization or AHP describes the hyperpolarization 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. Afterhyperpolarization in part causes the refractory period, the time period following an action potential during which a neuron cannot initiate a subsequent action potential.

During the action potential, the raised voltage opens many more potassium channels than usual, and some of these do not close right away when the membrane returns to its normal resting voltage. In addition, further potassium channels open in response to the influx of calcium ions during the action potential. The potassium permeability of the membrane is transiently unusually high, driving the membrane voltage V_m even closer to the potassium equilibrium voltage E_K. Hence, there is an undershoot or hyperpolarization that persists until the membrane potassium permeability returns to its usual value.

Both medium and slow afterhyperpolarization currents can be demonstrated in neurons. The SK channel, a calcium-activated potassium channel, is the major mediator of afterhyperpolarization currents, although other candidate mediators have been suggested.

See also

• Action Potential
• Refractory period (physiology)