Afterhyperpolarization, or AHP, is the hyperpolarizing phase of a neuron's

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

where the cell's

membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...

falls below the normal

resting potential A relatively static membrane potential which is usually referred to as the ground value for trans-membrane voltage. The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as opp ...

. 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. During single action potentials, transient depolarization of the membrane opens more voltage-gated K⁺ channels than are open in the resting state, many of which do not close immediately when the membrane returns to its normal resting voltage. This can lead to an "undershoot" of the membrane potential to values that are more polarized ("hyperpolarized") than was the original resting membrane potential. Ca²⁺-activated K⁺ channels that open in response to the influx of Ca²⁺ during the action potential carry much of the K⁺ current as the membrane potential becomes more negative. The K⁺ permeability of the membrane is transiently unusually high, driving the membrane voltage ''V''_''M'' even closer to the K⁺ equilibrium voltage ''E''_K. Hence, hyperpolarization persists until the membrane K⁺ permeability returns to its usual value.Purves ''et al.'', p. 37;

Bullock Bullock may refer to: Animals * Bullock (in British English), a castrated male bovine animal of any age * Bullock (in North America), a young bull (an uncastrated male bovine animal) * Bullock (in Australia, India and New Zealand), an ox, an adu ...

, Orkand, and Grinnell, p. 152. Medium and slow AHP currents also occur in neurons. The ionic mechanisms underlying medium and slow AHPs are not yet well understood, but may also involve M current and

HCN channel Hyperpolarization-activated cyclic nucleotide–gated (HCN) channels are integral membrane proteins that serve as nonselective voltage-gated cation channels in the plasma membranes of heart and brain cells. HCN channels are sometimes referred to ...

s for medium AHPs, and ion-dependent currents and/or ionic pumps for slow AHPs. The afterhyperpolarized (sAHP) state can be followed by an afterdepolarized state (which is not to be confused with the cardiac

afterdepolarization Afterdepolarizations are abnormal depolarizations of cardiac myocytes that interrupt phase 2, phase 3, or phase 4 of the cardiac action potential in the electrical conduction system of the heart. Afterdepolarizations may lead to cardiac arrhythm ...

) and can thus set the phase of the subthreshold oscillation of the membrane potential, as reported for the stellate cells of the

entorhinal cortex The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time.Integrating time from experience in the ...

. This mechanism is proposed to be functionally important to maintain the spiking of these neurons at a defined phase of the theta cycle, that, in turn, is thought to contribute to encoding of new memories by the

medial temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in pro ...

of the brain

References

{{Reflist Electrophysiology Electrochemistry Cellular processes Membrane biology Cellular neuroscience