afterhyperpolarization

Afterhyperpolarization, often abbreviated as AHP, refers to a transient increase in the membrane potential of a neuron that occurs after an action potential has been generated and repolarized. Following the peak of the action potential and the subsequent repolarization phase, the membrane potential briefly becomes more negative than the resting membrane potential. This period of hyperpolarization is a key feature of neuronal excitability and plays a significant role in regulating the firing rate of neurons.

The mechanism underlying AHP is primarily attributed to the activation of voltage-gated potassium channels. During the action potential, these channels open, allowing potassium ions to flow out of the cell, contributing to repolarization. However, some potassium channels remain open for a short period even after the membrane potential has returned to its resting level. This sustained outward flow of potassium ions causes the membrane potential to dip below the resting potential, creating the afterhyperpolarization. The duration and amplitude of the AHP can vary depending on the type of neuron and the specific ion channels involved. For instance, different types of potassium channels, such as calcium-activated potassium channels, can contribute to various forms of AHP with distinct kinetics.

The functional significance of AHP is multifaceted. It contributes to the refractory period of a neuron, preventing it from firing another action potential immediately after the previous one. This ensures that action potentials are discrete events and helps to regulate the frequency of neuronal firing. In some neurons, particularly in the central nervous system, a prolonged form of AHP can limit sustained high-frequency firing and shape complex firing patterns, influencing synaptic integration and information processing. Therefore, AHP is an essential component of neuronal electrophysiology that modulates neuronal excitability and contributes to the dynamic behavior of neural circuits.