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Hyperpolarization (biology)
Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization. It inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold.
Hyperpolarization is often caused by efflux of K+ (a cation) through K+ channels, or influx of Cl– (an anion) through Cl– channels. On the other hand, influx of cations, e.g. Na+ through Na+ channels or Ca2+ through Ca2+ channels, inhibits hyperpolarization. If a cell has Na+ or Ca2+ currents at rest, then inhibition of those currents will also result in a hyperpolarization. This voltage-gated ion channel response is how the hyperpolarization state is achieved. In neurons, the cell enters a state of hyperpolarization immediately following the generation of an action potential. While hyperpolarized, the neuron is in a refractory period that lasts roughly 2 milliseconds, during which the neuron is unable to generate subsequent action potentials. Sodium-potassium ATPases redistribute K+ and Na+ ions until the membrane potential is back to its resting potential of around –70 millivolts, at which point the neuron is once again ready to transmit another action potential.〔Pack, Phillip E. "Cliffs AP Biology 3rd Edition"〕
==Voltage gated ion channels and hyperpolarization==
Voltage gated ion channels respond to changes in the membrane potential, generating the action potential as well as hyperpolarization. These channels work by selecting an ion based on electrostatic attraction or repulsion allowing the ion to bind to the channel.〔Becker, W. M., Kleinsmith, L. J., Hardin, J., & Bertoni, G. P. (2009). Signal Transduction Mechanisms: I. Electrical and Synaptic Signaling in Neurons. The World of the Cell (7th ed., ). San Francisco: Pearson/Benjamin Cummings.〕 This releases water molecules attached to the ion, and the ion passes through the pore.
At resting potential, both the voltage gated sodium and potassium channels are closed, but as the cell membrane becomes depolarized the voltage gated sodium channels begin to open and the neuron begins to depolarize, creating a current feedback loop known as the Hodgkin cycle.〔 However, potassium ions naturally move out of the cell and if the original depolarization event was not significant enough then the neuron does not generate an action potential. If all the sodium channels are open, however, then the neuron becomes ten times more permeable to sodium than potassium, quickly depolarizing the cell to a peak of +40 mV.〔 At this level the sodium channels begin to inactivate (ball and chain model) and voltage gated potassium channels begin to open. This combination of closed sodium channels and open potassium channels leads to the neuron repolarizing. The neuron continues to repolarize until the cell reaches approximately -75 mV,〔 which is the equilibrium potential of potassium ions. At this point the potassium channels close and the natural permeability of the neuron to sodium and potassium allows the neuron to return to its resting potential of -70 mV. During the following refractory period, which is accompanied by an afterhyperpolarization, the neuron is incapable of triggering an action potential due to the sodium channels inability to open.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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