Hyperpolarization

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Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization.

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.

Because hyperpolarization is a change in membrane voltage, electrophysiologists measure it using current clamp techniques. In voltage clamp, the membrane currents giving rise to hyperpolarization are either an increase in outward current, or a decrease in inward current.

Examples

Diagram of membrane potential changes during an action potential

1. During the afterhyperpolarization period after an action potential, the membrane potential is more negative than when the cell is at the resting potential. In the figure to the right, this undershoot occurs from approximately 3 to 4 milliseconds (ms) on the time scale. The afterhyperpolarization is the time when the membrane potential is hyperpolarized relative to the resting potential.
2. During the rising phase of an action potential, the membrane potential changes from negative to positive, a depolarization. In the figure, the rising phase is from approximately 1 to 2 ms on the graph. During the rising phase, once the membrane potential becomes positive, the membrane potential continues to depolarize (overshoot) until the peak of the action potential is reached at about +40 millivolts (mV). After the peak of the action potential, a hyperpolarization returns the membrane potential to its resting value, first by making it less positive, until 0 mV is reached, and then by continuing to make it more negative. This hyperpolarization occurs in the figure from approximately 2 to 3 ms on the time scale.

External links

• Neuroscience - online textbook by Purves, et al.
• Basic Neurochemistry Molecular, Cellular, and Medical Aspects by Siegel, et al.