Hyperpolarization is a change in a
cell'sThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
membrane potentialMembrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
that makes it more negative. It is the opposite of a
depolarizationIn biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
.
Hyperpolarization is often caused by efflux of
K+Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
(a cation) through
K+ channelsIn the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes...
, or influx of
Cl–The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
(an anion) through
Cl– channelsChloride channels are a superfamily of poorly understood ion channels consisting of approximately 13 members.Chloride channels display a variety of important physiological and cellular roles that include regulation of pH, volume homeostasis, organic solute transport, cell migration, cell...
. On the other hand, influx of cations, e.g.
Na+Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
through Na
+ channels or
Ca2+Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
through
Ca2+ channelsA Calcium channel is an ion channel which displays selective permeability to calcium ions. It is sometimes synonymous as voltage-dependent calcium channel, although there are also ligand-gated calcium channels.-Comparison tables:...
, inhibits hyperpolarization. If a cell has Na
+ or Ca
2+ currents at rest, then inhibition of those currents will also result in a hyperpolarization.
Because hyperpolarization is a change in membrane
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
,
electrophysiologistsElectrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...
measure it using current clamp techniques. In
voltage clampThe voltage clamp is used by electrophysiologists to measure the ion currents across the membrane of excitable cells, such as neurons, while holding the membrane voltage at a set level. Cell membranes of excitable cells contain many different kinds of ion channels, some of which are voltage gated...
, the membrane currents giving rise to hyperpolarization are either an increase in outward current, or a decrease in inward current.
Examples
- During the afterhyperpolarization
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...
period after an action potentialIn physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
, the membrane potential is more negative than when the cell is at the resting potentialThe relatively static membrane potential of quiescent cells is called the resting membrane potential , as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane 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.
- 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.
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