In biology, depolarization is a positive-going change in a cell's membrane potential, making it more positive, or less negative, and thereby removing the polarity that arises from the accumulation of negative charges on the inner membrane and positive charges on the outer membrane of the cell. In neurons and some other cells, a large enough depolarization may result in an action potential. Hyperpolarization is the opposite of depolarization, and inhibits the rise of an action potential.
If, for example, a cell has a resting potential of –70mV, once the membrane potential changes to –50mV, then the cell has been depolarized. Depolarization is often caused by influx of cations, e.g. Na+ through Na+ channels, or Ca2+ through Ca2+ channels. These channels, also known as voltage-dependent ion channels, open when an action potential begins, or at the threshold potential.  On the other hand, efflux of K+ through K+ channels inhibits depolarization, as does influx of Cl– (an anion) through Cl– channels. If a cell has K+ or Cl– currents at rest, then inhibition of those currents will also result in a depolarization.
Because depolarization is a change in membrane voltage, electrophysiologists measure it using current clamp techniques. In voltage clamp, the membrane currents giving rise to depolarization are either an increase in inward current, or a decrease in outward current.
There are drugs, called depolarization blocking agents, that cause prolonged depolarization by opening channels responsible for depolarization and not allowing them to close, preventing repolarization. Examples include the nicotinic agonists suxamethonium and decamethonium.
Depolarization in the Heart
Depolarization occurs in the four chambers of the heart: both atria first, and then both ventricles.
- The sinoatrial (SA) node on the wall of the right atrium initiates depolarization in the right and left atria, causing contraction, which is symbolized by the P wave on an electrocardiogram.
- The SA node sends the depolarization wave to the atrioventricular (AV) node which –- with a delay of about 100 milliseconds to allow the atria to finish contracting –- then causes contraction in both ventricles, seen in the QRS wave. At the same time, the atria are repolarized and relaxed.
- The ventricles are repolarized and relaxed at the T wave.
This process continues regularly, unless there is a problem in the heart.
- Purves D, Augustine GJ, Fitzpatrick D, et al., ed. (2001). Neuroscience (2. ed.). Sunderland, Mass: Sinauer Assoc. ISBN 0-87893-742-0.
- "Depolarization (Animation)". Psychology Department, Hanover College. Retrieved 18 May 2013.
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