In biology, depolarization is a positive-going 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 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 two chambers of the heart, the atria and the ventricles. The SA node initiates depolarization in the atria causing contraction, which is symbolized by the P wave on an electrocardiogram. In the ventricles, depolarization is the contraction of the ventricles, also seen in the QRS wave. At the same time, the atria are repolarized, or relaxed 
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- "Depolarization (Animation)". Psychology Department, Hanover College. Retrieved 18 May 2013.
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