Afterdepolarization
Encyclopedia
Afterdepolarizations are abnormal depolarization
Depolarization
In 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...

s of cardiac myocytes that interrupt phase 2, phase 3, or phase 4 of the cardiac action potential
Cardiac action potential
In electrocardiography, the cardiac action potential is a specialized action potential in the heart, necessary for the electrical conduction system of the heart....

 in the electrical conduction system of the heart
Electrical conduction system of the heart
The normal intrinsic electrical conduction of the heart allows electrical propagation to be transmitted from the Sinoatrial Node through both atria and forward to the Atrioventricular Node. Normal/baseline physiology allows further propagation from the AV node to the ventricle or Purkinje Fibers...

. Afterdepolarizations may lead to cardiac arrhythmias.

Early afterdepolarizations

Early afterdepolarizations (EADs) occur with abnormal depolarization during phase 2 or phase 3, and are caused by an increase in the frequency of abortive action potential
Action potential
In 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...

s before normal repolarization is completed. Phase 2 may be interrupted due to augmented opening of calcium channel
Calcium channel
A 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:...

s, while phase 3 interruptions are due to the opening of sodium channels. Early afterdepolarizations can result in torsades de pointes
Torsades de pointes
Torsades de pointes, or simply torsades, is a French term that literally means "twisting of the points". It was first described by Dessertenne in 1966 and refers to a specific, rare variety of ventricular tachycardia that exhibits distinct characteristics on the electrocardiogram .- Presentation...

, tachycardia
Tachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...

, and other arrhythmias..

Afterhyperpolarizations can also occur in cortical pyramidal neurons. There, they typically follow an action potential and are mediated by voltage gated sodium or chloride channels. This phenomenon requires potassium channels to close quickly to limit repolarization. It is responsible for the difference between regular spiking and intrinsically bursting pyramidal neurons.

Delayed afterdepolarizations

Delayed afterdepolarizations (DADs), on the other hand, begin during phase 4 - after repolarization is completed, but before another action potential would normally occur. They are due to elevated cytosolic calcium
Calcium
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...

 concentrations, as might be seen with digoxin
Digoxin
Digoxin INN , also known as digitalis, is a purified cardiac glycoside and extracted from the foxglove plant, Digitalis lanata. Its corresponding aglycone is digoxigenin, and its acetyl derivative is acetyldigoxin...

toxicity. The overload of the sarcoplasmic reticulum may cause spontaneous Ca2+ release during repolarization, causing the released Ca2+ to exit the cell through the 3Na+/Ca2+-exchanger which results in a net depolarizing current.
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