- Afterdepolarization
Afterdepolarizations are abnormal
depolarization s ofcardiac myocyte s that interrupt phase 2, phase 3, or phase 4 of thecardiac action potential in theelectrical conduction system of the heart . Afterdepolarizations may lead tocardiac arrhythmia s.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 s before normal repolarization is completed. Phase 2 may be interrupted due to augmented opening ofcalcium channel s, while phase 3 interruptions are due to the opening ofsodium channel s. Early afterdepolarizations can result intorsades de pointes ,tachycardia , and otherarrhythmia s. [Cranefield, PF: The Conduction of the Cardiac Impulse. New York, Future Publishing Co. 1975] .Afterhyperpolarisations 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 repolarisation. It is responsible for the difference between regular spiking and intrinsically bursting pyramidal neurons. [Nelson Spruston, "Pyramidal Neurons: dendritic structure and synaptic integration", 2008. Nature Reviews. Neuroscience.]
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 concentrations, as might be seen withdigoxin toxicity.Katzung, B: "Basic & Clinical Pharmacology", chapter 14: "Agents Used in Cardiac Arrhythmias", The McGraw-Hill Companies, 2007, ISBN 978-0-07-145154-6] Lilly, L: "Pathophysiology of Heart Disease", chapter 11: "Mechanisms of Cardiac Arrhthmias", Lippencott, Williams and Wilkens, 2007]References
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