Editing Bradycardia (section)

== Normal cardiac conduction ==
The heart is a specialized muscle containing repeating units of [[Cardiac muscle|cardiomyocytes]], or heart muscle cells. Like most cells, cardiomyocytes maintain a highly regulated negative voltage at rest and are capable of propagating [[action potential]]s, much like neurons.<ref name="Loscalzo-2022">{{Cite book |title=Harrison's Principles of Internal Medicine |vauthors=Loscalzo J, Keaney JF, MacRae CA |publisher=McGraw Hill |year=2022 |isbn=978-1-264-26850-4 |veditors=Loscalzo J, Fauci AS, Kasper DL, Hauser S, Longo D, Jameson JL |edition=21st |chapter=Basic Biology of the Cardiovascular System}}</ref> While at rest, the negative cellular voltage of a cardiomyocyte can be raised above a certain threshold (so-called [[depolarization]]) by an incoming action potential, causing the myocyte to [[Myocardial contractility|contract]]. When these contractions occur in a coordinated fashion, the atria and ventricles of the heart will pump, delivering blood to the rest of the body.<ref name="Loscalzo-2022" />

Normally, the origination of the action potential causing cardiomyocyte contraction originates from the [[sinoatrial node]] (SA node). This collection of specialized conduction tissue is located in the right atrium, near the entrance of the [[superior vena cava]].<ref name="Sauer-2022a">{{Cite book |title=Harrison's Principles of Internal Medicine |vauthors=Sauer WH, Koplan BA, Zei PC |publisher=McGraw Hill |year=2022 |isbn=978-1-264-26850-4 |veditors=Loscalzo J, Fauci AS, Kasper DL, Hauser S, Longo D, Jameson JL |edition=21st |chapter=Principles of Clinical Cardiac Electrophysiology}}</ref> The SA node contains pacemaker cells that demonstrate "automaticity" and can generate impulses that travel through the heart and create a steady heartbeat.<ref name="Sauer-2022a" />

At the beginning of the cardiac cycle, the SA node generates an electrical action potential that spreads across the right and left [[Atrium (heart)|atria]], causing the atrial contraction of the [[cardiac cycle]].<ref name="Sauer-2022a" /> This electrical impulse carries on to the [[atrioventricular node]] (AV node), another specialized grouping of cells located in the base of the right atrium, which is the only anatomically normal electrical connection between the atria and ventricles. Impulses coursing through the AV node are slowed before carrying on to the ventricles,<ref>{{Cite journal |vauthors=Kurian T, Ambrosi C, Hucker W, Fedorov VV, Efimov IR |date=June 2010 |title=Anatomy and electrophysiology of the human AV node |journal=Pacing and Clinical Electrophysiology |volume=33 |issue=6 |pages=754–762 |doi=10.1111/j.1540-8159.2010.02699.x |pmc=2889145 |pmid=20180918}}</ref> allowing for appropriate filling of the ventricles before contraction. The SA and AV nodes are both closely regulated by the autonomic nervous system's fibres, allowing for adjustment of cardiac output by the central nervous system in times of increased metabolic demand.{{cn|date=May 2025}}

Following slowed conduction through the atrioventricular node, the action potential produced initially at the SA node now flows through the His-Purkinje system. The [[bundle of His]] originates in the AV node and rapidly splits into a left and right branch, each destined for a different ventricle. Finally, these bundle branches terminate in the small [[Purkinje fibers]] that innervate myocardial tissue. The His-Purkinje system conducts action potentials much faster than can be propagated between myocardial cells, allowing the entire ventricular myocardium to contract in less time, improving pump function.<ref name="Sauer-2022a" />