Editing Cardiac conduction system (section)

==Structure==
[[File:ConductionsystemoftheheartwithouttheHeart-en.svg|thumb|upright=1.4|Graphical representation of the electrical conduction system of the heart that maintains the [[heart rate]] in the [[cardiac cycle]]]]
Electrical signals arising in the [[sinoatrial node|SA node]] (located in the right [[Atrium (heart)|atrium]]) stimulate the atria to contract. Then the signals travel to the [[atrioventricular node]] (AV node), which is located in the [[interatrial septum]]. After a  short delay that gives the ventricles time to fill with blood,  the electrical signal diverges and is conducted through the left and right [[bundle branches]] of [[bundle of His|His]] to the respective [[Purkinje fibers]] for each side of the heart, as well as to the [[endocardium]] at the apex of the heart, then finally to the ventricular epicardium; causing the ventricles to  contract.<ref name="NIH2022">{{cite web |title=How the Heart Works – How the Heart Beats  |url=https://www.nhlbi.nih.gov/health/heart/heart-beats |website=www.nhlbi.nih.gov – NHLBI, NIH|date=24 March 2022 |access-date=24 August 2022}}</ref> These signals are generated rhythmically, which results in the coordinated rhythmic contraction and relaxation of the heart.

On the microscopic level, the wave of [[depolarization]] propagates to adjacent cells via [[gap junction]]s located on the [[intercalated disc]]. The heart is a [[Syncytium#Cardiac muscle|functional syncytium]] as opposed to a skeletal muscle [[syncytium]]. In a functional syncytium, electrical impulses propagate freely between cells in every direction, so that the myocardium functions as a single contractile unit. This property allows rapid, synchronous depolarization of the myocardium. While advantageous under normal circumstances, this property can be detrimental, as it has potential to allow the propagation of incorrect electrical signals. These gap junctions can close to isolate damaged or dying tissue, as in a [[myocardial infarction]] (heart attack).

===Development===
{{Main|Heart development#Pacemaker and conduction system}}
Embryologic evidence of generation of the cardiac conduction system illuminates the respective roles of this specialized set of cells. Innervation of the heart begins with a brain only centered [[parasympathetic]] [[cholinergic]] first order. It is then followed by rapid growth of a second order [[Sympathetic nervous system|sympathetic]] [[adrenergic]] system arising from the formation of the [[thorax|thoracic]] [[spinal ganglion|spinal ganglia]]. The third order of electrical influence of the heart is derived from the [[vagus nerve]] as the other peripheral organs form.<ref>{{cite web | url = http://www.embryology.ch/anglais/pcardio/funktion02.html | work = Human Embryology: Organogenesis: Functional development of the heart | title = Innervation of the heart|archive-url=https://web.archive.org/web/20200218164512/http://www.embryology.ch/anglais/pcardio/funktion02.html |archive-date=February 18, 2020}}</ref>