Editing Cardiac pacemaker (section)

==Control==
[[Image:bundleofhis.png|thumb|300px|right|Schematic representation of the sinoatrial node and the atrioventricular bundle of His. The location of the SA node is shown in blue. The bundle, represented in red, originates near the orifice of the coronary sinus, undergoes slight enlargement to form the AV node. The AV node tapers down into the bundle of His, which passes into the ventricular septum and divides into two bundle branches, the left and right bundles. The ultimate distribution cannot be completely shown in this diagram.]]

===Primary pacemaker===
The [[sinoatrial node]] (SA node) is the primary pacemaker of the heart. It is a region of cardiac muscle on the wall of the upper [[right atrium]] near to the [[superior vena cava]] entrance. The cells that make up the SA node are specialized [[cardiomyocyte]]s known as '''pacemaker cells''' that can spontaneously generate [[cardiac action potential]]s. These signals are propagated through the heart's [[electrical conduction system of the heart|electrical conduction system]].<ref>{{cite web| vauthors = Kashou AH, Basit H, Chhabra L | title = Physiology, Sinoatrial Node (SA Node) | publisher = StatPearls | date=January 2020 | url = https://www.ncbi.nlm.nih.gov/books/NBK459238/ | access-date = 10 May 2020 | pmid = 29083608}}</ref><ref name=Campbell_473>{{cite book |author=Neil A. Campbell  |display-authors=etal |title=Biology : concepts & connections |url=https://archive.org/details/studycardforcamp00camp  |url-access=limited  |year=2006 |publisher=Pearson/Benjamin Cummings |location=San Francisco |isbn=0-13-193480-5 |pages=[https://archive.org/details/studycardforcamp00camp/page/n514 473] |edition=5th}}</ref> Only one percent of the heart muscle cells are conductive, the rest of the cardiomyocytes are [[Muscle contraction|contractile]]. 

The SA node controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the quickest. The action potential generated by the SA node passes down the [[electrical conduction system of the heart]], and depolarizes the other potential pacemaker cells at the AV node to initiate action potentials before these other cells have had a chance to generate their own spontaneous action potential, thus they contract and propagate electrical impulses to the pace set by the cells of the SA node. This is the normal conduction of electrical activity in the heart.

The pacemaker cells are connected to neighboring contractile cells via [[gap junction]]s, which enable them to locally depolarize adjacent cells. Gap junctions allow the passage of positive cations from the depolarization of the pacemaker cell to adjacent contractile cells. This starts the depolarization and eventual action potential in contractile cells. Having cardiomyocytes connected via gap junctions allow all contractile cells of the heart to act in a coordinated fashion and contract as a unit. All the while being in sync with the pacemaker cells; this is the property that allows the pacemaker cells to control contraction in all other cardiomyocytes. 

Cells in the SA node spontaneously [[depolarization|depolarize]], ultimately resulting in contraction, approximately 100 times per minute. This native rate is constantly modified by the activity of [[sympathetic nervous system|sympathetic]] and [[Parasympathetic nervous system|parasympathetic]] nerve fibers via the [[autonomic nervous system]], so that the average resting [[heart rate]] in adult humans is about 70 beats per minute.

===Secondary pacemaker===
Impulses from the sinus node reach the [[atrioventricular node]] which acts as the secondary pacemaker. The cells of the AV node normally discharge at about 40–60 beats per minute. 

====Atrioventricular junction====
The atrioventricular node and the [[Bundle of His]], a little further down, are located in the region separating the atria from the ventricles known as the atrioventricular junction. The Bundle of His transmits signals to the [[bundle branches]], which send them on to the [[Purkinje fibers]]. These will also produce a spontaneous cardiac action potential at a rate of 30–40 beats per minute, so if the SA and AV node both fail to function, these cells can also become pacemakers but with a much lower rate of conduction than either the primary or secondary pacemakers.