Editing Cardiac conduction system (section)

==Electrical activity==
{{Further|Electrocardiogram}}
[[File:Shapes of the cardiac action potential in the heart.svg|thumb|upright=1.4|Different wave shapes generated by different parts of the [[Cardiac action potential|heart's action potential]]]]
[[File:ECG Intervals.svg|thumb|The ECG complex. P=P wave, PR=PR interval, QRS=QRS complex, QT=QT interval, ST=ST segment, T=T wave]]
[[File:ECG Principle fast.gif|thumb|Principle of ECG formation. The red lines represent the depolarization wave, not bloodflow.]]
An [[electrocardiogram]] is a recording of the electrical activity of the heart.

===SA node: P wave===
Under normal conditions, electrical activity is spontaneously generated by the [[SA node]], the cardiac pacemaker.  This electrical impulse is propagated throughout the right [[atrium (anatomy)|atrium]], and through [[Bachmann's bundle]] to the left [[atrium (anatomy)|atrium]], stimulating the [[myocardium]] of the atria to contract.  The conduction of the electrical impulses throughout the atria is seen on the [[ECG]] as the [[P wave (electrocardiography)|P wave]].<ref name=":0" /><ref>{{Cite web|url = http://sitemaker.umich.edu/ecgtutorial/cardiac_cycle|archive-url = https://archive.today/20150103040232/http://sitemaker.umich.edu/ecgtutorial/cardiac_cycle|url-status = dead|archive-date = January 3, 2015|title = Cardiac Cycle|access-date = January 2, 2015|website = ECG Tutorial|publisher = University of Michigan Health System}}</ref>

As the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as ''internodal tracts'', from the SA node to the [[AV node]].

===AV node and bundles: PR interval===
The AV node functions as a critical delay in the conduction system.  Without this delay, the [[atrium (anatomy)|atria]] and [[Ventricle (heart)|ventricles]] would contract at the same time, and blood wouldn't flow effectively from the atria to the ventricles. The delay in the AV node forms much of the '''PR segment''' on the ECG, and part of atrial repolarization can be represented by the PR segment.

The distal portion of the AV node is known as the [[bundle of His]].<ref name="AndersonMori2016">{{cite journal|last1=Anderson|first1=Robert H.|last2=Mori|first2=Shumpei|title=Wilhelm His Junior and his bundle|journal=[[Journal of Electrocardiology]]|year=2016|volume=49|issue=5|pages=637–643|issn=0022-0736|doi=10.1016/j.jelectrocard.2016.06.003|pmid=27324867}}</ref> The bundle of His splits into two branches in the interventricular septum: the left bundle branch and the right bundle branch. The left bundle branch activates the [[left ventricle]], while the right bundle branch activates the [[right ventricle]].

The left bundle branch is short, splitting into the left anterior fascicle and the left posterior fascicle. The left posterior fascicle is relatively short and broad, with dual blood supply, making it particularly resistant to ischemic damage. The left posterior fascicle transmits impulses to the papillary muscles, leading to [[mitral valve]] closure. As the left posterior fascicle is shorter and broader than the right, impulses reach the papillary muscles just prior to depolarization, and therefore contraction, of the left ventricle myocardium. This allows pre-tensioning of the chordae tendinae, increasing the resistance to flow through the mitral valve during left ventricular contraction.<ref name=":0" /> This mechanism works in the same manner as pre-tensioning of car seatbelts.

===Purkinje fibers/ventricular myocardium: QRS complex===
The two bundle branches taper out to produce numerous [[Purkinje fibers]], which stimulate individual groups of myocardial cells to contract.<ref name=":0" />

The spread of electrical activity through the ventricular myocardium produces the [[QRS complex]] on the ECG.

Atrial repolarization occurs and is masked during the [[QRS complex]]  by ventricular depolarization on the ECG.

===Ventricular repolarization===
The last event of the cycle is the repolarization of the [[ventricle (heart)|ventricles]]. It is the restoring of the resting state. In the ECG, repolarization includes the J point, ST segment, and T and U waves.<ref name="pmid12906963">{{cite journal | vauthors = Yan GX, Lankipalli RS, Burke JF, Musco S, Kowey PR | title = Ventricular repolarization components on the electrocardiogram: cellular basis and clinical significance | journal = J Am Coll Cardiol | volume = 42 | issue = 3 | pages = 401–409 | date = August 2003 | pmid = 12906963 | doi = 10.1016/s0735-1097(03)00713-7 | doi-access = free }}</ref>
The transthoracically measured PQRS portion of an electrocardiogram is chiefly influenced by the [[sympathetic nervous system]]. The T (and occasionally U) waves are chiefly influenced by the [[parasympathetic nervous system]] guided by integrated [[brainstem]] control from the [[vagus nerve]] and the thoracic [[spinal accessory ganglia]].

An impulse ([[action potential]]) that originates from the SA node at a relative rate of 60–100&nbsp;bpm is known as a normal [[sinus rhythm]]. If SA nodal impulses occur at a rate less than 60&nbsp;bpm, the heart rhythm is known as [[sinus bradycardia]]. If SA nodal impulses occur at a rate exceeding 100 bpm, the consequent rapid heart rate is [[sinus tachycardia]]. These conditions are not necessarily bad symptoms, however. Trained athletes, for example, usually show heart rates slower than 60 bpm when not exercising. If the SA node fails to initialize, the AV junction can take over as the main pacemaker of the heart. The AV junction consists of the AV node, the bundle of His, and the surrounding area; it has a regular rate of 40 to 60 bpm. These "junctional" rhythms are characterized by a missing or inverted P wave. If both the SA node and the AV junction fail to initialize the electrical impulse, the ventricles can fire the electrical impulses themselves at a rate of 20 to 40&nbsp;bpm and will have a QRS complex of greater than 120&nbsp;ms. This is necessary for the heart to be in good function.