Editing Cardiac conduction system (section)

===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.