The heartbeat originates in a specialized cardiac conduction system and spreads via this system to all parts of the heart muscle. The structures that form the conducting system are the sinoatrial node (SA node); anterior, middle and posterior internodal tracts; the atrioventricular node (AV node); the bundle of His; right and left bundle branches; anterior-superior and posterior-inferior brances of the left bundle; and the Purkinje network. Conduction speeds in cardiac tissue
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