The Cardiac Electrical System - How the Heart Beats

Figure 4: The specialized AV conduction system consists of the AV node (AVN), the "His bundle," and the right and left bundle branches (RBB and LBB). The AV node conducts the electrical impulse very slowly, and passes it to the His bundle (pronounced "hiss"). The His bundle penetrates the AV disk, and passes the signal to the right and left bundle branches. The right and left bundle branches, in turn, send the electrical impulse to the right and left ventricles, respectively. (The figure also shows that the LBB itself splits into the left anterior fascicle (LAF) and the left posterior fascicle (LPF).)

Because the impulse travels only very slowly through the AV node, there is a pause in the electrical activity on the ECG, referred to as the PR interval. (The PR interval is indicated by the solid purple line on the ECG in Figure 3.) This "pause" in the action allows the atria to contract fully, emptying their blood into the ventricles before the ventricles begin to contract.)