The electrical activation of the heart begins within the SA, or sinoatrial, node, a collection of specialized pacemaker cells located in the right atrium near its junction with the superior vena cava. Subsequently, the stimulus spreads to the right and left atrium causing atrial contraction, and then, to the AV node. The electrical stimulus then spreads into the intraventricular conduction system, composed of Purkinje fibers, the His-Purkinje system. These fibers are organized into the His bundle and the left and right bundle branches. The left bundle branch has two major subdivisions supplying the left ventricle, the anterior and posterior fascicles. Note that once the electrical impulse enters the His bundle, it is insulated from ventricular muscle and it is not until it reaches the distal Purkinje fibers, Purkinje terminals, that it begins to activate ventricular muscle and cause ventricular contraction.

The following animation illustrates what we have just described.