Your heartbeat occurs because of electrical impulses that travel in a rhythmic pattern
through your heart muscle. The rhythm is controlled by an area in your right atrium called the sino-atrial (SA) node
(1) which functions like a "natural pacemaker." Each heartbeat is the result of an electrical impulse that begins in
the SA node. This impulse travels across both atria causing the atria to contract and then to the atrio-ventricular
(AV) node(2). The AV node is near the point in the septum where all four chambers meet and can be thought of as an
electrical relay station.
The impulse then continues down through the septum that divides the two ventricles, and around to their outer walls
causing them to contract. As the impulse is transmitted to the ventricles, it travels along specialized conduction
pathways called the bundle branches(3). There is both a right and left bundle branch. This electrical process
repeats itself for each heartbeat. The normal heart usually beats 50 to 90 times per minute while at rest
(see illustration).
