Cardiac Conduction

Cardiac conduction is the rate at which the heart conducts electrical impulses. Cardiac muscle cells contract spontaneously. These contractions are coordinated by the sinoatrial (SA) node which is also referred to as the pacemaker of the heart. The SA node is composed of nodal tissue that has characteristics of both muscle and nervous tissue. The SA node is located in the upper wall of the right atrium. When the SA node contracts it generates nerve impulses that travel throughout the heart wall causing both atria to contract.

Another section of nodal tissue lies on the right side of the partition that divides the atria, near the bottom of the right atrium. It is called the atrioventricular (AV) node. When the impulses reach the AV node they are delayed for about a tenth of a second. This delay allows the atria to contract and empty their contents first.
The impulses are then sent down the atrioventricular bundle. This bundle of fibers branches off into two bundles and the impulses are carried down the center of the heart to the left and right ventricles.
At the base of the heart the atrioventricular bundles start to divide further into Purkinje fibers. When the impulses reach these fibers they trigger the muscle fibers in the ventricles to contract.