A node is a specialized type of tissue that behaves as both muscle
tissue. When nodal tissue contracts (like muscle tissue) it generates nerve impulses (like nervous tissue) that travel throughout the heart
wall. The heart has two nodes that are instrumental in cardiac conduction
, which is the electrical system that powers the cardiac cycle
. These two nodes are the sinoatrial (SA) node and the atrioventricular (AV) node.
Sinoatrial (SA) Node
The sinoatrial node
, also referred to as the pacemaker of the heart, coordinates heart contractions. Located in the upper wall of the right atrium
, it generates nerve impulses that travel throughout the heart wall causing both atria to contract.
Atrioventricular (AV) Node
The atrioventricular node
lies on the right side of the partition that divides the atria, near the bottom of the right atrium. When the impulses generated by the SA node reach the AV node, they are delayed for about a tenth of a second. This delay allows the atria to contract, thereby emptying blood into the ventricles
. The AV node then sends the impulses 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.