Sinoatrial node: The heart's natural pacemaker, one of the major elements in the cardiac conduction system, the system that controls the heart rate. Abbreviated SA node. The SA node consists of a cluster of cells that are situated in the upper part of the wall of the heart's right atrium, where the electrical impulses are generated. An electrical signal generated by the SA node moves from cell to cell, down through the heart, until it reaches the atrioventricular (AV) node, a cluster of cells situated in the center of the heart, between the atria and ventricles. The AV node serves as a gate, slowing the electrical current before the signal is permitted to pass down to the ventricles. This delay ensures that the atria have a chance to fully contract before the ventricles are stimulated. After passing the AV node, the electrical current travels to the ventricles along special fibers embedded in the walls of the lower part of the heart. The autonomic nervous system controls the firing of the SA node to trigger the start of this cardiac cycle. The autonomic nervous system can transmit a message quickly to the SA node, so it in turn can increase the heart rate to twice the normal rate within only 3 to 5 seconds. This rapid response is important during exercise, when the heart has to increase its beating speed to keep up with the body's heightened demand for oxygen. Also known as sinus node.