Autonomic nervous system: Part of the nervous system that was once thought to be functionally independent of the brain. The autonomic nervous system regulates key functions of the body including the activity of the heart muscle (see below), the smooth muscles (e.g., the muscles of the intestinal tract), and the glands.
The autonomic nervous system has two divisions: (1) the sympathetic nervous system that accelerates the heart rate, constricts blood vessels, and raises blood pressure; and (2) the parasympathetic nervous system slows the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles.
The autonomic system, together with the SA (sinoatrial) and AV (atrioventricular) nodes, is a major element in the cardiac conduction system, the system that controls the heart rate. This stunningly designed system generates electrical impulses and conducts them throughout the muscle of the heart, stimulating the heart to contract and pump blood.
The SA node is the heart's natural pacemaker. The SA node consists of a cluster of cells that are situated in the upper part of the wall of the right atrium (the right upper chamber of the heart). The electrical impulses are generated there. The SA node is also called the sinus node.
The electrical signal generated by the SA node moves from cell to cell down through the heart until it reaches the 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 that slows the electrical current before the signal is permitted to pass down through 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 the 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 normal within only 3 to 5 seconds. This quick response is important during exercise when the heart has to increase its beating speed to keep up with the body's increased demand for oxygen.