sinus node, a cluster of hundreds of cells located in the right atrial wall of the heart, near the opening of the superior vena cava. It comprises a knot of modified heart muscle that generates impulses that travel swiftly throughout the muscle fibers of both atria, causing them to contract. Specialized pacemaker cells in the node have an intrinsic rhythm that is independent of any stimulation by nerve impulses from the brain and the spinal cord. Slender fusiform cells making up the sinoatrial node are largely filled with sarcoplasm but contain a few striated fibrillae. The cells are irregularly grouped together and, at the edge of the node, merge with the atrial musculature. The sinoatrial node will normally “fire” at a rhythmic rate of 70-75 beats/min. If the node fails to generate an impulse, pacemaker function will shift to another excitable component of the cardiac conduction system, such as the atrioventricular node or Purkinje’s fibers. Certain hormones and various autonomic impulses can affect the sinoatrial node and cause it to “fire” faster, such as during strenuous physical activity. During a lifetime of 70 years the node generates about 2 billion impulses. Surgical implantation of an artificial pacemaker is a common procedure for individuals suffering from a defective sinoatrial node. Also called Keith’s bundle, Keith-Flack node, pacemaker, sinus pacemaker, sinoatrial node. Compare atrioventricular (AV) node, Purkinje’s network.
