The sinus node, sometimes called the SA node is a grouping of cells that initiate electrical impulses and control heart rate. It is a central component of the cardiac conduction system. Also known as the sinoatrial node or the sinuatrial node, this intricate arrangement of muscle cells serves as an innate pacemaker for the heart.
On the upper wall of the right atrium of the heart, the sinus node is positioned near the entry point of the vein called the superior vena cava. The cluster is comprised of muscle cells known as modified cardiac myocytes, which trigger and lead electrical impulses through the heart chamber. It is embedded with nerves from both the spinal nerve system and the vagus nerve system.
When the sinus node produces an electrical impulse, the impulse travels through the cells of the heart but is significantly slowed once it reaches the atrioventricular node, or AV node. It is the function of the AV node to slow the impulse until the ventricles are ready to receive it. Once the ventricles have sufficiently contracted and are prepared for the impulse, the AV node allows the impulse to pass, and it moves along the fibrous walls through the heart and into the ventricles.
If the sinus node becomes compromised, its duties can be performed by the AV node. Should the AV node, in turn, be unable to function properly, heart fibers known as Purkinje fibers can take over impulse-production responsibilities. These internal processes ensure the heart beats at a normal rate and rhythm and keeps blood flowing to the rest of the body.
The series of actions instigated by the SA node makes it a natural pacemaker for the heart. The node itself is regulated by the automatic nervous system, which also controls blood pressure, digestion, respiration rate, perspiration, and numerous other automated functions. When the automatic nervous system sends communication to the sinus node, it springs into action and begins generating impulses. If the heart needs to beat faster, the automatic nervous system alerts the sinus node, and the heart can double its beating rate in a matter of seconds.
The SA node can initiate defective impulses that lead to a condition known as sick sinus syndrome, or sinus node dysfunction. These malfunctioning signals cause abnormal heartbeats. The implementation of an artificial pacemaker is a popular course of treatment and helps to re-regulate the electrical impulses produced by the SA node.