Sinoatrial Node

The sinoatrial node is the impulse-generating tissue located in the upper part of right atrium of the heart, and thus the generator of sinus rhythm. It is an importan part of the cardiac conduction system which controls the heart rate. The sinoatrial node is a group of modified cardiac myocytes (cells), which are situated on the wall of the right atrium, near the entrance of the superior vena cava. Although they have some contractile filaments, they do not contract.

All of the heart's myocytes can generate the electrical impulses that trigger cardiac contraction, but it is the sinoatrial node that initiates it, as it generates impulses slightly faster than the other areas with pacemaker potential. Because cardiac myocytes, like all muscle cells, have refractory periods following contraction during which additional contractions cannot be triggered, their pacemaker potential is overridden by the sinoatrial node.

In the absence of extrinsic neural and hormonal control, cells in the sinoatrial node will naturally discharge at about 60-100 beats/minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker. The electrical impulses from the sinoatrial node triggers a sequence of electrical events in the heart to control the orderly sequence of muscle contractions that pump the blood out of the heart.

If the sinoatrial node stopped functioning, a group of cells further down the heart will become the heart's pacemaker. These cells form the atrioventricular node, which is an area between the atria and ventricles, within the atrial septum. The autonomic nervous system controls the firing of the sinoatrial node to trigger the initiation of the cardiac cycle. The autonomic nervous system can transmit a message quickly to the sinoatrial node so it can increase the heart rate to twice normal within only 3 to 5 seconds.