Sinoatrial Node

The sinoatrial node or sinus node (SA node) is a minute area in the human heart muscle which plays a crucial role in its overall functioning. Major actions of the heart (contraction or beats) originate from this small region. From the SA node, cardiac impulses are transmitted to the heart's conduction system for the coordinated cardiac contractions.

The sinus node along with other factors influence the heart rate of an individual. It is a primary component of the cardiac conduction system, which is solely responsible for controlling the heart rate of an individual.

Located in the upper right chamber (right atrium) of the heart, it comprises a group of cells and is innervated with nerve fibers (vagus and sympathetic). Stimulation via vagal fiber causes slow heart rate; while stimulation at the sympathetic nerve leads to rapid heart rate.

It is commonly known as the heart's natural or primary pacemaker. The main function of this node is to regulate the heart rate according to the necessities of the body. It remains in association with the autonomic nervous system (ANS).

A part of the main nervous system, the ANS performs the function of regulating the involuntary actions of the cardiac muscles, smooth muscles, and various glands. The SA node acts in accordance to the command or input made by the autonomic nervous system.

In order to start the cardiac cycle, the ANS first triggers this node for generation of cardiac impulses. Electrical impulses that have their roots in the SA node are then spread from one cell to the adjacent cell, until they are received by the atrioventricular node (AV node is another group of cells, present between the atria and the ventricles.).

By this time, the atria are contracted fully, and the signals are further passed on to the walls of the ventricle to initiate ventricular contraction. Generally, the normal heart rate falls between 60-100 beats per minute.

Accordingly, the SA node generates electrical impulses at a fast pace. These impulses are then passed on to the cardiac conduction system and the heart muscles, which stimulate contraction. The same steps are repeated many times, resulting in faster heartbeats. This increases the oxygen supply to the other body parts.

Thus, in order to meet oxygen requirements of the body, the heart rate of a person is increased during exercise. On the contrary, the oxygen needed during rest is low, and accordingly resting heart rate is slow (60-70 beats per minute).

Being a major element of the cardiac conduction system, normal functioning of the SA node is responsible for maintaining the optimal heart health. Any abnormality of the SA node can lead to irregular heart rate, or cardiac arrhythmia.

The conditions developed may be slow heartbeats (bradycardia), rapid heartbeats (tachycardia), or a combination of both (bradycardia-tachycardia). This is due to wrong electrical impulses generated by the sinus node.

Sinus node reentrant tachycardia or SA nodal reentrant tachycardia (SANRT) is a rare type of arrhythmia that usually remains asymptomatic. People who have structural heart problems are more prone to SANRT.

The abnormalities in the sinus node can be managed by implanting an artificial pacemaker inside the heart. For introducing the pacemaker inside the body, a traditional surgery or non-invasive laparoscopy is performed.