The sinus node is a group of cells located in the wall of the right atrium of the heart. These cells can initiate electrical impulses that stimulate the rhythmic contraction of the heart. These impulses then travel from the SA node through specialized heart muscle cells to the AV node. The AV node consists of another group of cells located between the atria and ventricles.
From this point on, the impulses travel through the bundle of His towards the bundle branches, stimulating the entire heart. Because of the direction that the electrical impulses follow, the atria contract first. This contraction is followed by the contraction of the ventricles when the impulses reach the bundle branches. The impulses are generated continuously by the SA node, which creates the well-known heart rhythm. As a result, the sinus node is also called the natural pacemaker of the heart.