S-A meaning in Medical ?

The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is a group of cells known as pacemaker cells, located in the wall of the right atrium of the heart. These cells have the ability to spontaneously produce an electrical impulse (action potential), that travels through the electrical conduction system of the heart causing it to contract. In a healthy heart, the SA node continuously produces action potentials, setting the rhythm of the heart (sinus rhythm), and so is known as the heart's natural pacemaker. The rate of action potentials produced (and therefore the heart rate) is influenced by the nerves that supply it.