A small mass of specialized muscle cells in the mammalian heart, found in the wall of the right auricle near the opening of the superior vena-cava is called pacemaker or sinu-atrial node (SA-node). It initiates and maintains the heart beat.
Sinu-auricular node is called pacemaker of the heart because it initiates the heart beat. A wave of contraction originates from it and spreads over to the whole heart. Allowing it to contract and relax for normal functioning of heart.