Myocardium is one of three heart-only muscle groups in vertebrates. Cardiac muscle has sarcomeres, like skeletal muscle, but this distinguishes it from smooth muscle. Regular contractions and lack of voluntary control distinguish cardiac from skeletal muscle. Sinoatrial node, the heart’s pacemaker, controls rhythmic muscle contraction. Cardiac muscle cells make up the heart (or myocardium). The heart’s pumping function depends on its contractility and rhythmicity.