Cardiac muscle. Coloured scanning electron micrograph (SEM) of cardiac muscle from a healthy heart. Part of the muscle's lining, the sarcolemma (grey and green), has been removed to show the muscle fibrils. Mitochondria (orange) supply the muscle cells with energy. The muscle fibrils, or myofibrils, are crossed by transverse tubules (horizontal lines). These tubules mark the division of the myofibrils into contractile units (sarcomeres). Cardiac muscle is under subconscious control and continuously contracts to pump blood around the body without tiring.