Cardiac muscle. Coloured scanning electron micrograph (SEM) of a freeze-fracture through cardiac muscle fibrils (yellow) from a healthy heart. Mitochondria (pink) 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.