Cardiac muscle. Coloured transmission electron micrograph (TEM) of cardiac muscle fibrils from a healthy heart. Mitochondria (red) supply the muscle cells with energy. The muscle fibrils, or myofibrils, are crossed by transverse tubules 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. Magnification: 6000x when printed 10 centimetres across.