STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Cardiac muscle. Coloured scanning electron micrograph (SEM) of a bundle of cardiac muscle fibrils (pink) from a healthy heart. Mitochondria (round, between fibrils) supply the muscle cells with energy. The muscle fibrils, or myofibrils, are crossed by transverse tubules (vertical lines). These tubules mark the division of the myofibrils into contractile units (sarcomeres). Connective tissue, composed mainly of collagen, is seen at the top and bottom of the image. Cardiac muscle is under subconscious control and continuously contracts to pump blood around the body without tiring.
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