BSIP, JACOPIN / SCIENCE PHOTO LIBRARY BSIP, JACOPIN / SCIENCE PHOTO LIBRARY
Skeletal muscle. Artwork of the structure of human skeletal muscle. Skeletal muscle (upper right) consists of bundles of long, thin muscle fibres. A bundle of these fibres with surrounding blood vessels is seen at lower left. Each muscle fibre contains numerous myofibrils, one of which is seen extending from the bundle at lower left. The myofibril comprises two types of protein myofilament: myosin (left, with "arms") and actin (right, helical). When the muscle is stimulated, the arms on the myosin filaments hook onto actin filaments and pull them along. As a result, myosin and actin filaments slide over each other, shortening the muscle and making it contract.
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