Skeletal muscle. Illustration of the structure of human skeletal muscle. Skeletal muscle (top right) consists of bundles of long, thin cells called muscle fibres. A group of five muscle fibres with surrounding blood vessels is shown at centre. Each muscle fibre contains numerous myofibrils", one of which is shown at lower centre. The myofibril is a bundle of two types of protein "myofilament": myosin (green) and actin (red). When the muscle is stimulated, tiny cross-bridges (lower left) in 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."