Skeletal muscle and tendons. Coloured transmission electron micrograph (TEM) of a section through skeletal, or striated, muscle (pink) and tendon fibres (green). The striations are due to muscle fibrils, which run left to right. These are made up of the proteins actin and myosin. The lighter regions are stretches of actin, the darker regions are actin and myosin. Muscles contract when actin slides over the myosin. Tendons are made up of collagen fibres. Their parallel structure makes the tendon inelastic but flexible. Tendons attach muscle to bone.