How do muscles contract and how is this controlled?

Skeletal muscle is composed of myofibrils. The myofibrils contain repeating units called sarcomeres. This also called the operational units. The sarcomeres contain actin filaments and myosin filaments. The actin fibres are thin and the myosin fibres are thick. The contraction of the skeletal muscle is achieved by the sliding of actin and myosin filaments and each other. This pulls the ends of the sarcomeres together, making the muscle shorter. This sliding of the filaments is an active process and requires ATP. ATP binds to myosin heads which causes the cross bridges to break. ATP is then hydrolysed to ADP + P released and myosin heads to change angle. Myosin heads attach to binding sites further along the sarcomere ADP + P release and myosin head push actin filaments along. The muscle contraction is controlled by the release of calcium ions from the sarcoplasmic reticulum. Ca 2 + ions are released when a nerve impulse arrives at the muscle. They expose the myosin binding sites causing the movement of tropomyosin. Cross bridges form between actin and myosin molecule.