Explain the process of muscle contraction

Nerve impulses travel along the muscle fibre membrane (sarcolemma). The impulse is carried down into the fibre through T-tubules. Impulses are spread along the sarcoplasmic reticulum membrane causing calcium ions to be released. Before contraction, binding sites for myosin heads on the actin filaments are covered by 2 molecules troponin & tropomyosin. At rest the myosin head has ADP and inorganic phosphate (Pi) bound to it. Calcium binds to the actin filaments causing troponin and tropomyosin to change shape & expose myosin binding sites. Myosin heads bind to actin filaments forming cross-bridges. This causes inorganic phosphate (Pi) to be released. ADP is then released as cross-bridges form and causes the myosin heads bend towards centre of sarcomere and pull actin filaments inwards past the myosin filaments (power stroke)New ATP molecules bind to myosin heads and break the cross bridges, detaching them from the actin filaments. The ATP is hydrolysed to ADP and Pi and the myosin head returns back to original position. If another impulse arrives the process repeats causing further contraction