Describe the role of calcium ions and ATP during muscle contraction

Calcium ions diffuse from the sarcoplasmic reticulum and bind to troponin. This binding to troponin causes tropomyosin- which at rest blocks the actin-myosin binding sites- moves to expose these binding sites. The myosin heads have a molecules of ATP attatched which is hydrolysed into ADP and Pi causing the myosin heads to bind to the actin to form an actin-myosin cross bridge. The release of the ADP and Pi causes a 'power stroke' in the myosin, pulling the actin filaments along and causing the muscle to contract. The attatchment of a new ATP to the myosin molecules causes them to be released from the actin and return to their resting position.