Tropomyosin molecules cover the myosin binding site on actin, when the muscle is at rest. Contraction begins when intracellular calcium ions bind to troponin, causing a change in the position of the troponin-tropomyosin complex. This change exposes active sites along the F actin strand. A single myosin head contains bound ADP and phosphate, and seeks to bind to the exposed active site on actin. Once binding occurs, phosphate is released. This release strengthens the myosin-actin bond. This initiates the power stroke that pivots the myosin head and pulls the actin filament toward the center of the sarcomere. After pivoting, ADP dissociates. A new ATP molecule attaches to the empty nucleotide binding site. This binding causes the myosin head to detach from the actin filament. The ATP is hydrolyzed into ADP and phosphate, releasing energy that cocks the myosin head back to its pre-stroke state. The myosin uses the energy from the new ADP and phosphate to form more cross-bridges with actin, and begin another contraction cycle. The two myosin heads walk along the actin filaments towards the barbed end. This shortens the sarcomere and contracts the muscle. Cross-bridge formation can continue as long as calcium ions keep the actin binding sites exposed and ATP are present. Once calcium is depleted or reabsorbed, calcium ions detach from troponin. Troponin and tropomyosin return to their original positions, covering the actin binding site. Myosin can no longer bind to the actin. Therefore, actin slides back to its original position, causing relaxation of the muscle.
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