The release of calcium ions initiates muscle contractions. Please describe how actin-binding sites are made available for cross-bridging with myosin heads during contraction.
Calcium ions bind to the protein complex troponin and induce conformational changes of the protein. This process stimulates the exposure of the active-binding sites on the actin. As soon as the actin-binding sites are uncovered, the high-energy myosin head bridges the gap, forming a cross-bridge.