Final answer:
To enable muscle contraction, calcium ions bind to troponin, causing tropomyosin to move away from the myosin binding sites on actin, allowing the myosin heads to bind and form cross-bridges. This process continues until calcium ions and ATP are depleted.
Step-by-step explanation:
To enable a muscle contraction, tropomyosin must change conformation, uncovering the myosin-binding site on an actin molecule and allowing cross-bridge formation. This can only happen in the presence of calcium, which is kept at extremely low concentrations in the sarcoplasm.
If present, calcium ions bind to troponin, causing conformational changes in troponin that allow tropomyosin to move away from the myosin binding sites on actin. Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering contraction. Cross-bridge cycling continues until Ca²+ ions and ATP are no longer available and tropomyosin again covers the binding sites on actin.