Final answer:
The protein tropomyosin covers the active sites for myosin on actin molecules when muscle is at rest, while the troponin-tropomyosin complex prevents contraction. Calcium ions binding to troponin is key for initiating muscle contraction.
Step-by-step explanation:
When a muscle is at rest, the majority of the active sites for myosin binding on the actin molecule are covered by the protein tropomyosin. Tropomyosin is a regulatory protein that winds around the actin filaments, preventing the myosin heads from attaching to actin and initiating muscle contraction. It does so by blocking the myosin binding sites on the actin molecules.
Additionally, the protein complex troponin binds to tropomyosin and helps maintain its position over the active sites. Troponin also has a specific binding site for calcium ions, which is crucial for initiating muscle contraction upon the arrival of a neural signal. When calcium ions bind to troponin, a conformational change occurs that shifts tropomyosin away from the myosin-binding sites on actin, allowing for muscle contraction to occur.