Final answer:
Tropomyosin blocks the myosin-binding site on actin when Ca²⁺ levels are low; it moves away when calcium binds to troponin, permitting muscle contraction.
Step-by-step explanation:
When cytoplasmic Ca²⁺ levels are low, the myosin-binding site on actin is blocked by tropomyosin. Tropomyosin is a protein that winds around the actin filament chains, covering the myosin-binding sites to prevent interaction with myosin. It is part of the troponin-tropomyosin complex, which is regulated by calcium ions, Ca²⁺. When calcium binds to troponin within the complex, tropomyosin moves away from the myosin-binding sites on actin, allowing for muscle contraction to occur.