Which substance regulates muscle actions?

• A. Potassium
• B. Calcium
• C. Troponin
• D. Tropomyosin

Answer: (B)

Calcium regulates muscle actions. When a muscle is stimulated, calcium ions are released into the cytosol and bind to troponin C, a component of the troponin complex on actin filaments. This binding causes troponin to change shape and move tropomyosin away from the myosin-binding sites on actin. With those sites exposed, myosin heads can attach to actin and perform cross-bridge cycling, powered by ATP, producing contraction. If calcium levels drop, the troponin-tropomyosin complex covers the binding sites again and the muscle relaxes. Potassium helps establish membrane excitability but does not directly regulate the contraction process, while troponin and tropomyosin are the regulatory proteins that mediate the calcium-triggered exposure of binding sites.