When the sarcomere is at rest, which of the following best describes the position of the myosin-binding sites on actin filaments?

• A. They are covered by tropomyosin, preventing cross-bridge formation.
• B. They are degraded by acetylcholinesterase.
• C. They are bound to calcium ions, initiating contraction.
• D. They are exposed, allowing myosin heads to bind freely.

Answer: (A)

Verified step by step guidance

1. Understand the structure of the sarcomere: The sarcomere is the functional unit of muscle contraction, composed of actin (thin filaments) and myosin (thick filaments). Tropomyosin and troponin are regulatory proteins associated with actin filaments.
2. Learn the role of tropomyosin: Tropomyosin is a protein that covers the myosin-binding sites on actin filaments when the muscle is at rest, preventing the myosin heads from binding to actin and forming cross-bridges.
3. Understand the role of calcium ions: During muscle contraction, calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin, exposing them for cross-bridge formation.
4. Clarify the role of acetylcholinesterase: Acetylcholinesterase is an enzyme that breaks down acetylcholine in the synaptic cleft, terminating the signal for muscle contraction. It does not degrade myosin-binding sites on actin.
5. Conclude the resting state: When the sarcomere is at rest, the myosin-binding sites on actin filaments are covered by tropomyosin, preventing cross-bridge formation and ensuring the muscle remains relaxed.