Which of the following causes the active site on actin to be exposed or uncovered during muscle contraction?

• A. Release of acetylcholine at the neuromuscular junction
• B. Binding of calcium ions to troponin
• C. Hydrolysis of ATP by myosin
• D. Phosphorylation of myosin light chains

Answer: (B)

Verified step by step guidance

1. Understand the context: Muscle contraction involves the interaction between actin and myosin filaments. For this interaction to occur, the active sites on actin must be exposed. This process is regulated by proteins such as troponin and tropomyosin, as well as calcium ions (Ca²⁺).
2. Recall the role of calcium ions: Calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum in response to a signal from the nervous system. These ions play a critical role in muscle contraction by binding to troponin.
3. Explain the function of troponin: Troponin is a regulatory protein that, when bound to calcium ions, undergoes a conformational change. This change causes the tropomyosin protein to shift its position on the actin filament.
4. Describe the exposure of the active site: The shift in tropomyosin exposes the active sites on actin, allowing myosin heads to bind to these sites and initiate the cross-bridge cycle, which is essential for muscle contraction.
5. Connect the correct answer: The binding of calcium ions to troponin is the specific event that leads to the exposure of the active sites on actin, enabling the process of muscle contraction to proceed.