Which of the following is the correct sequence of events for muscle contraction?

• A. Cross-bridge formation, power stroke, action potential, calcium release
• B. Action potential, calcium release, cross-bridge formation, power stroke
• C. Power stroke, cross-bridge formation, calcium release, action potential
• D. Calcium release, action potential, power stroke, cross-bridge formation

Answer: (B)

Verified step by step guidance

1. Understand the role of the action potential: It is the electrical signal that travels along the nerve to the muscle fiber, initiating the process of muscle contraction.
2. Recognize the importance of calcium release: Once the action potential reaches the muscle fiber, it triggers the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm of the muscle cell.
3. Identify the process of cross-bridge formation: Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the binding sites on actin, allowing myosin heads to attach to actin filaments, forming cross-bridges.
4. Understand the power stroke mechanism: The myosin heads pivot, pulling the actin filaments toward the center of the sarcomere, which is known as the power stroke. This action shortens the muscle fiber, leading to contraction.
5. Sequence the events correctly: The correct order of events in muscle contraction is action potential, calcium release, cross-bridge formation, and then power stroke.