Which of the following correctly arranges the steps of the muscle contraction cycle in order from first to last?

• A. Cross-bridge formation → Power stroke → Detachment of myosin from actin → Reactivation of myosin head
• B. Detachment of myosin from actin → Cross-bridge formation → Power stroke → Reactivation of myosin head
• C. Reactivation of myosin head → Detachment of myosin from actin → Power stroke → Cross-bridge formation
• D. Power stroke → Cross-bridge formation → Detachment of myosin from actin → Reactivation of myosin head

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the muscle contraction cycle, which involves the interaction between actin and myosin filaments within the sarcomere. This process is driven by ATP and calcium ions.
2. Step 2: Recall the sequence of events in the muscle contraction cycle: (1) Cross-bridge formation occurs when the myosin head binds to actin, facilitated by calcium ions exposing binding sites on actin. (2) The power stroke follows, where the myosin head pivots, pulling the actin filament toward the center of the sarcomere. (3) Detachment of myosin from actin happens when ATP binds to the myosin head, causing it to release from actin. (4) Reactivation of the myosin head occurs as ATP is hydrolyzed into ADP and inorganic phosphate, re-cocking the myosin head for another cycle.
3. Step 3: Analyze the options provided in the question and compare them to the correct sequence of events in the muscle contraction cycle.
4. Step 4: Eliminate incorrect options by identifying sequences that do not follow the physiological order of the muscle contraction cycle.
5. Step 5: Select the option that correctly arranges the steps in the order: Cross-bridge formation → Power stroke → Detachment of myosin from actin → Reactivation of myosin head.