General Physiology

Step 1: Understanding the Concept:
The muscle contraction cycle, also known as the cross-bridge cycle, describes the series of molecular events that occur between actin and myosin filaments during muscle contraction.

Step 2: Detailed Explanation of the Sequence:
The cycle can be thought of as starting with the myosin head in a state ready to bind.
1. C. ATP hydrolysis: The myosin head has an ATP binding site and ATPase activity. It hydrolyzes ATP into ADP and inorganic phosphate (Pi). The energy released from this hydrolysis "cocks" the myosin head into a high-energy, ready position.
2. B. Attachment of myosin to actin: The high-energy myosin head attaches to an available binding site on the actin filament, forming a cross-bridge.
3. A. Power stroke: The release of the inorganic phosphate (Pi) from the myosin head triggers the power stroke. The myosin head pivots and pulls the actin filament towards the center of the sarcomere (the M line). ADP is then released.
4. D. Detachment of myosin from actin: A new molecule of ATP binds to the myosin head. This binding causes the myosin head to detach from the actin filament. The cycle is now ready to begin again with the hydrolysis of this new ATP molecule (back to step C).

Step 3: Final Answer:
The correct sequence of these four events is C B A D. This corresponds to option (C).