The following is a list of events that occur during a muscle contraction. Place them in order: 4. The myosin head breaks down ATP and returns to the cocked position. 3. Binding sites on the actin become available and myosin creates a cross-bridge. 5. A molecule of ATP bind to the myosin head, breaking the crossbridge, detaching myosin from actin. 1. Calcium ions are released from the sarcoplasmic reticulum. 6. The myosin head pivots toward the center of the sarcomere, pulling the thin filament. 2. Calcium ions bind to troponin on the thin filaments.
The Correct Answer and Explanation is:
The correct sequence of events during muscle contraction is as follows:
- Calcium ions are released from the sarcoplasmic reticulum.
- This is the first step in muscle contraction. The release of calcium ions from the sarcoplasmic reticulum (SR) into the muscle fiber’s cytoplasm initiates the contraction process.
- Calcium ions bind to troponin on the thin filaments.
- Calcium ions bind to troponin, a protein that is part of the thin filament (actin). This binding causes a conformational change in troponin, which shifts tropomyosin, another protein that covers the actin binding sites. This shift exposes the binding sites on actin.
- Binding sites on the actin become available, and myosin creates a cross-bridge.
- With the binding sites on actin now exposed, the myosin heads, which are part of the thick filaments, attach to these sites, forming a cross-bridge between the actin and myosin filaments.
- The myosin head breaks down ATP and returns to the cocked position.
- The myosin head hydrolyzes ATP to ADP and inorganic phosphate (Pi), which provides the energy for the myosin head to “cock” or position itself for the next power stroke.
- A molecule of ATP binds to the myosin head, breaking the cross-bridge, detaching myosin from actin.
- After the power stroke, a new ATP molecule binds to the myosin head, causing it to detach from actin. The cross-bridge is broken, and the muscle relaxes momentarily, before it can form another cross-bridge if needed.
- The myosin head pivots toward the center of the sarcomere, pulling the thin filament.
- This is the power stroke. Once the myosin head is in the cocked position and reattaches to actin, it pivots toward the center of the sarcomere. This motion pulls the thin filaments inward, shortening the muscle, which is what creates muscle contraction.
The sequence starts with calcium release and ends with the power stroke, where the muscle fiber contracts. Each of these steps relies on ATP to facilitate the movement of myosin along the actin filament.
