Which of the following happens as actin and myosin filaments slide past each other during muscle contraction?

During muscle contraction, the following steps occur as actin and myosin filaments slide past each other:

1. The process begins with the arrival of a nerve signal and the release of calcium ions (Ca2+) into the muscle cell.

2. The calcium ions bind to a protein called troponin on the actin filament. This causes the tropomyosin to move away from the binding sites for the myosin heads, exposing them.

3. The myosin heads, which are in a high-energy configuration due to the breakdown of ATP (adenosine triphosphate) to ADP (adenosine diphosphate) and inorganic phosphate, bind to the exposed sites on the actin filament, forming a cross-bridge.

4. The myosin heads then change to a low-energy configuration, pulling the actin filament along with them. This is known as the power stroke.

5. Another ATP molecule binds to the myosin head, causing it to detach from the actin filament.

6. The ATP is then broken down again, returning the myosin head to its high-energy configuration and allowing it to reattach to a further point along the actin filament.

7. This process repeats, causing the actin and myosin filaments to slide past each other and the muscle to contract.

8. When the nerve signal stops, calcium ions are pumped back out of the muscle cell, the binding sites on the actin filament are covered again, and the muscle relaxes.

This process is known as the sliding filament theory of muscle contraction.