Final answer:
Muscle contraction at the molecular level involves the interaction of actin and myosin filaments in a process sustained by ATP. Myosin heads pivot and pulls actin filaments toward the sarcomere center after binding ATP, shortening the muscle fibers and causing contraction.
Step-by-step explanation:
At the molecular level, muscle contraction involves the interaction of actin and myosin filaments within the muscle cell. When a muscle fiber is stimulated, these protein filaments engage in a process called cross-bridge cycling, powered by the hydrolysis of ATP. This process shortens the sarcomeres, the structural units of muscle fibers, causing the muscle to contract.
Following the release of calcium ions, myosin heads bind to specific sites on the actin filaments, forming cross-bridges. The bound ATP is then hydrolyzed, releasing energy that causes the myosin heads to pivot and pull the actin filaments toward the center of the sarcomere in what is known as the power stroke. As the actin filaments are pulled together, the muscle sarcomeres shorten, leading to the contraction of the muscle fiber. After the power stroke, a new molecule of ATP binds to the myosin, causing the cross-bridge to detach, and the cycle can repeat as long as ATP is available.
In the event of ATP depletion, as seen in rigor mortis, myosin heads remain attached to actin, locking the muscles in their last state of contraction or relaxation.