Final answer:
The sliding filament theory explains muscle contraction by the sliding of actin and myosin filaments past each other, driven by ATP hydrolysis and the formation of cross-bridges, initiated by calcium binding to troponin.
Step-by-step explanation:
The sliding filament theory is a biological explanation for how muscle contraction occurs. According to this theory, muscle fibers contract when the actin and myosin filaments within the sarcomeres slide past one another. This sliding is initiated when calcium ions (Ca2+) bind to troponin, which causes tropomyosin to move away from the myosin-binding sites on actin filaments. Once these sites are exposed, myosin heads can bind to them, forming cross-bridges.
Upon binding, the myosin heads use energy from ATP hydrolysis to pivot and pull the actin filaments towards the center of the sarcomere, known as the M line. This action shortens the sarcomere and causes muscle contraction. Following this 'power stroke', a new ATP molecule binds to the myosin head, leading to the release of the myosin from actin, and upon hydrolysis of ATP, the myosin head is 're-cocked' for another cycle of binding and pulling.