Sliding-Filament Mechanism

The sliding-filament mechanism is the fundamental model explaining how striated muscle produces force and, when mechanical conditions allow, shortens. During shortening, the thin actin filaments and thick myosin filaments of each sarcomere slide past one another, increasing their overlap. The filaments themselves remain approximately the same length, while the sarcomere becomes shorter .^1,2

Sliding is generated by repeated interactions between myosin heads and actin. After calcium-dependent changes in the thin filament permit myosin binding, myosin can attach to actin and undergo an ATP-dependent sequence called the cross-bridge cycle. Structural changes in the myosin head generate force and can move the thin filament relative to the thick filament. ATP is also required for myosin to detach and begin another cycle .^1,2

As thin filaments move toward the center of the sarcomere during shortening, the Z discs move closer together. The I band and H zone become narrower because these regions contain portions of the filaments that do not overlap. In contrast, the A band remains approximately constant in length because it corresponds to the length of the thick filaments .^1,2

The sliding-filament mechanism should be distinguished from the cross-bridge cycle. The sliding-filament mechanism describes the overall relative movement of thick and thin filaments during shortening, whereas cross-bridge cycling describes the molecular actin-myosin interactions that generate force and can produce filament sliding .¹