Structure and function of a sarcomere | Physiology of Sport and Exercise

When muscle contracts, muscle fibers shorten. How do they shorten? The explanation for this phenomenon is termed the sliding filament theory. When the myosin cross-bridges are activated, they bind with actin, resulting in a conformational change in the cross-bridge, which causes the myosin head to tilt and to drag the thin filament toward the center of the sarcomere. This tilting of the head is referred to as the power stroke. The pulling of the thin filament past the thick filament shortens the sarcomere and generates force. When the fibers are not contracting, the myosin head remains in contact with the actin molecule, but the molecular bonding at the site is weakened or blocked by tropomyosin.

Immediately after the myosin head tilts, it breaks away from the active site, rotates back to its original position, and attaches to a new active site farther along the actin filament. Repeated attachments and power strokes cause the filaments to slide past one another, giving rise to the term sliding filament theory. This process continues until the ends of the myosin filaments reach the Z-disks, or until the Ca2+ is pumped back into the SR. During this sliding (contraction), the thin filaments move toward the center of the sarcomere and protrude into the H-zone, ultimately overlapping. When this occurs, the H-zone is no longer visible.

Recall that the sarcomeres are joined end to end within a myofibril. Because of this anatomical arrangement, as sarcomeres shorten, the myofibril shortens, causing muscle fibers within a fascicle to shorten. The end result of many such fibers shortening is an organized muscle contraction.

[tariqnaeem]

Thanks for the clearing this complex system ❤️