What is the sliding filament mechanism?

• A. Actin and myosin filaments slide past each other to shorten the sarcomere while their lengths remain constant
• B. Filaments shorten during contraction
• C. Filaments lengthen during contraction
• D. Filaments detach from cross-bridges

Answer: (A)

Contraction happens because the thick and thin filaments slide past each other, shortening the sarcomere while the filaments themselves stay the same length. Calcium release exposes myosin-binding sites on actin, allowing myosin heads to attach and pull actin inward through cross-bridge cycling powered by ATP. As this sliding occurs, the I-band and H-zone shrink and the sarcomere length decreases, while the A-band remains unchanged since the length of the thick filaments doesn’t change. This makes the sliding filament mechanism the correct description: contraction results from filaments sliding past each other, not from filaments getting shorter or longer. Detachment from cross-bridges happens during the cycle, but it isn’t the driving description of how contraction occurs.