What is the sliding filament model?

• A. The muscle remains same length
• B. The thick filament shortens
• C. The sliding filament model is irrelevant
• D. A muscle contraction when the thin filament slides over the thick filament

Answer: (D)

The sliding filament model explains muscle contraction by the actin (thin) filaments sliding past the myosin (thick) filaments, shortening the sarcomere without the filaments themselves shortening. When calcium exposes the myosin-binding sites on actin, the myosin heads form cross-bridges and pull on actin, using energy from ATP. This cross-bridge cycling causes the actin filaments to slide toward the center of the sarcomere, so the Z-lines come closer together and the muscle contracts. The thick filaments stay the same length; it’s the sliding of thin over thick that shortens the sarcomere.

So the best description is that a muscle contraction occurs when the thin filament slides over the thick filament.