Which protein forms the thin filaments in skeletal muscle?

• A. Myosin
• B. Actin
• C. Tropomyosin
• D. Nebulin

Answer: (B)

Thin filaments in skeletal muscle are built mainly from actin. Actin polymerizes into filamentous actin (F-actin), creating the long, flexible strand that runs along the length of the sarcomere and provides the binding sites for myosin heads during contraction. When calcium rises, regulatory proteins on the actin filament (tropomyosin and troponin) shift to expose these sites, allowing cross-bridge cycling with myosin. Myosin, not actin, forms the thick filament that interacts with actin during sliding. Tropomyosin is a regulatory component that blocks binding sites in a relaxed state, while nebulin helps stabilize and set the length of the thin filament but does not form the filament itself.