Thin filaments are composed of which components?

• A. Myosin
• B. Actin, troponin, tropomyosin
• C. Titin
• D. Keratin

Answer: (B)

Thin filaments in muscle are built around actin, with regulatory proteins troponin and tropomyosin closely associated along the filament. Actin forms the main structural backbone of the thin filament and provides the binding sites that myosin heads grab during contraction. Tropomyosin wraps around actin and blocks those binding sites when the muscle is relaxed. Troponin, a complex of three subunits, binds calcium when a stimulus arrives, causing troponin to shift tropomyosin away from the binding sites and exposing them for myosin to attach during contraction.

This combination—actin plus the regulatory troponin-tropomyosin complex—defines the thin filament. In contrast, myosin makes up thick filaments, titin helps anchor and elasticize the sarcomere, and keratin is a structural protein in skin, not part of the muscle contractile apparatus.