Which protein forms the thick filament in muscle fibers?

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

Answer: (B)

In muscle fibers, the contractile machinery relies on two main filament types organized in the sarcomere. The thick filament is built from myosin molecules, with long tails forming the filament backbone and heads that extend outward to form cross-bridges with the thin filaments. These myosin heads pull on actin when ATP is used, driving shortening of the sarcomere during contraction. The thin filament is made mainly of actin, arranged with regulatory proteins that control contraction. Other structural proteins like Nebulin help set the length of the thin filament, and Titin provides elasticity and helps anchor the thick filaments within the sarcomere. So, the protein that forms the thick filament is myosin.