What are thick filaments composed of and what are thin filaments composed of?

• A. Thick filaments are actin; thin filaments are myosin.
• B. Thick filaments are myosin; thin filaments are actin.
• C. Thick filaments are titin; thin filaments are nebulin.
• D. Thick filaments are dystrophin; thin filaments are spectrin.

Answer: (C)

In muscle, the main contractile filaments are organized as thick and thin filaments. Thick filaments are made mainly of myosin, while thin filaments are made mainly of actin. Myosin molecules have motor heads that form cross-bridges with actin and pull on the filaments during contraction, powered by ATP. Actin filaments carry regulatory proteins (troponin and tropomyosin) that control when myosin heads can bind, depending on calcium levels.

Other proteins like titin and nebulin play important roles in the sarcomere’s structure and length regulation, but they are not the primary components of the thick and thin filaments. Dystrophin and spectrin are membrane-associated cytoskeletal proteins, not the main contractile filaments.

So the correct pairing is thick filaments made of myosin and thin filaments made of actin.