What is the contractile unit within a muscle fibre called?

• A. Myofibril
• B. Sarcomere
• C. Actin
• D. Myosin

Answer: (B)

The contractile unit is the sarcomere. A sarcomere is the segment of a muscle fiber between two Z-discs and contains the thin actin filaments and thick myosin filaments that interact during contraction. When a muscle contracts, calcium activates the system and ATP powers the myosin heads to pull on actin, sliding the filaments past one another and shortening the sarcomere. Because many sarcomeres align end to end within a myofibril, the shortening of all of them together produces the overall muscle contraction. The other options are not the contractile unit: a myofibril is a long bundle of many sarcomeres, and actin and myosin are the filament proteins themselves, not the unit that shortens.