What is the functional unit of skeletal muscle?

• A. Sarcomere
• B. Myofibril
• C. Fibers
• D. Endomysium

Answer: (A)

The functional unit of skeletal muscle is the sarcomere, the segment between two Z-discs that contains the overlapping thick (myosin) and thin (actin) filaments. This is the smallest unit capable of producing contraction: when cross-bridges form and cycles between attachment and detachment, the sarcomere shortens, generating force. Sarcomeres are lined up end-to-end along each myofibril, so many of them shorten in unison during a contraction, while the individual filaments themselves don’t change length. The other options aren’t the contractile unit: a myofibril is a long chain of sarcomeres; a muscle fiber is a single cell that contains many myofibrils; the endomysium is a connective tissue wrapping around a fiber.