What is a muscle fiber?

• A. A muscle cell
• B. A tendon
• C. A nerve
• D. A blood vessel

Answer: (A)

A muscle fiber is the basic contractile cell of a skeletal muscle. These are long, cylindrical cells packed with many myofibrils, which contain the repeating units called sarcomeres where actin and myosin slide past one another during contraction. When a fiber contracts, its sarcomeres shorten, generating force that pulls the muscle as a whole. The fiber is wrapped in the sarcolemma, and its interior, the sarcoplasm, holds mitochondria for energy and a network of sarcoplasmic reticulum that releases calcium to trigger contraction. Fibers group into fascicles and together form a muscle.

A tendon is dense connective tissue that links muscle to bone, not a muscle cell. A nerve is tissue that transmits electrical signals to muscles, and a blood vessel supplies blood but is not a muscle cell. Therefore, the correct interpretation is that a muscle fiber is a muscle cell.