What is a muscle fiber?

• A. A bundle of fibers
• B. A tissue connecting bone
• C. A cell containing many myofibrils
• D. A nerve cell

Answer: (C)

A muscle fiber is a single muscle cell. Inside that cell are many parallel contractile threads called myofibrils, each containing repeating units called sarcomeres—the actual contractile units where actin and myosin interact to shorten the fiber during contraction. Because a muscle fiber holds many myofibrils, it can generate a lot of force. A bundle of fibers forms a fascicle, not an individual fiber; a tissue that connects bone to bone or bone to muscle is a tendon or ligament, and a nerve cell is a neuron. So the defining idea is that a muscle fiber is a single muscle cell packed with numerous myofibrils responsible for contraction.