Which term best describes a muscle fiber?

• A. Sarcomere
• B. Myofibril
• C. Muscle fiber
• D. Myofilament

Answer: (C)

Understanding the hierarchical structure of skeletal muscle: a muscle fiber is the actual cell that contracts. It is a single, elongated muscle cell with its own membrane and cytoplasm, and it can contain many nuclei. Within that fiber are many myofibrils, long bundles of proteins arranged in repeating units called sarcomeres. The sarcomere is the functional contractile unit, with actin and myosin myofilaments sliding past each other to produce tension. Therefore, the term that best describes a muscle fiber is the muscle cell itself. The other terms refer to components inside the muscle fiber: a sarcomere is a contractile unit within a myofibril; a myofibril is a bundle of sarcomeres inside the fiber; a myofilament is one of the protein filaments (actin or myosin) that make up the sarcomeres.