What is a myofilament?

• A. Protein filaments that make up a sarcomere
• B. The energy source for muscle contraction
• C. The outer covering of a muscle
• D. A nerve impulse conductor

Answer: (A)

Protein filaments that make up a sarcomere are myofilaments. In skeletal muscle, the sarcomere is the basic contractile unit and contains thick filaments of myosin and thin filaments of actin, along with regulatory proteins. When contraction occurs, myosin heads form cross-bridges with actin and pull the thin filaments past the thick ones, shortening the sarcomere in a process called the sliding filament mechanism. This action requires ATP and calcium to proceed, but the myofilaments themselves are not the energy source. They also are not the outer covering of the muscle or the carrier of nerve impulses—that role belongs to connective tissue layers encasing the muscle and to neurons, respectively.