The striated pattern that is so characteristic of striated muscle directly results from the structure of the contractile units of the muscle. Each fiber of the muscle is striated and made up of many myofibrils which are also striated in the same pattern of alternating dark and light bands called the A bands and I bands, respectively. In the center of each A band is a lighter zone called the H zone; in the center of each I band is a dark, thin line called the Z line. The portion of a myofibril between two Z lines constitutes a single contractile unit termed a sarcomere. Each sarcomere is composed of two sets of protein filaments. The thick myosin filaments are located in the A band. The filaments are composed of myosin proteins. Each protein strand has a globular head at each end and a long center section. The thin actin filaments are located primarily in the I bands but extend into the A bands. The overlap of the actin and myosin filaments causes the dark coloration of the A bands; actin's absence from the center of the A bands results in the lighter H zone of each A band. Contraction begins when the myosin is stimulated by a nerve impulse; when stimulated, the globular head of a myosin protein binds with the actin to form actomyosin. Once bound, the myosin head is able to break ATP into ADP and Pi releasing energy which causes the myosin head to pull the actin toward the center of the sarcomere. The myosin head then releases from the actin and repeats the binding and pulling action
