Muscle is composed of thousands of muscle fibers, each composed of a single muscle cell. As shown in Figure 1 , a muscle cell contains a series of ultramicroscopic filaments called myofibrils. Each myofibril is a muscle cell that contains units called sarcomeres.Sarcomeres contain thick microfilaments composed of the protein myosin.Sarcomeres also contain thin microfilaments composed of the protein actin. The actin and myosin filaments are arranged parallel to one another, with the myosin filaments' molecular “heads” protruding toward the actin filaments. In skeletal muscle, the overlapping actin and myosin filaments give the muscle fiber a banded, or striated, appearance. Hence the muscle is striated muscle.