A sarcomere is the contractile element and functional unit of a myofibril and is delineated by two Z lines.Overlapping and alternating actin and myosin filaments are responsible for the striated appearance or the presence of light and dark bands. The dark bands are called A-bands (anisotropic) and contain both actin and myosin. The middle of the dark band is the H-zone containing myosin only. The light bands are the I-bands (isotropic) that contain actin only.