Myofibril fibers are present along the length of the cell. Myofibril fibers are composed of actin and myosin filaments. Actin and myosin filaments together are called sarcomere. Myofibrils have two types of lines or bands. anisotropic band (A) and isotropic band (I). Darker bands are called anisotropic or A bands and lighter bands are isotropic or I bands.