Striated muscle. Coloured transmission electron micrograph (TEM) of a longitudinal section through a striated, or skeletal, muscle. The arrangement of the muscle's myofibrils produces the regular banding pattern. The wide, dark purple A bands are bisected by narrow, pink H bands, which contain thick filaments of the protein myosin. The wide, beige I bands contain thin filaments of the protein actin. According to the widely-accepted sliding filament theory, it is these thick and thin filaments that slide past one another during contraction, causing the H and I bands to narrow.