At higher magnification, you may be able to see a lighter band in the middle of the A band. This is the H band (present as a faint band in HA8), that portion of the A band where there are no thin filaments. Alternating with the A bands, there are bands, once again in register, that stain much more lightly. These are the I bands, the location of much of the length of the thin filaments. Bisecting the I band, there is a narrow dark line, the Z band, composed of a-actinin and other proteins that serve as the anchor for the thin filaments.