Transmission electron micrograph of the papillary muscle in a cat's heart. Here one sarcomere length of two adjacent myofibrils is shown at higher magnification. The A-band is seen to be composed of a parallel array of myosin filaments 10 to 11 nm thick and about 1.5 micrometers long . The light I-band consists of actin filaments about 6 nm thick extending in either direction from the central Z-line. Actin filaments are not confined to the I-band but penetrate some distance into the A-band, occupying the interspaces between the myosin filaments. The bracket marked X indicates the region of overlap of the two sets of interdigitating filaments. The bracket marked Y indicates the central region of the A-band, where there are only myosin filaments.