The banding pattern along the fibre divides it into repeating units called sarcomeres which lie between thin dark lines (the Z-lines - or better, the Z-discs) found in the middle of the light region (the I-band or isotropic band). In this isotropic region of the fibre, there are aligned thin filaments made of the protein actin, attached to the protein (alpha actinin) of the Z-disc.
Alpha actinin of the Z-disc.
Thin actin filaments are attached
on both sides, and form the I-band.
In the darker A-band (anisotropic band) toward the middle of the sarcomere, are the thick, myosin containing filaments. The these are held in place partly by a meshwork of protein filaments in the middle region, which forms the slightly darker M-line (grey). The region of overlap between the actin and myosin filaments is very dark. The lighter central region around the M-line has only myosin and is called the H-zone.
The thick filaments are also held in place by the longitudinal S-filaments of the protein titin.