A myofibril is a long cylindrical organelle found in muscle cells formed by two transverse filament systems: the thick and thin filaments. The thin filament is composed primarily of actin; it is tethered at one end to the Z-disk, and it interdigitates with the thick filaments. The main constituent of the thick filament is myosin; the direction of the myosin heads changes polarity at the M-line, allowing interaction with the thin filaments anchored from the next adjacent Z-disk. A third and a fourth filament complex exist, comprising the giant proteins titin and nebulin.
