The sarcomere represent the basal contractile unit of striated mucles. A single myofibril is composed of these short structural units arranged end to end, which contract due to the relative sliding of thick (myosin) over thin (actin) filaments. The mammalian sarcomere is ~ 2 mm in length, and can shorten to ~ 70% of its original length during contraction. Structural features of the sarcomere include bundles of parallel thick and thin filaments assembled by two transverse structures, the Z-disks and M-bands. The Z-disks and the M-band are connected by transverse filaments to the sarcolemma or to the neighboring myofibrils. Sarcomeres give to skeletal and cardiac muscles their striated appearance with I-bands surrounding the Z-disks, followed by A-bands. The A-bands contain a paler region called the H-band and in the middle the M-line.
