A characteristic feature of photographs of longitudinal sections of skeletal muscles when seen under the microscope is the regular transverse bandings or striation. These have given the name ‘ striated ’as an alternative to ‘ skeletal ’ muscle. Unless specially stained with dyes, muscles do not usually appear striated when in correct focus under the ordinary light microscope. However, the striations can be seen by using either a polarizing or a phase contrast microscope. The reason for this is that the striations are caused by alternating bands with high refractive index are birefringent. Birefringent materials alter the plane of vibration of light passing through them and this can be detected in the polarizing microscope.
Birefringent materials are described as optically anisotropic. Non-birefringent materials are isotropic . The dark bands in muscle seen under the polarizing microscope therefore became known as A-bands (after anisotropic) and the intervening (clear) bands as I-bands (after isotropic). It is now known how these bands relate to the structural relationships of the thin filaments. The A-band is formed by the thick filament, together with the overlapping thin filaments, and the I-band by mainly just the thin filaments. Because the myofibrils are exactly aligned, the banding pattern continues across the fibres.
Early microscopists also identified various other lines and zones, including the H zone, and the H zone, and the M and Z lines. The Z line is of particular functional significance. It is really a disc through which the thin filaments pass (and is often therefore also referred to as the Z disc ) Adjacent Z lines delineate the functional unit of the myofibril referred to as a sarcomere. A myofibril consists of thousands of sarcomeres. The sorcomere length, defined by the distance between Z lines,