Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
Human, Zenker's fluid, Verhoeff & Van Gieson stain, 1446 x.
I band: Isotropic band determined by polarization microscopy. Note the darker Z line bisecting the I band. Electron microscopy has shown that the I band contains thin filaments 50 Å or 5 nm in diameter and approximately 2 µm in length. The contractile protein actin is found in the thin I band filaments.
A band: Anisotropic band determined by polarization microscopy is bisected by the lighter H zone. The A band contains filaments of the protein myosin, which are 100 Å or 10 nm in diameter and 1.6. µm in length, as well as thin (actin) filaments extending from the I band into the A band. The A band filaments are composed of myosin molecules, which, through enzymatic activity (adenosine triphosphatase), release energy essential for the contractile process.
Z line: Bisects the I band. Z from the German word Zwischenscheibe, or in-between line.
H zone: Variable in width, it bisects the A band and vanishes in contraction as the thin I band filaments are drawn further into and through the middle of the A band. When seen, it contains only thick filaments. H for the German word Hell (bright) and also for the zone's discoverer, Hensen.*
Sarcomere: The contractile substance between two Z lines constitutes a convenient structural unit but not the precise functional or primary contractile unit of muscle fibers. See Plate 68.
Nucleus: Elongated and located beneath the sarcolemma. The sarcolemma is seen in Plate 64.
Collagen: Bundles of this connective tissue separate individual muscle fibers (endomysium), bind fascicles, or bundles, of muscle fibers (perimysium), and invest the entire muscle (epimysium). Through this tough and inelastic connective tissue, contractile forces are transmitted to bone and skin.
*Hensen, 1835-1924, was a German anatomist and physiologist.
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