Answer
Skeletal muscle cells are also called myofibers or myocytes ( diameter 10-100 um, length 10-30 cm). Myofilaments are smaller structures within the cylindrical myofibers. There are two kinds of myofilaments the thick filaments ( length 1-2 um, dia. 16 nm) and the thin filaments ( length 1-2 um, dia 8 nm).
Thick filaments lie only in the A band and are crossed by the M line. The main protein of thick filaments is myosin. These molecules have a shape like a golf club. In the thick filaments they molecules are twisted together to form a helical bipolar structure with the myosin heads pointing to the ends of the thick filament, and the shafts of the "golf clubs" positioned in the bare middle of the filament.
Thick filaments are fewer in number than thin filaments ; unlike thin filaments they are free at both ends and they do not slide during muscle contractions
Thin filaments contain three proteins-- actin, tropomyosin, and troponin ; in the relaxed muscle the tropomyosin obscures the myosin binding sites on the actin molecule. Thin filaments more numerous than thick filaments. They exist as long twisted chains that are free only at one end , the other end being attached to the Z disc. During muscle contraction the thin filaments slide into the H zone.
Work Step by Step
Z discs, or Z zones, are narrow saucer-like structures that separates sarcomeres, the functional units of myofibers.
1 nm = 0.001 um ; 1 um= 1/25,000 inch
