Harpers

558 / CHAPTER 49H bandA. ExtendedI bandA bandZ line2300 nmActin filaments6-nm diameterα-ActininMyosin filaments16-nm diameterCross section:B. ContractedThinfilament6-nm diameterThickfilament1500 nm16-nm diameterFigure 49–2. Arrangement of filaments in striated muscle. A: Extended. The positions of theI, A, and H bands in the extended state are shown. The thin filaments partly overlap the ends of thethick filaments, and the thin filaments are shown anchored in the Z lines (often called Z disks). Inthe lower part of Figure 49–2A, “arrowheads,” pointing in opposite directions, are shown emanatingfrom the myosin (thick) filaments. Four actin (thin) filaments are shown attached to two Z linesvia α-actinin. The central region of the three myosin filaments, free of arrowheads, is called theM band (not labeled). Cross-sections through the M bands, through an area where myosin andactin filaments overlap and through an area in which solely actin filaments are present, are shown.B: Contracted. The actin filaments are seen to have slipped along the sides of the myosin fibers towardeach other. The lengths of the thick filaments (indicated by the A bands) and the thin filaments(distance between Z lines and the adjacent edges of the H bands) have not changed. However,the lengths of the sarcomeres have been reduced (from 2300 nm to 1500 nm), and thelengths of the H and I bands are also reduced because of the overlap between the thick and thinfilaments. These morphologic observations provided part of the basis for the sliding filamentmodel of muscle contraction.