Arrangement of filaments in striated muscle. (A) Extended Arrangement of filaments in striated muscle. (A) Extended. The positions of the I, A, and H bands in the extended state are shown. The thin filaments partly overlap the ends of the thick filaments, and the thin filaments are shown anchored in the Z lines (often called Z disks). In the lower part of Figure 51–2A, “arrowheads,” pointing in opposite directions, are shown emanating from the myosin (thick) filaments. Four actin (thin) filaments are shown attached to two Z lines via α-actinin. The central region of the three myosin filaments, free of arrowheads, is called the M band (not labeled). Cross sections through the M bands, through an area where myosin and actin 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 toward each 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 to 1500 nm), and the lengths of the H and I bands are also reduced because of the overlap between the thick and thin filaments. These morphologic observations provided part of the basis for the sliding filament model of muscle contraction. Source: Muscle & the Cytoskeleton, Harper's Illustrated Biochemistry, 30e Citation: Rodwell VW, Bender DA, Botham KM, Kennelly PJ, Weil P. Harper's Illustrated Biochemistry, 30e; 2015 Available at: http://accessmedicine.mhmedical.com/DownloadImage.aspx?image=/data/books/1366/rod_ch51_f002.png&sec=73246934&BookID=1366&ChapterSecID=73246920&imagename= Accessed: December 21, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved