Structural Changes in the Actin- and Myosin-eontaining Filaments during Contraction

Earlier studies on the low-angle X-ray diffraction diagrams given by striated muscles showed that some features of the diagrams remained virtually unchanged during contraction, whereas others changed in a characteristic manner. Thus the subunit repeat and the pitch of the actin helices in the thin filaments remained apparently constant, and the subunit repeat in the myosin filaments (the 143 Å cross-bridge spacing) remained almost constant too (Elliott et al., 1965, 1967; Huxley et al., 1965; Huxley and Brown, 1967), although more extensive measurements revealed a small increase in spacing during contraction, by about 1 % (Huxley and Brown, 1967; Haselgrove, 1967, 1970). On the other hand, there was a very substantial decrease in the intensity of the X-ray diagram given by the helical arrangement of cross-bridges on the thick filaments, especially in the off-meridional parts of the pattern, which was interpreted as showing that the cross-bridges on any given filament...