Cross-bridge movement during muscle contraction

Abstract

WHEN vertebrate striated muscle contracts at constant length, the 1,0 reflection decreases in intensity while the 1,1 reflection increases in intensity^1,12. These changes have been interpreted as arising from the movement of the cross bridges from the vicinity of the thick filaments in relaxed muscle to the vicinity of the thin filaments in the contracting state, and this movement has been thought to involve both radial and azimuthal components^2,3. Lymn⁴ has claimed that models with cross bridges near the thick filament do not simulate the intensity distribution of the higher order equatorial reflections of relaxed muscle, while models in which the cross bridges are extended to near the radius of the thin filaments do give good simulations. This has led him to contest the interpretation of the intensity changes given above, and to propose instead a model in which these changes are brought about by azimuthal movement alone. In this article we challenge Lymn's interpretation on several grounds.