Equatorial X-ray reflections and cross arm movement in skeletal muscle

Abstract

WHEN resting skeletal muscle is excited or goes into rigor, there is a characteristic change in the ratio of the intensities of the two principal equatorial reflections, I₁₁/I₁₀. This has been interpreted simply as reflecting movement or transfer of mass in the radial direction from the thick to thin filaments^1,2. Other, higher orders, have, however, been observed^1,3, and a more detailed interpretation is possible, which should eliminate some of the ambiguity that results from using only two reflections. Since there is considerable evidence that the myosin filament is actually three or four stranded^4–6, rather than two stranded as originally proposed, reinterpretation is necessary. Further, studies of X-ray diagrams from muscle in the presence of ATP analogues suggest that at least in some cases, equatorial intensity ratios intermediate between rigor and relaxed do not arise from a mixture of those states, but rather from a state with different structural properties^7–9.