DISCOVERY OF THE SLIDING MECHANISM

The first hints of the sliding-filament mechanism of contraction were given by the low-angle X-ray diffraction patterns obtained by H. E. Huxley with living and glycerol-extracted muscle. As we have seen, high-angle X-ray diffraction patterns had not differentiated between resting and contracted muscle; on several occasions the need for the low-angle technique had been emphasised, but before 1953 this had been used only on dried material, as in the work of Astbury (quoted by MacArthur (1)) in 1943 and of Bear in 1945. H. E. Huxley (2) began in 1951 by showing with dried muscle that stretching by 60 % had no effect on the repeat pattern found by Bear (of which the largest spacing was about 58 Å); he surmised that if the structure producing this pattern existed in living muscle and if this structure were affected by stretching, then the effect could not be on the 58 Å units themselves but must consist in the pulling apart of the long molecules containing these units. Equipped with newly designed apparatus Huxley (3) went on to study the effect of stretch in living rabbit and frog muscle. Here the axial pattern at very low angles showed reflections corresponding to orders of a 420 Å repeat pattern, of which previous workers had seen only the reflections at 50–60 Å.