BACKGROUND TO THE THEORIES

MUSCLE STRUCTURE. Before introducing some of these conceptions entertained during the ten or fifteen years after the discovery of the interaction of myosin, actin and ATP, we may consider the re-orientation of ideas concerning interpretation of visible muscle structure. This closer look was necessitated by the discovery of actin, the more exact knowledge of the relative quantities of the muscle proteins and the early observations by means of the electron microscope. As we have seen, ‘myosin’ had been allotted by Noll & Weber (1) in 1935 to the A band, and the double refraction of the fibre had been explained as due to the rod and intrinsic double refraction of this protein. Weber (6) in 1956 remarked that this would mean that the I band must consist of other proteins – perhaps including globulin X and stroma. The assumption however was frequently made that it consisted of disordered myosin. Some observers recorded that the I band rather than the A band shortened on contraction, but the general opinion seems to have been that the material of the A band was that primarily concerned in the mechanism of movement, the changes in the I band being passive.

It is striking to see how many of the observations of classical histology were confirmed by the electron microscope – for example, the A and I bands, the H zone, and the Z and M lines could all be distinguished.