A comparative study of the supramolecular structure of frog sartorius and dorsal semitendinosus muscle

This report describes a comparative X-ray diffraction study of the supramolecular structure of frog sartorius and semitendinosus muscles. For sarcomere lengths of 2.7 microns and below the X-ray diffraction diagrams of each muscle type are very similar; the only differences being that the diffraction diagram for semitendinosus muscles exhibit the presence of a broad diffraction band or a cluster of diffraction orders at a spacing of ca. 230.0 nm and, also, they lack a periodicity of ca. 102.0 nm. For sarcomere lengths greater than 2.7 microns disruption of the sarcomere from sartorius muscle occurs as seen by the loss of sampling in the diffraction diagram. The semitendinosus muscle can be stretched to much longer lengths (in excess of 3.0 microns) before a loss of sampling is detected. The data also shows that in the case of the semitendinosus muscle for long sarcomere lengths transverse bands of mass are able to move without retaining a defined distance to either the Z or the M lines. This is not observed in the case of the sartorius muscle. Thus, at resolutions between ca. 3.6 microns and 7.50 nm significant ultrastructural differences between these two muscles are apparent. The data suggest that the ability of these mass bands to move may be responsible for the differences in the development of passive tension exhibited by these two muscles.