Constancy of Axial Spacings in Frog Sartorius Muscle during Contraction

Abstract

STRIATED muscles give a characteristic system of low-angle X-ray reflexions. These reflexions can be seen using preparations maintained in a normal physiological condition and still able to contract^1,2. A number of axial reflexions can be observed in addition to the equatorial reflexions which arise from the hexagonal lattices of overlapping actin and myosin filaments^1,3. Part of the axial pattern comes from the actin filaments and closely resembles the patterns given by oriented samples of purified actin^4–6; in muscle, the most prominent feature here is a strong off-meridional reflexion at 59 Å. At smaller angles a number of reflexions corresponding to an axial period of approximately 435 Å are observed, and Worthington⁷, whose measurements are more accurate than the original ones¹, has given good reasons for believing that they come from the myosin filaments. According to the sliding-filament model, passive changes in the length of the muscle are not accompanied by any changes in the lengths of either type of filaments, and so the original observation¹ that all the axial reflexions are unchanged by passive stretch is precisely what we should expect.