Constancy of axial spacings in frog sartorius muscle during contraction

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 contract1,2. 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 filaments1,3. Part of the axial pattern comes from the actin filaments and closely resembles the patterns given by oriented samples of purified actin4–6; 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