Diffraction Spectra of Striated Muscle and the Mechanism of Contraction

Abstract

Previous work 1 , 2 , 3 on the diffraction patterns produced by striated muscle at normal, stretched, and contracted lengths, has shown in general that s, the sarcomere length, can be calculated from measurements of the angles of diffraction, Θ_n, by means of the diffraction grating equation nλ = s sin Θ_n, where n = the spectral order, and λ = the wave length of the light. In the present work records have been obtained of the variations of the diffraction pattern, along with the simultaneously produced tension response, of the frog sartorius during isometric twitch. A set of only about 400 striations along the length of the muscle near the nerve plexus is used to form the diffraction spectra. It is thus possible, using the above equation, to compare changes in the length of the sarcomeres of these striations, with changes in the state of the whole muscle at each instant of muscle activity.

Fig. 1 shows 2 records obtained from different sartorii stimulated with supermaximal break shocks. Starting with the lowest band each record includes: the undiffracted beam, the first order spectrum, and the tension record, the dashes indicating 0.01 sec. intervals. The arrows mark the instant at which the tension response begins.∗

Although the unsharp edges of the spectra permit only a rough analysis, several features are nevertheless quite plain. The first order spectra persist during contraction; hence it follows that the striations likewise persist. The upward displacement of the spectra is the consequence of an increase in the angle of diffraction; since n and λ are constant, s must decrease. The rough measurements show that at the peak of contraction the percentage decrease in s is of the same order as that of the length of the entire muscle.