A study of myofibril sarcomere structure during contraction.

In the present investigation of cross-striated muscle fibers of axolotl, we succeeded in observing in one field of vision of the electron microscope all the stages of myofibril contraction. This allowed us to avoid errors in establishing the sequence of individual contraction stages. Our studies reveal a new contraction stage which appears at the shortening of the sarcomere below 74 per cent of the "resting length" but prior to the formation of typical "maximally shortened" sarcomeres, characterized by strong "contraction bands." At this stage, in the center of the sarcomere, at either side of the M line, a "secondary anisotropic" band arises which widens with further sarcomere contraction. At either side of this band, at the place of the former ("primary") anisotropic band, a "secondary isotropic" band is formed. A scheme of successive stages of contraction of the sarcomere is presented. The mechanisms of contraction for the first stage (from 100 to 79 per cent of the "resting length") and for the last stage (from 74 to 58 per cent of the "resting length") seem to be different. While the sliding of myofilaments with respect to one another can be assumed for the first stage, it is the spiralization of these structures which is the most likely explanation for the last stage. (An Abstract in German also appears at the end of this article.)