Optical diffraction study of muscle fibers.

Skeletal muscle fibers act as a one-dimensional grating to electromagnetic waves in the optical frequency range. Results are presented of the optical diffraction study of frog sartorius muscle fibers. 1. A simple theoretical consideration predicts that, at a sarcomere length near 3 mum, the second-order reflection is contributed only by the thin filaments due to the destructive interference between scattered rays from thick filaments. At that sarcomere length, both the first-order and the third-order reflections are contributed by both kinds of filaments. 2. Intensity measurements of diffraction lines of muscle fibers at rest at various sarcomere lengths confirmed the validity of our theoretical prediction.3. The isometric contraction of skinned fibers was realized by the phoretic injection of Ca2+ from a micropipette. During the Ca2+ injection intensity drops of the diffraction lines were observed. At a very narrow range of sarcomere lengths near 3 mum, the intensity drop of the second-order diffraction line became very small on Ca2+ injection, whereas the intensity drops of both the first-order and the third-order diffraction lines were very large. 4. A theoretical analysis concludes that the intensity drops on Ca2'njection are solely due to small random fluctuations of the position of thick filaments in the sarcomeres.