Laser diffraction patterns during isometric and auxotonic contractions in frog skeletal muscle

The changes in laser diffraction patterns during contraction of frog skeletal muscle are investigated under auxotonic as well as isometric conditions. Muscle fiber is connected to the double lever system of Huxley and Peachey's type so that the illuminated portion can be kept immobile during the muscle shortening. Under isometric conditions, the first order diffraction lines show rapid decreased in intensity at the start of contraction, but they stay at a nearly steady level during maintained tension. Meanwhile, noticeable changes in line width are not observed. The first order diffraction lines show further transient decrease in intensity at the onset of relaxation after the end of stimulation and then return almost to the initial resting state when relaxation is completed. These changes in diffraction lines are not obvious in sarcomeres longer than 3.3 micrometers, where only small tensions are developed. If the muscle is allowed to shorten under auxotonic conditions, not only decrease in intensity but also expansion in width is observed. The possible origins of changes in diffraction lines are considered to be non-uniform activation in excitation-contraction coupling, in addition to the displacement, bending, or transformation of the diffractive gratings. The disordering of the sarcomere arrangements also contributes to the change in diffraction patterns during muscle shortening.