Polarization changes in light diffracted from contracting muscle fibers

Single fibers were isolated from the semitendinosus muscle of frog and illuminated with an He-Ne laser. The polarization of the laser beam was varied by a photoelastic modulator. The time course of the degree of polarization of light diffracted from the muscle fiber during an isometric contraction was measured directly with a time resolution of 1 ms. Tension, sarcomere length, and diffraction intensity were also measured. During the contraction cycle, the degree of polarization of the active fiber exhibited a biphasic variation relative to that of the resting fiber. Analysis identifies the movement of heavy meromyosin toward actin and the rise in myoplasmic calcium ion concentration as the main contributors to the polarization transient of active fibers. A quantitative theory describing the polarized diffraction from muscle fibers is formulated. There is good agreement between the theory and measurements.