The optical properties of birefringence signals from single muscle fibres.

Abstract

The optical retardation of single muscle fibers at rest and the optical properties of the large, early birefringence signal detectable during a twitch were investigated. The resting birefringence, B, which was the factor relating resting retardation, R, to the light path length through the fiber, L, was 2.25 .times. 10-3 (i.e., R = 2.25 .times. 10-3 .times. L) and was independent of wavelength (.lambda. = 480-660 nm). When the angle of incidence, .psi., of the crossed polarizers with respect to the fiber axis was varied, the resting light intensity and large, early change in light intensity were related by the function sin2 .psi. .cntdot. cos2 .psi.. When the net phase shift, .vphi..lambda., of a narrow longitudinal strip of fiber plus compensator was varied, the resting light intensity was described by the function (1-cos .vphi..lambda.), whereas the early change in light intensity followed sin .vphi..lambda.. The optical mechanism underlying the early birefringence signal was a change in retardation. When a narrow longitudinal strip of fiber was illuminated by monochromatic light in the range 480-690 nm, the magnitude of the signal varied approximately as expected if the retardation change was independent of wave-length. The spatial characteristics of the signal were examined by moving a small slit of light across the fiber width and by measuring the signal collected from the entire fiber width as a function of wave-length. The large, early change in retardation was due to a volume-related rather than surface-related structure. Under the assumption that the retardation change was distributed as fiber volume, its average magnitude was calculated. For fibers in normal Ringer the peak of the early retardation change compared with resting was about 1.7 .times. 10-3, and for fibers in D2O Ringer about 0.7 .times. 10-3.