Properties and Distribution of Fast Twitch and Multiinnervated Slow Fibers in Glycerol-Extracted Extraocular Muscles of the Cat Examined by the Laser Diffraction Method
Mammalian extraocular muscles contain multiinnervated slow fibers. To investigate the properties of these slow fibers, laser diffraction patterns were examined at rest and during contraction. Kittens were anesthetized and the retractor bulbi (RB), the inferior oblique (IO), and the lateral rectus (LR) were isolated from the orbita, and they were treated by 50% glycerol at ― 20°C for 4-14 days. These muscle fibers were illuminated by a polarized He-Ne laser beam (632.8 nm) and the intensity of the diffracted light was monitored by moving a small photodiode in the direction perpendicular to the diffraction line. The diffraction patterns of a single fiber prepared from the RB exhibited spatially distinct distribution. The diffraction patterns of the LR and IO, however, were not as sharp as those of the RB. The ratio of peak amplitude to the width at 50% peak of the first order diffraction pattern was taken as an index to describe the sharpness quantitatively. It was observed that most of the muscle fibers from the RB showed high peak/width ratio values, whereas many fibers from the IO and LR exhibted low peak/width ratio values. The muscle fibers with high values were possibly due to fast fibers and those with low values to multiinnervated slow fibers. Based on these results, it was estimated that the slow fibers occupied 30-41% in the IO. 0-11% in the global layer and 36-47% in the orbital layer of the LR. In regard to the contraction, the total light intensity of one side of the firsl order line decreased remarkably during contraction. This came from structural changes in myosin filaments. It is expected that the decrease in the light intensity during contraction would be different for the two types of muscle fibers.