A comparison of the fine structure of extraocular and interosseus muscles in the monkey

¹ Department of Ophthalmology, New York University Medical Center New York, N.Y. 10016; This investigation was conducted under a Fight for Sight Postdoctoral Research Fellowship from the National Council to Combat Blindness, Inc., New York, N.Y.
² Department of Ophthalmology, New York University Medical Center New York, N.Y. 10016

The fine structure of the lateral rectus muscle in the rhesus monkey was compared with that of the palmar interosseus muscle. Since the interosseus possessed muscle spindles, it was classified into extra- and intrafusal muscle fibers. Although both of these fibers corresponded to the twitch fiber, the intrafusal fibers were smaller in diameter (15 to 30µ) than the extrafusal (50 to 60µ), and both the transverse tubular system (T system) and the sarcoplasmic reticuluim were less developed in the intrafusal than in the extrafusal fibers. The lateral rectus, however, lacked muscle spindles but possessed the slow fiber as well as the twitch. The slow fibers, smaller in diameter (9 to 15µ), showed irregular, ill-defined myofibrils and poorly developed sarcoplasmic reticulum, and no T system. The twitch fibers, larger in diameter (25 to 50µ), which showed regular arrangement of myofibrils and well developed T system, were further classified into two subtypes: the fiber with numerous mitochondria (red) and the fiber with few mitochondria (white). The physiological significance of the ultrastructural characteristics in each type of muscle fiber was discussed