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

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 extraand intrafusal muscle fibers. Although both of these fibers corresponded to the twitch fiber, the intrafusal fibers were smaller in diameter (15 to 30(i) than the extrafusal (50 to 60fi), and both the transverse tubidar system (T system) and the sarcoplasmic reticulum 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 15fi), showed irregular, ill-defined myofibrils and poorly developed sarcoplasmic reticulum, and no T system. The twitch fibers, larger in diameter (25 to 50fi), 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 ultrastmctural characteristics in each type of muscle fiber ivas discussed.