Histochemical analysis of muscle fiber types of rat superior rectus extraocular muscle

SummaryExtraocular muscles (EOMs) represent a distinctive class among mammalian skeletal muscles in exhibiting unique anatomical and physiological properties. To gain insight into the basis for the unique structural/functional diversity of EOM fiber types and to explain their high fatigue resistance, rat superior rectus muscle (SRM) was studied using histochemical techniques. Muscle fibers were typed with regard to their oxidative and glycolytic profiles generated from succinic dehydrogenase (SDH) and phosphorylase activities in combination with their morphologic characteristics. Superior rectus muscle is organized into two layers, a central global layer (GL) of mainly large diameter fibers and an outer C-shaped orbital layer (OL) of principally small diameter fibers. Five muscle fiber types were recognized within the SRM: I, II, III, IV and V. In the global layer, four muscle fiber types were identified: type I (18.25±0.96 μm; 32%) showed intermediate SDH (coarse type) and high phosphorylase activity. Type II fibers (14.45±0.82 μm; 22%) exhibited high SDH (fine type) and intermediate phosphorylase activity. Low SDH (granular type) and high phosphorylase activity were demonstrated by type III fibers (22.65±1.73 μm; 36%). Type IV fibers (26.24±1.32 μm; 10%) were recognized by their low oxidative and glycolytic reactions. In the orbital region, only three muscle fiber types were recognized; fiber types I and II were found to compose approximately two-thirds of the layer. The third orbital fiber type (type V, 10.05±0.99 μm) exhibited low SDH and low phosphorylase profiles. In this paper, the functional significance of the histochemical characteristics of the EOM fiber types is discussed.