Fiber composition of the superior rectus extraocular muscle of the rhesus macaque.

The fiber composition of the superior rectus extraocular muscle of adult rhesus monkeys (Macaca mulatta) was examined in serial sections by light and electron microscopy as well as morphometry. Analyses allowed separation of singly versus multiply innervated fibers, and disclosed the morphology of these fibers and their topographical distribution within the muscle. Six morphologically distinct fiber types were found. The global layer of the muscle contains three singly innervated fibers (SIFs) and one multiply innervated fiber (MIF). The overlying orbital surface layer contains one SIF and one MIF, both showing morphological variation along their length. The MIFs of the orbital surface layer had not only multiple, relatively superficial endings along their length, but also endplatelike junctions in which the axonal terminals lie within synaptic clefts. These endplatelike junctions were confined to that region of the MIFs that had a morphology typical of "twitch" fibers; away from this region these MIFs had a morphology characteristic of "tonic" fibers. In contrast, the global MIFs showed constant morphology, and did not exhibit endplatelike endings.