Innervation and structure of extraocular muscles in the monkey in comparison to those of the cat

Motoneurones that innervate the medial rectus, lateral rectus, and accessory lateral rectus muscles in the monkey have been identified and localized by retrograde transport of horseradish peroxidase. Medial rectus motoneurones were located within both dorsal and ventral regions of the oculomotor nucleus, with a differential distribution along the rostral-caudal axis of the nucleus. Lateral rectus motoneurones were located predominantly within the abducens nucleus, and were distributed throughout the rostral-caudal extent of the nucleus. Motoneurons that innervate the accessory lateral rectus muscle comprised a group of large cells located approximately 0.5 mm ventral to the rostral portion of the abducens nucleus, corresponding to the ventral abducens nucleus of Tsuchida ('06). The ventral subgroup of abducens motoneurones, which innervate both the lateral rectus and accessory lateral rectus muscles, thus do not occupy a motoneurones innervate the retractor bulbi muscle, to which the accessory lateral rectus muscle presumably is homologous. A few accessory lateral rectus motoneurones also were located within the abducens nucleus, overlapping the distribution of lateral rectus motoneurones. Electron microscope examination of the lateral rectus muscle revealed the presence of three morphological types of singly innervated muscle fibers and two morphological types of multiply innervated muscle fibers that exhibited a differential distribution within the orbital, intermediate, and global regions of the muscle. The accessory lateral rectus muscle resembled the global portion of the lateral rectus muscle in containing two morphological types of singly innervated fibers and one type of multiply innervated fiber. These findings indicate that the central differences in the brainstem locations of motoneurones that innervate the cat retractor bulbi and monkey accessory lateral rectus muscles are correlated with peripheral differences not only in the morphology, but also possibly in the mechanical roles, of the muscles they innervate. The accessory lateral rectus muscle thus appears to have evolved both structurally and functionally toward more of a role in patterned eye movement. Furthermore, with the phylogenetic regression of the retractor bulbi muscle, the various types of eye movement with which this muscle is associated in lower vertebrates may be assumed by the other extraocular muscles in higher mammals, including humans.