Motoneurons of twitch and nontwitch extraocular muscle fibers in the abducens, trochlear, and oculomotor nuclei of monkeys

Eye muscle fibers can be divided into two categories: nontwitch, multiply i nnervated muscle fibers (MIFs), and twitch, singly innervated muscle fibers (SIFs). We investigated the location of motoneurons supplying SIFs and MIF s in the six extraocular muscles of monkeys. Injections of retrograde trace rs into eye muscles were placed either centrally, within the central SIF en dplate zone; in an intermediate zone, outside the SIF endplate zone, target ing MIF endplates along the length of muscle fiber; or distally, into the m yotendinous junction containing palisade endings. Central injections labele d large motoneurons within the abducens, trochlear or oculomotor nucleus, a nd smaller motoneurons lying mainly around the periphery of the motor nucle i. Intermediate injections labeled some large motoneurons within the motor nuclei but also labeled many peripheral motoneurons. Distal injections labe led small and medium-large peripheral neurons strongly and almost exclusive ly. The peripheral neurons labeled from the lateral rectus muscle surround the medial half of the abducens nucleus: from superior oblique, they form a cap over the dorsal trochlear nucleus; from inferior oblique and superior rectus, they are scattered bilaterally around the midline, between the ocul omotor nucleus; from both medial and inferior rectus, they lie mainly in th e C-group, on the dorsomedial border of oculomotor nucleus. In the medial r ectus distal injections, a "C-group extension" extended up to the Edinger-W estphal nucleus and labeled dendrites within the supraoculomotor area. We c onclude that large motoneurons within the motor nuclei innervate twitch fib ers, whereas smaller motoneurons around the periphery innervate nontwitch, MIF fibers. The peripheral subgroups also contain medium-large neurons whic h may be associated with the palisade endings of global MIFs. The role of M IFs in eye movements is unclear, but the concept of a final common pathway must now be reconsidered. J. Comp. Neurol. 438:318-335, 2001. (C) 2001 Wile y-Liss, Inc.