Brainstem terminations of extraocular muscle primary afferent neurons in the monkey.

The central terminations of afferent nerve fibers from the extraocular muscles of the monkey were investigated by means of transganglionic transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA/HRP). Following injections of selected extraocular muscles with WGA/HRP, terminal labeling was apparent in the ipsilateral trigeminal sensory and cuneate nuclei. The density of trigeminal projections varied markedly from one rostrocaudal level to the next, being heaviest within the ventrolateral portion of pars interpolaris of the spinal trigeminal nucleus. A second extraocular muscle afferent representation was noted in ventrolateral portions of the cuneate nucleus. This projection was restricted to rostral portions of pars triangularis of the cuneate nucleus, partially overlapping the afferent termination from dorsal neck muscles. It is likely that some of the problems encountered in formulating conclusions regarding the functional role of extraocular muscle proprioception are due to a lack of detailed information of the central termination pattern of muscle afferents. Taken together, the present findings should provide a basis for further anatomical and physiological studies designed to elucidate the role played by extraocular muscle proprioceptors in vision and oculomotor control.