Accessory oculomotor nuclei in the monkey: Projections and effects of discrete lesions

Abstract

A study in the monkey was made of the degeneration and effects of lesions in the accessory oculomotor nuclei. Lesions destroyed: (1) the interstitial nucleus (INC), (2) regions lateral and dorsal to the INC, (3) the posterior commissure (PC), and (4) the central gray. Degeneration was studied in Nauta‐Gygax stained sections. Efferent fibers from the INC project: (1) contralaterally, to specific subdivisions of the oculomotor complex (OMC), (2) bilaterally, to the trochlear nuclei (TN), and (3) ipsilaterally, to the medial vestibular nucleus, to certain perihypoglossal nuclei and to the spinal cord. Projections of the INC to the oculomotor complex cross in ventral parts of the PC and project differentially to the dorsal, medial and intermediate cell columns via the contralateral INC and MLF. No fibers from the INC project to the ventral somatic cell columns, the visceral nuclei or the caudal central nucleus of the OMC. Discrete lesions in the INC produce transient head tilt to the opposite side. Unilateral and bilateral lesions of the PC interrupt crossing interstitio‐oculomotor fibers and produce: (1) symmetrical differential degeneration in the OMC and TN similar to that resulting from discrete lesions of the INC, (2) bilateral eyelid retraction, and (3) impairment of vertical gaze, particularly upward gaze. Horizontal gaze and pupillary reflexes were normal. Evidence suggests that relationships between interstitio‐oculomotor fibers and the nuclei of the posterior commissure may underlie neural mechanisms that regulate vertical eye movements.