Neurons located in the trigeminal sensory complex and the lateral pontine tegmentum project to the oculomotor nucleus in the rabbit.

Neurons located in the trigeminal sensory complex (TSC) and the lateral pontine tegmentum (LPT) have been reported to project to both the accessory abducens and the facial nuclei, which innervate the retractor bulbi and orbicularis oculi muscles respectively, in order to control the nictitating membrane (NM) and eyelid defensive reflex. Since muscles innervated by the oculomotor nucleus (OCM) also appear to be involved in this reflex, retrograde and anterograde tracers were used in this study to determine whether there are projections from the TSC and LPT to the OCM in the rabbit. Injections of horseradish peroxidase (HRP) in the OCM nucleus labeled neurons in the LPT surrounding the trigeminal motor nucleus dorsally, laterally and ventrally. Only a few scattered neurons were found in the principal and spinal trigeminal nuclei. Injection of biocytin in the LPT area containing most of the HRP-labeled neurons caused anterograde labeling of fibers that crossed the midline and ascended just dorsal to the contralateral medial lemniscus. A proportion of these fibers coursed in a dorsal direction to enter and terminate within the OCM contralateral to the injection site. The location of the motoneuronal groups innervating the different extraocular muscles was studied by retrograde transport of HRP, and compared with the distribution of biocytin-labeled terminals. It was found that the terminals were located in the superior rectus and the levator palpebrae zone of the nucleus. We discuss the functional significance of this projection for the eyelid and NM response.