AFFERENTS TO THE ABDUCENS NUCLEUS IN THE MONKEY AND CAT

The abducens nucleus is a central coordinating element in the generation of conjugate horizontal eye movements. As such, it should receive and combine information relevant of visual fixation, saccadic eye movements, and smooth eye movements evoked by vestibular and visual stimuli. To reveal possible sources of these signals, we retrogradely labeled the afferents to the abducens nucleus by electrophoretically injecting horseradish peroxidase into an abducens nucleus in four monkeys and two cats. The histologic material was possessed by the tetramethyl benzidine (TMB) method of Mesulam. (1) In both species the largest source of afferents to the abducens nuclues was bilateral projections from the ventrolateral vestibular nucleus and the rostral pole of the medial vestibular nucleus. Scattered neurons were also labeled in the middle and caudal levels of the medial vestibular nucleus. (2) Large numbers of neurons were labeled in the ventral margin of the nucleus prepositus hypoglossi in the cat and in the common margin of the nucleus prepositus and the medial vestibular nucleus in the monkey, a region we call the marginal zone. (3) Substantial numbers of retrogradely labeled neurons were found in the dorsomedial pontine reticular formation both caudal and rostral to the abducens nuclei. (4) In the monkey, large numbers of labeled neurons were present in the contralateral medial rectus subdivision of the oculomotor complex, while smaller numbers occurred in the ipsilateral medial rectus subdivision and elsewhere in the oculomotor complex. In the cat, large numbers of retrogradely labeled cells were present in a small periaqueductal gray nucleus immediately dorsal to the caudal pole of the oculomotor complex, and a few labeled neurons were also dispersed through the caudal part of the oculomotor complex. (5) Occasional labeled neurons were present in the contralateral superior colliculus in both species. The size and distribution of the labeled neurons within the intermediate gray differed dramatically in the two species. In the cat, the retrogradely labeled neurons were very large and occurred predominantly in the central region of the colliculus, while in the monkey, they were small to intermediate in size and were distributed more uniformly within the middle gray. (6) Among the afferent populations present in the monkey, but not in the cat, was a group of scattered neurons in the ipsilateral rostral interstitial nucleus of the medial longitudinal fasciculus and a denser, bilateral population in the interstitial nucleus of Cajal. A few labeled neurons were also present in the nucleus reticularis tegmenti pontis in the monkey, but not in the cat. Small numbers of retrogradely labeled neurons were present in the monkey''s y-group and superior vestibular nucleus. It appears that while the monkey and the cat have similar sets of abducens afferents, there are definite differences in the details of the innervation of the abducens nucleus in the two species.