Afferent connections of the oculomotor nucleus in the chick.

Horseradish peroxidase was injected into the oculomotor nucleus of the chick in order to locate and characterize the neurons projecting to this nucleus. In the rostral mesencephalon, 120-180 neurons were labelled in the medial area of the ipsilateral nucleus campi Foreli; 190-220 in the interstitial nucleus of Cajal (most of them contralateral); and smaller numbers bilaterally in the medial mesencephalic reticular formation, the nucleus of the basal optic root complex, and the central grey matter. More caudally, numerous neurons were labelled in the contralateral abducens nucleus and the vestibular complex and a few in the nucleus reticularis pontis caudalis. Labelled neurons appeared ipsilaterally in the caudal region of the nucleus vestibularis superior and in the rostral tip of the nucleus descendens just lateral to the tractus lamino-olivaris. In the contralateral vestibular complex, a group of labelled cells observed in the dorsolateral area may be homologous to the mammalian cell group Y. At the level of the contralateral abducens nucleus, the most numerous group of cells (625-700) projecting to the oculomotor nucleus formed a lateromedial fringe that affected the nucleus tangentialis, the rostral tip of the nucleus descendens, and the ventrolateral region of the nucleus medialis. Only a few labelled neurons were seen in the contralateral nucleus vestibularis superior, the ipsilateral cell group A, and the ipsilateral nucleus vestibularis medialis.