Location of motoneurons and internuclear neurons within the rat abducens nucleus by means of horseradish peroxidase and fluorescent double labeling.

The distribution of abducens motoneurons and internuclear neurons was determined in the rat by injections of horseradish peroxidase or fluorochromes into the ipsilateral lateral rectus muscle and the contralateral oculomotor nucleus either separately or simultaneously. The labeled somata of abducens internuclear neurons were intermingled with the labeled motoneurons at the medial third of the nucleus, but they were more segregated at the rostral third, where the labeled interneurons were more numerous. Internuclear neurons were preferentially located around and ventral to the central part of the facial genu, while motoneurons were located more dorsomedially, closer to the midline than in other species of mammals. The evolutionary trend of the location of both populations of neurons is also discussed.