Cytoarchitectonic organisation of the abducens nucleus in the pigeon (Columbia livia).

The distribution of abducens nucleus motoneurons and internuclear neurons was determined in the pigeon (Columbia livia) by injecting horseradish peroxidase or fluorochromes into the ipsilateral lateral rectus muscle and/or in the contralateral oculomotor nucleus. A small degree of intermingling of motoneurons and internuclear neurons in the caudal two thirds of the nucleus and an almost complete segregation of both types of neurons in the rostral third was observed. Both labelled populations, motoneurons and internuclear neurons, were more numerous at the central part of the abducens nucleus. Motoneurons were preferentially located nearer the medial longitudinal fasciculus and less numerous at the rostral end of the nucleus. Internuclear neurons were preferentially located further from the medial longitudinal fasciculus and were less abundant at the caudal end of the nucleus. The evolutionary trend of the relative location of both types of neurons is discussed in relation to the degree of conjugate eye movements.