The abducens motor and internuclear neurons in the rabbit: retrograde horseradish peroxidase and double fluorescent labeling.

Horseradish peroxidase and the fluorochromes Fast blue and propidium iodide were injected into the lateral rectus and retractor bulbi muscles and/or the oculomotor nucleus of the rabbit to determine the locations and basic morphology of motoneurons and internuclear neurons in the abducens nucleus. The 1000-1100 motoneurons found were distributed throughout the nucleus except in the rostral and caudal tips, but were most densely clustered in the dorsomedial area, especially in the middle third of the nucleus, where 60% of these cells were found. The rostral and caudal tips were composed of internuclear neurons, 25% of which lay in the rostral third of the nucleus, 35% in the middle third and 40% in the caudal third. In the middle third, interneurons occupied the ventral and lateral areas of the nucleus (where they mingled with motoneurons); in the rostral and caudal thirds they were more widely distributed. At the level of the caudal half of the nucleus it was impossible to distinguish clearly between the most lateral abducens interneurons and the most rostromedial labeled vestibular neurons. The abducens interneurons of the rabbit (320-380) thus differ in interesting respects from those described previously in either lateral eyed or frontal eyed mammals.