A morphological study of abducens nucleus motoneurons and internuclear neurons in the goldfish (Carassius auratus).

The location and distribution of abducens (ABD) nucleus motoneurons (Mn) and internuclear neurons (Int) were determined in the goldfish (Carassius auratus) by means of horseradish peroxidase and fluorochrome retrograde labeling. ABD Mn were labeled following tracer injection into the ipsilateral lateral rectus muscle. These Mn were arranged in two ventrolateral clusters along the rostro-caudal axis of the posterior brainstem. Both groups of neurons showed a similar number of cells, and their axons ran ventrally to their respective nerve roots. ABD Int were labeled following the injection of the tracer into the contralateral oculomotor nucleus. They also formed two distinct groups in the rostro-caudal axis. The rostral group of Int formed a dorso-lateral cap around the caudal motoneuronal pool, with little if any intermingling. The caudal group of Int was located at the same position in the dorso-ventral and medio-lateral axis as the rostral group, but 500 microns behind it. Both groups of ABD Int had a similar number of neurons. Int axons ascended dorso-medially, then crossed the midline through the internal arcuate fibers, and entered the contralateral medial longitudinal fasciculus. The soma diameters of both ABD Mn and Int were not significantly different. The relative location of both types of neurons is discussed.