Distribution of neurons projecting to the trochlear nucleus in goldfish (Carassius auratus)

The distribution of neurons projecting to the trochlear nucleus of goldfish (Carassius auratus) was studied by the electrophoretic injection of horseradish peroxidase into the nucleus. The location of the injection site was electrophysiologically determined by the antidromic field potential elicited from the trochlear nucleus after the electrical stimulation of its nerve. Retrogradely labeled neurons were observed in the following structures: (1) anterior, tangential and descending nuclei of the octaval column--afferents to these nuclei were mainly ipsilateral for the former and exclusively contralateral for the other two; (2) cerebellum; (3) rhombencephalic reticular formation, near the abducens nucleus; and (4) nucleus of the medial longitudinal fasciculus. In addition, a few stained neurons were scattered in the nucleus of the posterior commissure and in nucleus pretectalis superficialis pars magnocellularis. These results are compared with the afferent sources to trochlear nucleus in mammals and with the set of structures projecting to the oculomotor and abducens nuclei in goldfish. It is suggested that the neuronal pathways involved in the compensatory vestibular and optokinetic reflexes, as well as in the saccadic system, are similarly organized in vertebrates but that the relative importance of some of the components of these pathways varies.