Divergent axon collaterals from fastigial oculomotor region to mesodiencephalic junction and paramedian pontine reticular formation in macaques.

Collateralization of efferent fibers from the fastigial oculomotor region (FOR) to the paramedian pontine reticular formation (PPRF) and the mesodiencephalic junction (MDJ) was studied in macaque monkeys using a fluorescent double-labeling technique. Retrogradely-labeled neurons in the contralateral FOR were examined following injections of fast blue (FB) into the MDJ and diamidino yellow (DY) into the PPRF, or vice versa. Some FOR neurons were labeled with FB, while some other FOR neurons were labeled with DY and intermingled within the FOR. While single-labeled cells in the FOR projected either to the PPRF or to the MDJ, the presence of double-labeled cells indicated that the FOR contains neurons whose axons collateralize to project to both the MDJ and PPRF. These are regarded as the preoculomotor nuclei responsible for vertical and horizontal saccades, respectively.