Zonal organization of flocculo-vestibular connections in rats.

Anatomical and electrophysiological evidence has contributed to the hypothesis that microzones in the mammalian flocculus are organized to reflect control of eye movements in the planes of semicircular canals. Adult male Long-Evans rats received iontophoretic injections of FluoroGold and/or tetramethylrhodamine dextran amine (10,000 molecular weight, "FluoroRuby") into the vestibular nuclei. The distribution of retrogradely labeled Purkinje cells revealed that efferent projections from the dorsal surface of the flocculus and the ventral paraflocculus to the superior vestibular nucleus, rostral medial vestibular nucleus, ventral lateral vestibular nucleus, and caudal aspect of the vestibular nuclear complex (caudal medial vestibular nucleus, inferior vestibular nucleus and nucleus prepositus hypoglossi) tended to correspond to previously identified climbing fiber zones [Ruigrok et al. (1992) J. comp. Neurol. 316, 129-150] in a manner consistent with other mammals. However, vestibular nucleus projections from the ventral surface of the flocculus did not appear to respect climbing fiber zonal boundaries. Rather, climbing fiber zones each contained interdigitated groups of Purkinje cells that project to different vestibular nuclear regions. It is suggested that this pattern of flocculus efferent organization is a specialization for controlling the activity of primary and accessory extraocular muscle pairs to confine vestibulo-ocular reflexes within semicircular canal planes when the "center of regard" is located at different eccentricities.