Anatomical and electrophysiological evidence has contributed to the hypothe
sis that microzones in the mammalian flocculus are organized to reflect con
trol of eye movements in the planes of semicircular canals. Adult male Long
-Evans rats received iontophoretic injections of FluoroGold and/or tetramet
hylrhodamine dextran amine (10,000 molecular weight, "Fluoro-Ruby") into th
e vestibular nuclei. The distribution of retrogradely labeled Purkinje cell
s revealed that efferent projections from the dorsal surface of the floccul
us and the ventral paraflocculus to the superior vestibular nucleus, rostra
l 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. (1
992) J. comp. Neurol. 316, 129-150] in a manner consistent with other mamma
ls. However, vestibular nucleus projections from the ventral surface of the
flocculus did not appear to respect climbing fiber zonal boundaries. Rathe
r, climbing fiber zones each contained interdigitated groups of Purkinje ce
lls that project to different vestibular nuclear regions.
It is suggested that this pattern of flocculus efferent organization is a s
pecialization for controlling the activity of primary and accessory extraoc
ular muscle pairs to confine vestibule-ocular reflexes within semicircular
canal planes when the "center of regard" is located at different eccentrici
ties. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved
.