Olivo-cortico-nuclear localizations within crus I of the cerebellum

Retrograde and anterograde tracers were microinjected into the folia of crus I of the cat cerebellum to investigate spatial localization in olivo‐cerebellar and cortico‐nuclear projections. The folia were shown to be mainly occupied in rostrocaudal succession by three zones receiving their olivo‐cerebellar climbing fiber afferents from parts of, respectively, the dorsal lamella of the principal olive, the ventral lamella of the principal olive, and the rostral half of the medial accessory olive. These zones are presumably parts of the D2, D1, and C2 cerebellar cortical zones, as earlier proposed by Rosina and Provini ([1982] Neuroscience 7:2657–2676). Their respective nuclear target territories were found to be in the rostroventral quadrant of nucleus lateralis, the caudoventral quadrant of nucleus lateralis, and the ventral half of nucleus interpositus posterior. The medial‐to‐lateral width of each zone was shown to be innervated by different groups of olive cells and to project respectively to medial and lateral parts of the nuclear territory for that zone, consistent with the existence in crus I of olivo‐cortico‐nuclear microcomplexes (cf. Ito [1984] New York: Raven Press). Parts of the length of each zone located within different folia were also shown to relate to different groups of olive cells and to different regions of the zone's overall nuclear territory. Interfolial localizations, which were heavily overlapping in nature, intersected orthogonally with those for zone width. The fine‐grain topography implies that individual microzones exist within each of the zones present within crus I. The results also have implications for the possibility that lateral cerebellar pathways are involved in cognition. J. Comp. Neurol. 497:287–308, 2006. © 2006 Wiley‐Liss, Inc.