Organization of inferior olivary projections to the flocculus and ventral paraflocculus of the rat cerebellum.

The climbing fiber projection to the rat flocculus and adjacent ventral paraflocculus was investigated by using Phaseolus vulgaris-leucoagglutinin as an anterograde and horseradish peroxidase as a retrograde tracer. Large injections of horseradish peroxidase in the flocculus and ventral paraflocculus indicated that the climbing fibers to this region are derived exclusively from any of the following contralateral olivary regions: the dorsal cap of Kooy, the ventrolateral outgrowth, the caudal half of the ventral leaf of the principal olive near its lateral bend, and the rostral pole of the medial accessory olive. Subsequent anterograde and retrograde studies with small injections demonstrated that the latter area projects to the C2 zone, which runs caudally in the ventral paraflocculus and enters the caudal most aspect of the flocculus. The ventral leaf of the principal olive is connected to a D zone in the cerebellar hemisphere and paraflocculus, which, upon entering the ventral paraflocculus, divides into a caudal and rostral strip, termed FD and FD', respectively. The dorsal cap and the ventrolateral outgrowth each project to two distinct zones in the flocculus and part of the ventral paraflocculus. Two floccular zones, which are continuous with the parafloccular FD and FD' zones, receive their climbing fibers from the ventrolateral outgrowth. Two other zones, (FE and FE') receive their climbing fibers from the dorsal cap. The FE' zone is found at the rostral pole of the flocculus and is followed caudalwards by the FD', FE, FD, and C2 zones, respectively. The rostromedial part of the dorsal cap is connected to the continuation of the FE zone into the ventral paraflocculus. The observation that the dorsal cap and the ventrolateral outgrowth are both connected to a set of two alternating zones of floccular/ventral parafloccular Purkinje cells is in agreement with recent studies in the rabbit, and suggests that these zones reflect functionally distinct and discrete units related to specific aspects of visuomotor control.