Afferent projections from the brainstem to the three floccular zones in cats. I. Climbing fiber projections.

The cat's flocculus can be divided into 3 zones on the basis of differences in their efferent projection sites. In the present study, climbing fiber projections from the inferior olive to each zone of the flocculus were studied by means of retrograde axonal transport of horseradish peroxidase (HRP). Following large injections of HRP into the flocculus, labeled cells appear in the dorsal cap and the ventrolateral outgrowth of the principal olive. No HRP-labeled somata are present in other parts of the inferior olive. Following microinjections of HRP into the rostral of caudal zones of the flocculus, labeled cells appear in the ventrolateral outgrowth and the rostral part of the dorsal cap, while, after injections into the middle zone, labeled cells are found in the caudal part of the dorsal cap. These findings show that there exists zonal organization in the climbing fiber projections to the flocculus; the rostral and caudal zones receive climbing fiber afferents from the ventrolateral outgrowth and the rostral part of the dorsal cap, while the middle zone receives those from the caudal part of the dorsal cap.