The title of this presentation refers to the unfortunate circumstance that very little is known about the anatomy of the human cerebellum, and the primate cerebellum in general. Most of our ideas about the morphology and the connections of the human cerebellum are derived from data obtained in rodents and carnivores.

Most structural elements, and the afferent and efferent fiber connections of the cerebellum are arranged in two perpendicular planes.

1. The conformation of the folial pattern of the human cerebellum to Bolk's (1906) fundamental Bauplan of the mammalian cerebellum will be illustrated.(Fig.1)
2. The "lattice structure" of the cerebellar cortex will be emphasized.
3. The concept of the longitudinal zonal organization of the Purkinje cells of the cerebellar cortex, based on the corticonuclear connections and the olivocerebellar climbing fiber projections will be introduced (Fig.2)
4. In the description of the cerebellar nuclei, the division of the human dentate into rostromedial ("macrogyric") and caudoventral ('microgyric') regions will be emphasized (Fig.3).
5. Differences in connections between different (sets of) Purkinje cell zones will be discussed. The B, C1, C and C3 zones are limited to anterior and posterior regions of the cerebellum. They receive climbing fibers from the dorsal accessory olive, possess a detailed somatotopical organization and, in rodents, consist of zebrin-negative Purkinje cells. Their brain stem connections with Deiters'nucleus, the red nucleus and the motor cortex are the substrate of most classical, systems-physiological studies of the cerebellum (Fig.4A). The C2 zone extends over the entire cerebellum, from the anterior lobe to the flocculus. It receives its climbing fiber afferents from the dorsal accessory olive, lacks a somatotopical organization, and consists of zebrin-positive Purkinje cells in rodents. It is connected with extensive regions of the frontal lobe and is provided with a recurrent pathway, originating from Darkschewitsch'nucleus, located next to the parvocellular red nucleus, terminating in the dorsal accessory olive. Nothing is known about the physiology of this system.
6. The major part of the primate cerebellar hemisphere consists of the D1 and D2 zones, which project to the ventrocaudal and rostromedial dentate resepectively. A correspondance with the two subdivisions of the human dentate has not been substantiated. D1 receives its climbing fibers from the ventral lamina of the principal olive, D2 from its dorsal lamina. Both zones are represented by zebrin-positive Purkinje cells in rodents. The rostromedial dentate (the target of the D2 zone) projects in a somatotopical manner to motor and premotor areas of the frontal lobe (Fig.4C). The caudomedial dentate (target of the D1 zone) projects to more frontal regions, including areas 9, 46 and the frontal eye fields and area 5a of the parietal lobe (Fig.4B). These data on cortical connections are based on the retrograde transneuronal tracer studies of Strick c.s. in the cebus monkey. In addition both subdivisions of the dentate give rise to a large, recurrent pathway from the parvocellular red nucleus (central tegmental tract) which terminates in both lamina of the principal olive. Like the C2 zone system, information on the electrophysiology of the D zones and their central connections is extremely scarce.
7. The cerebellum receives its main input as mossy fibers, which terminate on the granule cells of the cortex. Contrary to the parasagittal oreintation of the Purkinje cell output of the cortex, mossy fibers are oriented transversely (Fig.5). This orientation is enhanced by the transverse course of the parallel fibers, the axons of the granule cells. The parent fibers of the mossy fibers emit collaterals at specific, nultiple and bilateral locations (Wu et al., 1999). These collaterals terminate in longitudinal aggregates of mossy fiber terminals in the granular layer. The termination of moossy fibers in multiple, bilateral longitudinal aggregates appears to be a general feature of mossy fiber systems and has been confirmed in rat and/or cat for spinocerebellar, cuneocerebellar, vestibulocerebellar, reticulocerebellar mossy fibers, and by Serapide et al. (2001), for the pontocerebellar system,. Unfortunately no information on mossy fiber systems is available for primates.
8. There are many indications for a correspondance in the topography of the zonal organization of the Purkinje cells and their climbing fibers, and the mossy fiber terminal fields. In classical studies of Eccles c.s. and the more recent papers of Garwicz (2000) and Brown and Bower (2001) a corrrespondance in receptive field organization was noticed for the climbing fiber afferent of zones or patches of Purkinje cells and the mossy fibers terminating subjacent to them. The consequences of this organization for the processing of information by the cerebellar cortex have not yet been evaluated.

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