The Transcerebellar Loops

As already mentioned, many of the cerebellar circuits are organized in recurrent loops. This is particularly evident when the rubrocerebellar and cerebrocerebel-lar circuits are taken into account.

1. Rubrocerebellar Loop

A magnocellular and a parvicellular component can be recognized within the red nucleus. Both receive cerebellar fibers, the former from the interposed nucleus and the latter from the dentate nucleus. The two components project to different targets. Magno-cellular cells give origin to rubrospinal fibers, whereas parvicellular cells project to the inferior olivary neurons and thus reverberate the information to the cerebellum through the climbing fibers. Within the red nucleus, there is a convergence of cerebellar and cortical inputs over the same neuron. This is particularly true in the parvicellular part, where cerebellar terminals tend to concentrate in the soma, and

Figure 7 Representation of the fractured somatotopical pattern of multiple patches in the posterior lobe of the cerebellum of the rat. The gray scale code indicates the corresponding receptive fields for the different patches. Cr, crown; El, eyelid; FL, flocculus; Fpb, furry buccal pad; G, gingiva; I-X, lobules according to Larsell; Li, lower incisor; Lob. Ant., anterior lobule; Lob. sim., lobulus simplex; Nk, neck; P, pinna; PFL; paraflocculus; PML, paramedian lobule; PY, pyramis; Rh, rhinarium; Ui, upper incisor; Ul, upper lib; UV, uvula; V, vibrissae [modified with permission from M. Glickstein, C. Keo, and J. Stein (Eds.), Cerebellum and Neuronal Plasticity, Fig. 1, p. 114. Copyright © 1987 by Plenum Press].

Figure 7 Representation of the fractured somatotopical pattern of multiple patches in the posterior lobe of the cerebellum of the rat. The gray scale code indicates the corresponding receptive fields for the different patches. Cr, crown; El, eyelid; FL, flocculus; Fpb, furry buccal pad; G, gingiva; I-X, lobules according to Larsell; Li, lower incisor; Lob. Ant., anterior lobule; Lob. sim., lobulus simplex; Nk, neck; P, pinna; PFL; paraflocculus; PML, paramedian lobule; PY, pyramis; Rh, rhinarium; Ui, upper incisor; Ul, upper lib; UV, uvula; V, vibrissae [modified with permission from M. Glickstein, C. Keo, and J. Stein (Eds.), Cerebellum and Neuronal Plasticity, Fig. 1, p. 114. Copyright © 1987 by Plenum Press].

proximal dendrites while cortical synapses are present in the distal dendritic branches. This arrangement is not fixed but can be functionally modulated as shown by lesion and inactivation experiments.

2. Neocortical Cerebellar Loop

The ascending branches of the efferents from the interposed and lateral nuclei and partially from the fastigial nucleus reach the contralateral thalamus and, in a much smaller amount, recross the midline in the thalamus, terminating ipsilaterally. This latter component is formed by axon collaterals of the main contralateral projection. Within the thalamus the cerebellar fibers terminate in the ventrolateral nucleus and in the intralaminar nuclei. The different cerebellar nuclei have segregated terminal areas within the thalamus, especially in higher mammals. Nevertheless, the precise relationships between cerebellar terminals and cortical and basal ganglia projecting thalamic cells must still be clarified. In general terms, cerebellar recipient thalamic nuclei project mainly to the frontal and prefrontal area, with some components reaching the posterior parietal association areas. The returning loop originates from layer V pyramidal cells located in almost all cortical areas except for the pole of the temporal lobe. These fibers do not reach the cerebellum directly; they terminate in a topographic pattern in the pontine nuclei, which in turn reverberate the information over the cerebellar cortex and deep nuclei.

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