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Figure 5 Cerebellar corticonuclear microcomplex as proposed by Masao Ito. DCN, deep cerebellar nuclei; PN, pontine cerebellar nuclei, IO, inferior olive; MF, mossy fiber; CF, climbing fiber; LTD, long-term depression; White cells, excitatory neurons; gray cells; inhibitory neurons; O, excitatory synapses; K, inhibitory synapses.

Figure 5 Cerebellar corticonuclear microcomplex as proposed by Masao Ito. DCN, deep cerebellar nuclei; PN, pontine cerebellar nuclei, IO, inferior olive; MF, mossy fiber; CF, climbing fiber; LTD, long-term depression; White cells, excitatory neurons; gray cells; inhibitory neurons; O, excitatory synapses; K, inhibitory synapses.

nucleus of the solitary tract) and from higher brain structures (red nucleus, superior colliculus, pretectal area, and cerebral cortex) to the cerebellar cortex and deep nuclei. It is worth noting that a significant number of deep cerebellar nuclei fibers provide a return route from the cerebellum to the inferior olive cells.

Olivo cerebellaris projections are topographically organized in such a way that the different subnuclei of the inferior olive project to different longitudinal stripes of the deep cerebellar nuclei and the cerebellar cortex. This pattern coincides with the neurochemical pattern evidenced with zebrin and described previously. The overall organization is such that each inferior olive fiber sends one collateral to the deep cerebellar nuclei and one to the cerebellar cortex that project to the same sector of the deep nuclei.

B. Cerebellar Efferents

The only output system of the cerebellum is represented by the deep cerebellar nuclei. These nuclei receive massive projections from the Purkinje cells of the overlying cerebellar cortex. This connection is clearly topographic, with a general mediolateral and anteroposterior arrangement. Therefore, more medially placed Purkinje cells project to the medial nucleus, the most lateral ones project to the lateral nucleus, and the anterior lobe projects to the more anterior sectors of the nuclei, and the posterior lobe to the more posterior sectors. However, this general arrangement is not consistently maintained and some divergence and convergence have been reported. Virtually all climbing and mossy fibers give off a collateral for the cerebellar nuclei on their way to the cerebellar cortex. Thus, within the deep cerebellar nuclei there is a convergence of extracerebellar excitatory and Purkinje inhibitory synapses over the same projection neurons. The latter synapses greatly outnumber those of extracerebellar origin and are in a position, over the somata and proximal dendrites, to highly influence neuron firing. From the nuclei, efferent fibers leave the cerebellum mainly through the superior cerebellar peduncle; fewer fibers, mainly originating from the fastigial nucleus, leave the cerebellum through the inferior cerebellar peduncle. Once the brain stem is reached, both efferent systems give off an ascending and a descending branch.

Figure 6 Zonal arrangement of corticonuclear and olivocerebellar projections illustrated on a flattened cerebellar cortex. Three groups of cerebellar nuclei with their corticonuclear projection zones can be distinguished: (i) the fastigial nucleus (F; the target nucleus of the vermal A zone), which is continuous through the intermediate cell group (IC; X zone) with the globose or posterior interposed nucleus (IP; C2 zone); (ii) the emboliform or anterior interposed nucleus (IA; Cj and C3 zones) and the dentate nucleus, which can be subdivided into ventrocaudal (DC) and dorsomedial (DR) parts (target nuclei of the D1 and D2 zones, respectively); and (iii) the lateral vestibular nucleus of Deiters (LV; target nucleus of the vermal B zone). Zones in the flocculus and the nodulus project to the vestibular nuclei. The inferior olive is shown at the bottom of the figure in a horizontal projection. The zonal projections of the individual subnuclei are indicated with the same gray shading as shown in the top of the figure (reprinted from J. Voogd and M. Glickstein, The anatomy of the cerebellum. Trends Neurosci. 21, pp. 370-375, copyright 1998 with permission from Elsevier Science).

The descending branch of the fastigial nucleus efferents is formed by ipsilateral, contralateral, as well as bilateral projections directed to many precerebellar nuclei, namely, the vestibular nuclei, the inferior olive complex, and large sectors of the reticular formation. The ascending branch is smaller and reaches several structures of the midbrain, the superior colliculus, and the dorsal thalamus.

Efferents from the interposed nucleus exit through the superior peduncle and are also organized in an ascending and a descending branch. Ascending fibers reach the red nucleus, where they terminate in the magnocellular part in a topographic manner, and the dorsal thalamus terminating in the VL nucleus as well as in the intralaminar nuclei. Descending interposed fibers terminate in reticular and inferior olive precer-ebellar structures.

The lateral nucleus is the recipient of the projections from the lateral part of the hemisphere and its efferents reach the contralateral parvicellular red nucleus and ventrolateral and intralaminar thalamic nuclei with the ascending branch. The descending branch is formed by fibers directed to the precerebellar stations that reach the lateral cerebellum: reticular formation, inferior olive, and the pontine nuclei. It is worth noting that the deep cerebellar nuclei also give off recurrent projections to the cerebellar cortex.

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