Cerebellum Modular Organization

CEREBELLAR CORTICONUCLEAR PROJECTION

Output from the cerebellar cortex to the DCN is derived exclusively from PCs and is inhibitory. The vermis projects to the fastigial nucleus, the intermediate cortex project to globose and emboliform nuclei, and lateral hemispheres project to the dentate nucleus (DN). There is a reciprocal excitatory projection of the DCN neurons back onto the PCs (Plate 8-5).

INFERIOR OLIVARY NUCLEUS

This nuclear complex is a folded sheet of 1.5 millions neurons in the medulla, situated between the pyramidal tract and the lateral reticular nucleus. The medial accessory olive (MAO) and the dorsal accessory olive (DAO) each have rostral and caudal components. The principal olive (PO) has dorsal, lateral, ventral, and medial lamellae. Other subnuclei include the beta cell group, dorsomedial cell column (DMCC), and dorsal cap of Kooy. Proximal dendrites of olivary neurons have appendages that form the central core of a complex synaptic structure, the olivary glomerulus. These have gap junctions enabling electrotonic coupling between groups of olivary neurons. Each olivary axon provides 7 to 10 climbing fibers to the cerebellar cortex and DCN; one CF per PC (see Plate 8-6).

OLIVARY AFFERENTS AND PROJECTIONS TO CEREBELLUM

The olive receives multiple excitatory afferents, sends excitatory CFs to discrete longitudinally oriented parasagittal microzones in the cerebellar cortex, and sends collaterals to focal areas within the DCN that are linked to PCs in that cortical microzone. Olivary neurons receive inhibitory feedback projections from those DCN neurons to which they project, forming a closed-loop system.

Spinal cord projections are conveyed directly to the MAO and DAO in the crossed ventral spino-olivary tracts (SOT) and indirectly in the dorsal SOT that ascends in the ipsilateral dorsal column, synapses in the gracile and cuneate nuclei, decussates to the contralateral olive, and decussates again in the olivocerebellar projection, terminating in the cerebellum ipsilateral to its spinal cord origin. Spino-olivary fibers terminate in primary and secondary arm and leg representations in the spinocerebellum (zones A through C3).

The trigeminal sensory nucleus projects to the caudal MAO and rostromedial DAO; these project to lobule VI in zones C1 and C3, with collaterals to the emboliform nucleus.

Cerebral cortex projections are conveyed to the olive and cerebellum via the parvocellular component of the red nucleus (RNpc). Primary motor and premotor cerebral cortex are linked somatotopically with the caudolateral RNpc; this projects to the PO dorsal lamina and bend, which sends efferents to the lateral cerebellar D2 zone and dorsomedial dentate nucleus (paleodentate) and to the rostral MAO that targets cerebellar zone C2 and the globose nucleus. Frontal eye fields, premotor, and prefrontal areas project via the dorsomedial RNpc and the PO ventral lamina to the medial cerebellar D1 zone and the caudoventral dentate nucleus (neodentate).

Optokinetic information from pretectal nuclei, including the nucleus of the optic tract and accessory optic nuclei, is conveyed to the dorsal cap and the ventrolateral outgrowth, and relayed to lobule X (flocculonodular lobe).

Vestibular information is conveyed to vestibular nuclei, including the parasolitary nucleus, which project via DMCC and nucleus β to vermal and hemispheric regions of lobules IX (uvula, paraflocculus) and X (nodulus and flocculus).

Visual information from superior colliculus is relayed through the caudal MAO to the vermal visual area in lobule VII and fastigial nucleus. Other afferents originate in midbrain regions concerned with oculomotor control (nuclei of Darkschewitsch, Cajal, Edinger-Westphal, perihypoglossal) and are conveyed to vermal lobules IX and X. The lateral reticular nucleus, periaqueductal gray, and zona incerta convey motor, nociceptive/autonomic, and associative information to the olive.

Together with their connections with the inferior olive, the corticonuclear microcomplexes comprise cerebellar modules that are also linked with pontine and other afferents and efferents. The paracrystalline architectural uniformity of the modules likely supports a neural computation, the universal cerebellar transform, common to all cerebellar areas, which can be applied to multiple domains of neurologic function by virtue of the precise connections of each module with extracerebellar structures.

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