Cerebellar Cortex and Nuclei (Continued)

The unipolar brush cell (UBC) is the only excitatory interneuron in the cerebellum. The soma is 9 to 12 µm in diameter, with a single dendrite ending in a tight brushlike tip of dendrioles that have extensive synaptic contact with the mossy fiber rosette. Its axon synapses on granule and Golgi cells. The UBC is found in the vestibulocerebellum, vermis, and dorsal cochlear nucleus, and it is thought to amplify vestibular signals and provide feed-forward excitation to granule cells.

Glial cells in the cerebellum include protoplasmic astrocytes that envelope the PC perikaryon in a neuroglial sheath; Bergmann glial cells in the PC layer that are involved in neural migration and development of the cerebellar cortex, and that play a role in regulating glutamatergic neural transmission in the mature cerebellum; and oligodendroglia in cerebellar white matter and in the granular layer.

CEREBELLAR NUCLEI

The fastigial, globose, emboliform, and dentate nuclei are together termed the deep cerebellar nuclei (DCN) to differentiate them from the precerebellar nuclei. The fastigial nucleus is the homologue of the medial nucleus in lower primates, whereas the posterior and anterior interpositus nuclei are homologous with the globose and emboliform nuclei, respectively. Among the cerebellar nuclei, the dentate, or lateral nucleus in lower vertebrates, has evolved most. The posterior (dorsal) part with small narrow folds (microgyric) contains large cells and is phylogenetically older. The macrogyric anterior (ventral) and lateral part contains smaller neurons and has expanded greatly in concert with the association cortex of the cerebral hemispheres. This is important from the perspective of anthropology as well as cognitive neuroscience and behavioral neurology. Deiters lateral vestibular nucleus is located in the dorsal medulla. It receives PC axons directly from the vestibulocerebellum and part of the anterior vermis and is equivalent to a deep cerebellar nucleus.

The neurons of the DCN are outnumbered by the PCs of the cerebellar cortex by about 26 to 1. Each PC contacts approximately 35 nuclear neurons, and each DCN neuron receives inputs from more than 800 PCs. The cytologic features of the DCN suggest anatomic subdivisions that may have connectional and functional relevance, but these are not sufficiently definitive to formally subdivide the nuclei further. Neurons in the DCN are of three types. Large glutamatergic neurons convey excitatory output to the thalamus and brainstem, and nucleocortical projections back to the cerebellar cortex; small γ-aminobutyric acid (GABA)ergic neurons are inhibitory to the inferior olivary nucleus; and small glycinergic neurons in the DCN are thought to be inhibitory intranuclear interneurons. It is now possible to identify the DCN on magnetic resonance imaging (MRI) by taking advantage of their iron content, although detailed organization is not apparent with available technology.

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