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Review
, 66 (1-2), 220-45

The Unipolar Brush Cell: A Remarkable Neuron Finally Receiving Deserved Attention

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Review

The Unipolar Brush Cell: A Remarkable Neuron Finally Receiving Deserved Attention

Enrico Mugnaini et al. Brain Res Rev.

Abstract

Unipolar brush cells (UBC) are small, glutamatergic neurons residing in the granular layer of the cerebellar cortex and the granule cell domain of the cochlear nuclear complex. Recent studies indicate that this neuronal class consists of three or more subsets characterized by distinct chemical phenotypes, as well as by intrinsic properties that may shape their synaptic responses and firing patterns. Yet, all UBCs have a unique morphology, as both the dendritic brush and the large endings of the axonal branches participate in the formation of glomeruli. Although UBCs and granule cells may share the same excitatory and inhibitory inputs, the two cell types are distinctively differentiated. Typically, whereas the granule cell has 4-5 dendrites that are innervated by different mossy fibers, and an axon that divides only once to form parallel fibers after ascending to the molecular layer, the UBC has but one short dendrite whose brush engages in synaptic contact with a single mossy fiber terminal, and an axon that branches locally in the granular layer; branches of UBC axons form a non-canonical, cortex-intrinsic category of mossy fibers synapsing with granule cells and other UBCs. This is thought to generate a feed-forward amplification of single mossy fiber afferent signals that would reach the overlying Purkinje cells via ascending granule cell axons and their parallel fibers. In sharp contrast to other classes of cerebellar neurons, UBCs are not distributed homogeneously across cerebellar lobules, and subsets of UBCs also show different, albeit overlapping, distributions. UBCs are conspicuously rare in the expansive lateral cerebellar areas targeted by the cortico-ponto-cerebellar pathway, while they are a constant component of the vermis and the flocculonodular lobe. The presence of UBCs in cerebellar regions involved in the sensorimotor processes that regulate body, head and eye position, as well as in regions of the cochlear nucleus that process sensorimotor information suggests a key role in these critical functions; it also invites further efforts to clarify the cellular biology of the UBCs and their specific functions in the neuronal microcircuits in which they are embedded. High density of UBCs in specific regions of the cerebellar cortex is a feature largely conserved across mammals and suggests an involvement of these neurons in fundamental aspects of the input/output organization as well as in clinical manifestation of focal cerebellar disease.

Figures

Fig. 1
Fig. 1
The UBC participates in glomerular synaptic fields at its dendric and axonal endings.
Fig. 2
Fig. 2
The synaptic connectivities in regions of the granular layer containing (A) or devoid (B) of UBCs are fundamentally different. In red, UBC in synaptic contact with an extrinsic mossy fiber (eMF); in orange, UBC in synaptic contact with an intrinsic mossy fiber (iMF); in green granule cells and their axons ascending to the molecular layer; black dots, inhibitory terminals of Golgi axonal plexus. Arrows indicate direction of transmission;
Fig. 3
Fig. 3
The excitatory network within a cerebellar region enriched with UBCs is represented schematically. An extrinsic mossy fiber (MF, black) contacts a first order UBC (1° UBC, red) within a glomerulus containing dendrites belonging to several granule cells (GC, green). The axon of the 1° UBC provides branches innervating multiple glomeruli, one of which also involves another UBC (2°UBC). For clarity, only two of these glomeruli are represented, although in reality there would be more than ten. The axon of the second order UBC also provides branches innervating granule cells and another UBC (3° UBC). Granule cell axons form bundles that provide a strong excitatory drive to individual Purkinje cells (PC, blue) by means of synapses (dots) formed by the ascending portions before they bifurcate to give rise to parallel fibers (PFs). Arrows refer to the direction of information flow along the network. Purkinje cells targeted by parallel fibers driven by UBC-independent networks are also represented.

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