The unipolar brush cell (UBC) is a novel type of small neuron that is 
characterized by sets of morphological and chemical phenotypes. UBCs occur in 
the granular layer of the mammalian cerebellar cortex, particularly in folia of 
the vestibulocerebellum, and in the granule cell domains of the dorsal cochlear 
nucleus. The UBC is characterized by a single dendrite that terminates with a 
brush-like tip of dendrioles. The soma, the dendritic stem, and especially the 
dendrioles emit short, non-synaptic appendages. The dendrioles represent the 
main synaptic apparatus of the UBC and articulate tightly with a single mossy 
fiber rosette forming a glomerular array characterized by an extraordinarily 
extensive synaptic contact. Electron microscopic and electrophysiological 
observations indicate that the unusual synaptic ultrastructure may produce 
entrapment of neurotransmitter in the synaptic cleft. While ionotropic glutamate 
receptors are enriched in correspondence of the postsynaptic density, 
metabotropic glutamate receptors are situated extrasynaptically and are 
particularly enriched at the appendages, which usually do not bear synaptic 
junctions. Some of the UBCs receive their input from choline 
acetyltransferase-positive mossy rosettes originating from the vestibular 
nuclei, suggesting that ACh and glutamate are co-released at these synapses. The 
UBC brush occupies a glomerulus where granule cell dendrites are intermixed with 
the UBC dendrioles, both of which receive synapses from the same mossy fiber 
rosette and portions of the Golgi axonal plexus. In addition, the dendrioles are 
presynaptic to granule cell dendrites, forming dendrodendritic contacts that 
display features of excitatory synapses. Branches of the UBC axon in the 
granular layer bear large endings resembling mossy fibers. The UBCs may 
represent an extraordinary device for feedforward, excitatory links along the 
mossy fiber pathways of cerebellum and dorsal cochlear nucleus.