The unipolar brush cell: a neglected neuron of the mammalian cerebellar cortex.

We describe with a variant of the Golgi method a new type of neuron that is prominently represented in the granular layer of the mammalian vestibulocerebellum but is presently neglected in all major accounts on the cerebellum. These neurons, here termed unipolar brush cells, are intermediate in size between granule cells and Golgi cells. They typically have a thin and presumably myelinated axon, and a single and stubby dendrite whose tip forms a tightly packed group of branchlets resembling a paintbrush. The branchlets often intertwine with the digitiform claws of granule cell dendrites and are occasionally approached by Golgi cell dendrites, indicating that the unipolar brush cells may share the input of the other granular layer neurons. Branchlets of neighboring unipolar brush cells converging into the same neuropil island also occur. The brush-like tip of the unipolar cell engulfs one or two mossy fiber rosettes to form an extensive synapse that appears to close recurrent loops involving the vestibular nuclei. Positive feedback in these loops could help to explain several motor responses and drive mechanisms of extended duration that are controlled by the ventral cerebellum.