A direct cerebrocerebellar projection in adult birds and rats

The rostral Wulst of birds, like the somatosensory cortex of mammals, receives somatosensory information from the thalamus and projects to the brainstem and spinal cord via a pyramidal-like tract. Using anterograde and retrograde tract-tracers, we show here, in adult zebra finches, that the rostral Wulst also projects directly to the cerebellar cortex and deep nuclei. In the cortex, the cerebrocerebellar fibers resemble neither mossy nor climbing fibers, but more closely resemble the multilayer fibers shown to originate from the hypothalamus in mammals. We also show that a sparse projection to the cerebellum from the mammalian neocortex, originally thought to be lost during early development, is present in the adult rat. Although the functional implications of these results are obscure, they suggest a revision of the concept of the "cerebrocerebellar system", which is generally considered to involve a pontine relay.