Pontine and non-pontine pathways mediating early mossy fiber responses from sensorimotor cortex to cerebellum in the cat.

Stimulation of the cat's sensorimotor cortex evokes a short-latency mossy fiber field potential (N2) in the pars intermedia of the cerebellum. The fraction of the response carried by each of the six cerebellar peduncles was determined by comparing the field potentials before and after electrolytic lesions of individual cerebellar peduncles. The response evoked from the contralateral cerebrum is 3.5 times as large as that evoked from ipsilateral cerebrum. The size of the N2 wave from contralateral cerebral cortex is reduced to about half of the original size after electrolytic lesion of either the brachium pontis (BP) or the restiform body (RB) ipsilateral to the recording site, while only a small reduction in the size of N2 is observed after lesions of contralateral BP and RB. This indicates that inputs responsible for the generation of the N2 wave from contralateral cerebral cortex enter the cerebellum primarily through ipsilateral BP and RB. In contrast, the N2 wave from ipsilateral cerebral cortex is reduced by approximately one quarter following lesions of any one of the four RB's and BP's. Fibers in the brachium conjunctivum do not contribute to these responses. Observation on the P2 field potential, representing postsynaptic firing of granule cells, suggest that the cerebro-ponto-cerebellar and cerebro-reticulo-cerebellar mossy fiber pathways converge onto granule cells. Furthermore, these two inputs must arrive within 3 ms of each other in order to activate the granule cells. In addition, it is shown that BP lesions reducing the mossy fiber input lead to an increase in the climbing fiber input.