Convergence of cortico- and cuneopontine projections onto components of the pontocerebellar system in the rat: an anatomical and electrophysiological study

Abstract

Previous studies in the rat have demonstrated that corresponding peripheral tactile and somatosensory cortical inputs converge within the granule cell layer of various cerebellar lobules and further that descending corticopontine projections from the forelimb sensory cortex (FLSCx) partially overlap with the projection zones of ascending basilar pontine afferents from nucleus cuneatus (NC). The present study employed anatomical and electrophysiological procedures to determine whether cortical and dorsal column nuclear afferent projections converge on pontine neurons that, in turn, provide mossy fiber input to the granule cell layer of the paramedian lobule (PML), i.e., that portion of the rodent cerebellum shown to receive forelimb peripheral inputs. The combination of the orthograde and retrograde axonal transport of horseradish peroxidase (HRP) conjugated to wheat germ agglutinin (WGA) was used light microscopically to demonstrate that orthogradely labeled projections from injections of the FLSCx and NC converged with ponto-paramedian projection neurons that were retrogradely labeled from injections of the PML. These studies were also repeated in conjunction with ablations of either the FLSCx or NC which resulted in the ultrastructural identification of degenerating, as well as WGA-HRP labeled axonal boutons of these pontine afferent projections thus confirming that such projections actually formed synaptic contacts with the retrogradely labeled pontoparamedian projection neurons. Single unit recording analyses of neurons in the ventromedial region of the basilar pons following combined electrical stimulation of various regions of the sensorimotor cortex and the contralateral body surface indicated that approximately 40% of all cells recorded responded to electrical stimulation of corresponding regions of the cortex and periphery, particularly the FLSCx and the forepaw. Natural cutaneous stimuli applied to the forepaw that also elicited responses in these same groups of basilar pontine neurons and were associated with relatively small receptive fields. Taken together, these observations indicate that the previously observed convergence of peripheral and somatosensory cortical inputs within the granule cell layer of the cerebellar cortex may be at least partially organized at the level of the basilar pons.