Structure-function relations of two somatotopically corresponding regions of the rat cerebellar cortex: olivo-cortico-nuclear connections.

The somatotopical organization of the climbing fiber input to the paravermal region of lobulus simplex (LS, lobule Vla) was charted in the cerebellar cortex of anaesthetized rats. From medial to lateral in LS, zones a2, c1, c2 and c3 were identified. Forelimb responses were found in both LS and the paramedian lobule (PML) and simultaneous recordings from the c1 zone in both lobules showed that trial-by-trial fluctuations in climbing fiber field size evoked by ipsilateral forelimb stimulation did not occur in synchrony, suggesting that the two parts of the same zone are not closely linked by their climbing fiber input. Electrophysiological mapping in combination with a double fluorescent axonal tracing strategy (mix of Fluoro-Emerald and green beads, and mix of Fluoro-Ruby and red beads) revealed that the two parts of the c1 zone receive climbing fiber input from similar territories in the medial and dorsal accessory olives, but that only 4% of the total population of labelled cells have axons that branch to supply climbing fiber afferents to both regions of cortex. The corticonuclear output of the two parts of the zone was found in mainly overlapping regions of the transitional region between the anterior and posterior divisions of nucleus interpositus. Overall, the results suggest that the olivocerebellar and corticonuclear projections of cerebellar zones are similarly organized in rat and cat, implying that the function of individual zones is conserved between species.