Somatotopical organisation within the climbing fibre projection to the paramedian lobule and copula pyramidis of the rat cerebellum

Abstract

The climbing fibre projection to the paramedian lobule (lobule VII) and the copula pyramidis (lobule VIII) in the posterior lobe of the rat cerebellum was investigated in pentobarbitone-anaesthetised animals. Percutaneous electrical stimulation generated climbing fibre field potentials on the cerebellar surface that indicated a somatotopical organisation into five distinct cortical areas. Each area was identified by the site(s) of peripheral stimulation that evoked the largest response within that area, and from medial to lateral the cortex was subdivided as follows [principal site(s) of stimulation in parentheses with corresponding range of onset latencies]: in the paramedian lobule, area 1 (contralateral face, 17-18 ms), area 2 (ipsilateral forelimb, 10-15 ms) and area 3 (ipsi- and contralateral forelimbs, 16-26 ms and 15-30 ms, respectively); and in the copula pyramidis, area 4 (tail, 17-21 ms) and area 5 (ipsilateral hindlimb, 13-19 ms). In additional retrograde tracer experiments, small volumes of fluorescent-tagged beads were injected into each of the different areas and the location of retrogradely labelled olive cells was mapped. By comparison with other species, the results indicate that in the paramedian lobule area 1 corresponds to zone A2; area 2 corresponds, at least in part, to medial C1; and area 3 corresponds to C2; in addition, farther laterally, a C3 zone may be present. In the copula pyramidis, areas 4 and 5 correspond to subzones of medial C1. Overall, the results support the view that a general principle of cerebellar cortical organisation is a division into parasagittal zones, each characterised by its somatotopically organised climbing fibre input that arises from a specific, rostrocaudally oriented column of cells within the inferior olivary nucleus.