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Department of Physiology, School of Medical Sciences, University of Bristol, UK.

1. Chronically implanted microwires were used to record extracellular field potentials generated in the c1 and c3 zones in the cortex of lobules V and VI of the cerebellum by non-noxious stimuli delivered to the superficial radial nerve in the ipsilateral forelimb. Responses due to input via climbing fibre afferents were studied; their latency and other characteristics identified them as mediated mainly via the dorsal funiculus spino-olivocerebellar path (DF-SOCP). 2. Responses at individual sites were studied repeatedly with a range of stimulus intensities and during two different behaviours: quiet rest and steady walking on an exercise belt. For responses during walking, step histograms were constructed showing response mean size during different tenths of the step cycle in the ipsilateral forelimb, both in absolute terms and relative to mean size during rest. 3. Step histograms for the same site on different days or different stimulus intensities varied appreciably in form but in both cases the timing of the largest response was usually the same or shifted by only one step tenth. 4. In both zones the largest responses during walking occurred overwhelmingly during the E1 step phase when the limb is extended forwards and down to establish footfall. Least responses were much less uniform in timing but were mostly during stance, particularly its early (E2) part. 5. In many histograms the smallest responses were smaller in mean size than the responses during rest while the largest were larger. These changes were not paralleled by changes in nerve volley size, so presumably reflect step-related central changes in pathway excitability. Facilitations and depressions were differently affected by stimulus intensity and sometimes occurred independently, suggesting generation by separate mechanisms. 6. In both zones there were differences between recording sites which suggests that different DF-SOCP subcomponents innervate different parts of the zones. However, no systematic differences could be firmly established between the medial and lateral subzones of the c1 zone. 7. The results are discussed in relation to the hypothesis that the DF-SOCP constitutes the afferent limb of a transcerebellar mechanism involved in adapting the evolving step.