STEP PHASE-RELATED EXCITABILITY CHANGES IN SPINO-OLIVOCEREBELLAR PATHS TO THE C(1) AND C(3) ZONES IN CAT CEREBELLUM

1. Chronically implanted microwires were used to record extracellular field potentials generated in the c(1) and c(3) zones in the cortex of lobules V and VI of the cerebellum by non-noxious noxious stimuli del ivered to the superficial radial nerve in the ipsilateral forelimb. Re sponses due to input via climbing fibre afferents were studied; their latency and other characteristics identified them as mediated mainly v ia the dorsal funiculus spino-olivocerebellar path (DF-SOCP). 2. Respo nses at individual sites were studied repeatedly with a range of stimu lus intensities and during two different behaviours: quiet rest and st eady walking on an exercise belt. For responses during walking, step h istograms were constructed showing response mean size during different tenths of the step cycle in the ipsilateral forelimb, both in absolut e terms and relative to mean size during rest. 3. Step histograms for the same site on different days or different stimulus intensities vari ed appreciably in form but in both cases the timing of the largest res ponse was usually the same or shifted by only one step tenth. 4. In bo th zones the largest responses during walking occurred overwhelmingly during the E(1) step phase when the limb is extended forwards and down to establish footfall. Least responses were much less uniform in timi ng but were mostly during stance, particularly its early (E(2)) part. 5. In many histograms the smallest responses were smaller in mean size than the responses during rest while the largest were larger. These c hanges were not paralleled by changes in nerve volley size, so presuma bly reflect step-related central changes in pathway excitability. Faci litations and depressions were differently affected by stimulus intens ity and sometimes occurred independently, suggesting generation by sep arate mechanisms. 6. In both zones there were differences between reco rding sites which suggests that different DF-SOCP subcomponents innerv ate different parts of the zones. However, no systematic differences c ould be firmly established between the medial and lateral subzones of the c(1) zone. 7. The results are discussed in relation to the hypothe sis that the DF-SOCP constitutes the afferent limb of a transcerebella r mechanism involved in adapting the evolving step.