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Department of Physiology, Medical School, University of Bristol.

1. Extracellular microelectrodes were used in free-to-move cats to study the locomotor-related discharges of Purkinje cells in the intermediate part of lobule V of the cerebellar anterior lobe and of neurones in the underlying nucleus interpositus anterior. All cells studied discharged rhythmically during locomotion. 2. The discharges during walking at a speed of 0.5 m/s on a horizontal exercise belt were compared with those during (a) walking at 0.9 m/s (when the duration of the step cycle is shortened considerably and the amplitudes of the locomotor electromyograms (EMGs) recorded from flexor and extensor muscles of the limbs are markedly increased) and (b) during walking at 0.5 m/s with the belt tilted uphill by 30 deg (when step duration is little changed but locomotor EMGs are increased by 70-100%). 3. In each of thirty Purkinje cells the timing of the discharges relative to the forelimb step cycle showed no major difference between the two speeds of walking. Most cells discharged at slightly higher overall rates at the faster walking speed but the increase was usually modest, the average being only 5.6 impulses/s (i.e. an increase of 8%). Peak rates sometimes underwent larger increases but the average was only 11.9 impulses/s (+11%). Changes in minimum rate were generally small (an average increase of 0.3 impulses/s). 4. Among twenty-one interpositus neurones there was only one in which discharge timing relative to the step cycle was different between the two speeds. Like the Purkinje cells, most neurones discharged slightly faster at the higher speed but the average increase was only 5.5 impulses/s (+8.5%). Peak firing rates also usually showed a modest increase (averaging 6.2 impulses/s; +6.5%) while minimum rates were little changed. 5. Among nineteen Purkinje cells compared between walking uphill and on the flat only one showed any major difference in discharge phasing; overall firing rates were on average only 1.3 impulses/s (2%) higher for uphill locomotion. 6. Among twenty-one interpositus neurones discharge phasing differed markedly between walking uphill and on the flat in only two cells. Overall discharge rates were on average slightly higher uphill (by 3.5 impulses/s; 6.7%) and peak rates also usually increased slightly (on average, by 6 impulses/s; 7.7%). Minimum rates were higher, on average, by 1.6 impulses/s (+5%). 7. The findings are discussed in relation to current notions of how the intermediate part of the cerebellum may contribute to movement control and it is concluded that the neurones studies probably make little contribution to determining the vigour of the movements of steady walking.

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