The role of the motor cortex in the control of vigour of locomotor movements in the cat.

1. The impulse activity of single neurones in the motor cortex (MC) was recorded extracellularly using movable varnish-insulated tungsten microelectrodes in four adult freely moving cats. The cats walked inside the experimental box with various loadings in the swing or stance phases of the step cycle. The mean discharge rate (mR) and the depth of frequency modulation (dM) in each neurone were estimated over 10-100 steps. 2. The activity of thirty-one cells (including eighteen pyramidal tract neurones (PTNs)) was recorded during uphill walking on a 10 deg inclined floor. The mR in 68%, and the dM in 77% of neurones changed by less than 20% during uphill locomotion compared to walking on a level surface. 3. The activity of the same neurones was also recorded during downhill walking, also on a 10 deg inclined plane. The mR in 69% and the dM in 78% of neurones changed by less than 20% during downhill locomotion compared with walking on a level surface. 4. The activity of twenty-three (the left hemisphere) cells (sixteen PTNs) during walking with the floor swaying to the right (R) and to the left (L) was compared to activity during locomotion on a stable surface. The mR in 83% and the dM in 83% of cells in R-steps, and in 82 and 77% of cells, respectively, in L-steps changed by less than 20%. 5. The activity of thirty-seven cells was studied during locomotion at various speeds. The mR in 68% and the dM in 38% of cells changed by less than 20% during fast and slow locomotion compared to middle-speed locomotion. The dM in 46% of neurones increased with the transfer from slow to fast walking. 6. The activity of thirty-one MC cells was recorded during locomotion with loads of 85 g attached to the distal part of each elbow. The mR in 52% and the dM in 48% of neurones changed by more than 20%. 7. The activity of twenty-eight cells (six PTNs) was studied in steps when an animal turned. The swing of the limb contralateral to the recorded MC was shorter (condition 1) in turning steps in one direction, and was longer (condition 2) in turning steps in the opposite direction. The mR in 50% and the dM in 50% of cells in condition 1 and in 52% and 59%, respectively, of cells in condition 2 changed by more than 20%.(ABSTRACT TRUNCATED AT 400 WORDS)