Fig. 1.

Experimental setup. A: locomotion tasks: simple walking on a flat surface (simple locomotion) and walking along a horizontal ladder with crosspieces 18-, 12-, or 5-cm wide (complex locomotion). Paw prints on the crosspieces of the ladders schematically show placements of cat forelimb paws. The shaded area denotes a force platform. B: the cat model. Orientation of each segment was determined as the angle between the negative direction of the vertical axis and the longitudinal segment axis directed from the distal end of the segment to the proximal one, as indicated for the head/neck segment. C: a scheme of the recording area within the forelimb representation of the left motor cortex. The microelectrode entry points into the cortex (cortical plate openings through which penetrations have been made) were combined from all cats and shown by black circles (Cru, cruciate sulcus; Pcd, postcruciate dimple). D: a sample record of discharge of a pyramidal tract projecting neuron (PTN) during simple and complex locomotion. The bottom trace shows the swing (Sw, deflection up) and stance (St, deflection down) phases of the step cycle of the right forelimb recorded by an electromechanical sensor.