Recurrent inhibition of motoneurons in decerebrate cats during controlled treadmill locomotion.

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Abstract
Whether the recurrent inhibition pathway was altered during controlled treadmill locomotion was studied. The effects of antidromic stimulation of the proximal portion of a severed L7 ventral root (VR) on the firing patterns of single motoneurons isolated within an L7 VR filament were studied. Motoneuron firing was produced by cyclic passive limb movement and by stimulation of the mesencephalic locomotor region (MLR) to produce controlled treadmill locomotion in postmammillary cats. Antidromic stimulation produced significant decrements in motoneuron discharge during cyclic passive limb movement and controlled treadmill locomotion with mean reductions of 72 and 68%, respectively. Of 19 motoneurons studied, the rate of firing was significantly reduced by ventral root stimulation during cyclic passive limb movement in 15 and during locomotion in 13. There was a significant positive linear relationship between the initial firing rate of phasically active motoneurons and the amount by which their discharge was reduced by antidromically evoked recurrent inhibition. Previous reports on recurrent inhibition of tonically active motoneurons are not supported. The locomotor rhythm displayed by hindlimb motoneurons in the L7 ventral root filament was not affected by antidromic stimulation of the ipsilateral L7 ventral root. The 1st bursts of motoneuron activity following cessation of the stimulus train occurred within predicted latencies. The ensuing pattern of motoneuron activity was not significantly different from that observed in the filament prior to antidromic stimulation. The firing pattern of motoneurons that responded tonically to static muscle stretch was altered by antidromic stimulation of the ipsilateral L7 ventral root. In addition to significant reductions in motoneuron firing frequency, motoneuron activity often switched from single spikes appearing at a constant interspike interval to doublets with a variable interburst interval. Cyclic movement of the passive hindlimb and stimulation of the MLR evoked bursts of motoneuron activity in the L7 ventral root filament characterized by an initial doublet that was less effectively inhibited than the remainder of the burst during antidromic stimulation of the ipsilateral L7 ventral root. Renshaw cells responded to antidromic stimulation with equal effectiveness during cyclic passive limb movement and controlled treadmill locomotion. Renshaw cells apparently are not depressed during locomotion. The effects of Renshaw cell activity are not abolished during fictive locomotion and segmental afferents rhythmically active during locomotion in a nonparalyzed preparation do not suppress the recurrent inhibitory pathway to .alpha.-motoneurons.