Renshaw cell activity and recurrent effects on motoneurons during fictive locomotion.

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Abstract
Microelectrode recordings from Ia inhibitory interneurons, Renshaw cells and motoneurons were obtained in pre- or post-mamillary cats during fictive locomotion. Work by Feldman and Orlovsky showing that lumbar Ia inhibitory interneurons are rhythmically active during fictive locomotion was confirmed. Renshaw cells fired rhythmically in bursts of 1-22 spikes and with a maximum rate of 45 Hz during periods analogous to the stance or swing phase of the step cycle. Most Renshaw cells fired during 1 phase of the step cycle and were completely silent in the opposite phase. The bursts of Renshaw cell activity occurred during the periods of the step cycle when the motoneurons from which they received their maximum excitatory input were active. Whether Renshaw cells are inhibited during locomotion was tested by examining Renshaw cell discharges evoked by ventral root stimulation during fictive locomotion. A small but significant reduction in the response of Renshaw cells to ventral root stimulation occurred during fictive locomotion; 15.7 was the mean number of spikes evoked due to single shocks applied to the ventral root before fictive locomotion commenced. A mean of 13.6 spikes was evoked for each ventral root stimulus during fictive locomotion. Recurrent inhibitory postsynaptic potentials (IPSP) in motoneurons produced by ventral root stimulation were detected during all phases of the fictive step cycle. The amplitudes of the IPSP were inversely correlated with the motoneuron membrane potential. Recurrent facilitatory potentials could be evoked in motoneurons during all phases of the fictive step cycle. Single stimuli delivered to a cut ventral root during fictive locomotion evoked characteristic high-frequency Renshaw cell responses. If 200 ms or less separated the stimuli, MLR[mesencephalic locomotor region]-evoked rhythmic Renshaw cell activity could be abolished. The duration of this inhibition was usually about 200 ms and probably results from Renshaw cell-Renshaw cell inhibition. Renshaw cell activity and recurrent inhibition may contribute significantly to the control of locomotion at the spinal level.