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1. The output from Renshaw cells caused by a phasic motor volley was investigated when these neurones were submitted to a background firing secondary to a tonic motor discharge elicited by a repetitive stimulation of muscle group I afferents. It was invariably found in individual Renshaw cells that tonic excitation produced an increase in the additional ouput caused by the phasic motor volley. The curves displaying this increase exhibited a significant 'jump' when the output resulting from the combined tonic and phasic motor discharges ranged between 2 and 5 spikes during the first 10 msec following the phasic volley. 2. The whole pool of Renshaw cells was also considered by assessing the amount of recurrent inhibition in motoneurones following a phasic motor volley. Similarly it was found that additional recurrent inhibition elicited by a phasic motor volley was enhanced when the Renshaw cells received a tonic excitatory input. 3. The Renshaw cell discharges elicited by stimulation of two different nerves were compared when a conditioning stimulus was previously applied to only one of them. The results strongly suggest that a preceding volley caused a decrease in synaptic efficacy at the terminals of the recurrent collaterals. 4. The firing produced by current injected through the recording micro-electrode was investigated in one intracellularly recorded Renshaw cell. It was found that the current-frequency curve was linear for 'steady-state' firing but displayed a clearcut sigmoid shape for the earliest intervals before the final adaptation. 5. It is demonstrated that this sigmoid input-output relation in individual Renshaw cells is sufficient to explain how the controls acting on these neurones may change the gain in the recurrent pathway when Renshaw cells are fired by a phasic motor discharge. When Renshaw cells are fired by a longlasting tonic motor discharge the linear input-output relation in individual cells should not cause any modifications of the gain in the recurrent pathway. A change in this gain secondary to the effects of segmental and supraspinal control systems is, however, possible if the motor discharge creates a subliminal fringe within the pool of Renshaw cells.

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