After-effects of stochastic synaptic Renshaw cell excitation on their discharge probability

We have studied Renshaw cell (RC) responses to pseudo-Poisson stimulus sequences (small intervals below 5 ms excluded; mean rates between 10 and 45 pulses/s) delivered to motor axons in ventral roots or muscle nerves. Average RC responses to stimuli in a single stimulus channel depended upon the preceding stimulation history. The responses to pairs of stimuli separated by variable intervals (irrespective of intervening stimuli) generally showed a long depressant effect (from 25 to more than 250 ms) of the first ('conditioning') stimulus; in a number of cases a briefer facilitating effect preceded the depressant action. Several possible causes of these conditioning effects are discussed.