Differential effects of stimulation of the cat's red nucleus on lumbar alpha motoneurones and their Renshaw cells

Summary 1. The red nucleus region was stereotaxically stimulated with short trains of high-frequency alternating current pulses in anaesthetized cats. The effects were studied, in contralateral lumbar segments, on the responses of microrecorded individual Renshaw cells (RCs) to antidromic or orthodromic test shocks of ventral root or muscle nerve fibres. Monosynaptic reflexes (MRs) of their motoneurone pools were recorded from one of the cut lumbar ventral roots. Averages of 10–20 replicate test responses of the RC (converted into instantaneous frequency curves, IFCs) and of the MR shapes were computed and graphically displayed. 2. Orthodromic (afferent) test shocks induced simultaneously MRs as well as responses of a RC belonging to the same motor pool. From their paired records at systematically varied shock strengths, whole linkage characteristics of the relation between the two events could be obtained, representing the functional linkage from the motoraxon collaterals to the RC under study. The overall result of rubral conditioning was a change in the course of the characteristic, which indicated a reduction of this linkage (= relative inhibition of the RC against its recurrent input). 3. Sequential trials with test shocks of constant, submaximal strength were performed with 45 individual RCs. The clearest results were obtained with RC responses to antidromic ventral root shocks: 65% of the RCs were partially inhibited by rubral conditioning. Interposed minor facilitory subcomponents could be seen in the course of inhibited IFCs. Mixed sequences of manifest inhibitory/facilitory effects were observed in 11%; reversed sequences (facilitory/inhibitory) did not occur. A pure but weak facilitation was found in only one case, paralleled by an increase of the MR. RCs belonging to either extensor or flexor motor pools were affected about equally. A little over 20% of the tested RCs remained uninfluenced by rubral stimulation. 4. The MRs, induced by constant, submaximal, orthodromic test shocks, were usually enhanced with only few exceptions, by rubral stimulation. The effects on the orthodromic RC responses were mainly inhibitory, but could be more or less masked by the concurrent increase of the MR, providing a stronger recurrent input to the RC. Such inhibition could be uncovered, however, by observing the above described linkage change. 5. Variation of several parameters of rubral conditioning (train duration, timing of train with respect to test shock, strength of train) modified the inhibitory effects on antidromic RC responses to a certain extent without changing their principal character. Higher conditioning strengths frequently induced mass discharges of previously silent motoneurones, but at the same time an increased inhibition of the concurrent RC responses. 6. Spontaneous RC activity (in the absence of test stimuli) occurred infrequently and was weak and interrupted by silent periods. When this persisted long enough for testing repeated rubral stimulation, a strong initial inhibition lasting up to several hundred ms was found, sometimes followed by some oscillations of the average discharge rate. 7. The predominant combination of concurrent effects of the conditioning, namely, inhibition of RCs and facilitation of motoneurones, indicated independent (and mostly divergent) control of the two target neurones by the red nucleus. It is concluded that in this way the RCs can be flexibly and transiently decoupled to some degree from their recurrent motoneuronal input.Dedicated to Prof. Ragnar Granit on behalf of his 85th birthday