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Notes: 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.

Notes: Summary Successive motor unit (MU) twitches often do not sum linearly. Also, muscle spindle (MS) afferents may react nonlinearly to MU contractions occurring at short intervals. Little data is presently available on the interactions between two (or more) MUs regarding their effects on tension output and MS responses. We have studied these effects in cat Mm. gastrocnemius medialis (MG), soleus and semitendinosus. In adult anaesthetized cats, MUs of the muscle under study were electrically stimulated via their ventral root axons with random sequences of brief pulses having mean rates between 6 and 12 pulses per second. Isometric tension fluctuations were recorded from the muscle under study, and discharge patterns of MS afferents (Ia and group II) were recorded from dorsal root filaments. A crosscorrelation analysis was performed to display linear and nonlinear effects evoked by selected time constellations of MU activations. 1) 18 (67%) of 27 MG MUs showed marked potentiation of the second of two twitches in response to pairs of stimuli separated by 5 to about 25 ms. The remainder of these and 16 of the soleus MUs did not exhibit conspicuous nonlinearities. — 2) MS responses to such pairs of MU activations usually showed a prolonged spindle pause. — 3) About 28% of 36pairs of MG MUs produced twitch tension less than expected for linear summation if activated nearly simultaneously. — 4) If two MUs both produced a spindle pause and possibly a relaxation discharge in an MS afferent, the near-synchronous activation of the units produced respective discharge variations that were less than expected for linear summation. If one MU produced an early discharge, contraction of another MU would often prevent it. — These results are discussed in regard to mechanisms of tremor suppression.

Notes: Summary The effects of prolonged training of adult monkeys subjected to random, brief perturbations of alternating elbow flexions and extensions were studied over a period of four years. The training was intensive at first, for about one year, and then irregular, with long pauses, during the following three years. As a consequence of the prolonged training with the brief perturbations, the M2 component of the electromyographic (EMG) response of the biceps and triceps muscles became gradually smaller, and finally disappeared. The M1 component, on the other hand, progressively increased in amplitude and continued to do so after the loss of the M2, until it finally dominated the EMG response. The training had similar effects on the response of the biceps muscle to longer perturbations, but, only under certain conditions, did it affect the triceps muscle response. All changes occurred at earlier stages of the training in the flexor than in the extensor muscle. These observations demonstrate a long-term functional plasticity of the sensorimotor system of adult animals and suggest a growing role for fast segmental mechanisms in the reaction to external disturbances as motor learning progresses. Changes at various levels of the stretch reflex system could underlie the enlargement of the M1 component, while the lack of the M2 component should, at least partially, reflect a reduced cortical effect on αmotoneurones and/or changes in spinal systems processing afferent information.

Notes: Abstract We here present a method to study the interaction of parallel neural input channels regarding their effects on a neurone. In particular, the method allows to disclose the effects of oligosynaptic pathways that may exist in parallel to direct monosynaptic connections to the cell. Two (or more) inputs (nerves) are stimulated with random patterns of stimuli. The response of the cell to these patterns is evaluated by the computation of peristimulus-time histograms (PSTHs). One of the two stimulus trains is selected as the one to yield reference events for the PSTH computation. From this stimulus train are selected those stimuli as reference events which are preceded, at defined mean intervals, by stimuli in the same or a parallel channel. These “conditioning” stimuli are determined (1) separately from each single stimulus train and (2) concomitantly from the two trains as events occurring simultaneously in both. The effects exerted by these various conditioning events on the effects of the “test” pulses on the cell response yield insights into the interactions between the two (or more) inputs. These methods are demonstrated on spinal Renshaw cells activated by independent random stimulation of two muscle nerves and on dorsal horn neurones responding to cutaneous nerve stimulation.

Notes: Summary In 9 adult anaesthetized cats, 22 lumbosacral Renshaw cells recorded with NaCl-filled micropipettes were activated by random stimulation of ventral roots or peripheral nerves. The stimulus patterns had mean rates of 9.5–13 or 20–23 or 45 pulses per second and were pseudo-Poisson; short intervals below ca. 5 ms (except in two cases) were excluded. The Renshaw cell responses were evaluated by two kinds of peristimulus-time histograms (PSTHs). “Conventional” PSTHs were calculated by averaging the Renshaw cell discharge with respect to all the stimuli in a train. These PSTHs showed an early excitatory response which was often followed by a longer-lasting slight reduction of the discharge probability. These two response components were positively correlated. “Conditional” PSTHs were determined by averaging the Renshaw cell discharge with respect to the second (“test”) stimulus in pairs of stimuli which were separated by varied intervals, δ. The direct effect of the first “conditional” response was subtracted from the excitation following the second (“test”) stimulus so as to isolate the effect caused by the second stimulus per se. After such a correction, the effect of the first “conditioning” stimulus showed pure depression, pure facilitation or mixed facilitation/depression. Analysis of such conditioning curves yielded two time constants of facilitation (ranges: ca. 4–35 ms and 93–102 ms) and two of depression (ranges: ca. 7–25 ms and 50–161 ms). It is concluded that these time constants are compatible with processes of short-term synaptic plasticity known from other synapses. Other processes such as afterhyperpolarization and mutual inhibition probably are of less importance.