Problems of postsynaptic autogenous and recurrent inhibition in the mammalian spinal cord

Upper and lower motor neuron neurophysiology and motor control

This chapter considers the principles that underlie neurophysiological studies of upper motor neuron or lower motor neuron lesions, based on an understanding of the normal structure and function of the motor system. Human motor neurophysiology consists of an evaluation of the active components of the motor system that are relevant to volitional movements. Relatively primitive motor skills include locomotion, much dependent on the spinal cord central pattern generator, reaching, involving proximal and distal muscles activation, and grasping. Humans are well prepared to perform complex movements like writing. The role of motor cortex is critical for the motor activity, very dependent on the continuous sensory feedback, and this is essential for adapting the force and speed control, which contributes to motor learning. Most corticospinal neurons in the brain project to brainstem and spinal cord, many with polysynaptic inhibitory rather than excitatory connections. The monosynaptic connections observed in humans and primates constitute a specialized pathway implicated in fractional finger movements. Spinal cord has a complex physiology, and local reflexes and sensory feedback are essential to control adapted muscular contraction during movement. The cerebellum has a major role in motor coordination, but also consistent roles in sensory activities, speech, and language, in motor and spatial memory, and in psychological activity. The motor unit is the final effector of the motor drive. The complex interplay between the lower motor neuron, its axon, motor end-plates, and muscle fibers allows a relevant plasticity in the movement output.

Blind identification of the spinal cord output in humans with high-density electrode arrays implanted in muscles

Invasive electromyography opened a new window to explore motoneuron behavior in vivo. However, the technique is limited by the small fraction of active motoneurons that can be concurrently detected, precluding a population analysis in natural tasks. Here, we developed a high-density intramuscular electrode for in vivo human recordings along with a fully automatic methodology that could detect the discharges of action potentials of up to 67 concurrently active motoneurons with 99% accuracy. These data revealed that motoneurons of the same pool receive common synaptic input at frequencies up to 75 Hz and that late-recruited motoneurons inhibit the discharges of those recruited earlier. These results constitute an important step in the population coding analysis of the human motor system in vivo.

Modulation of motoneuron firing by recurrent inhibition in the adult rat in vivo

Recent reports show that synaptic inhibition can modulate postsynaptic spike timing without having strong effects on firing rate. Thus synaptic inhibition can achieve multiplicity in neural circuit operation through variable modulation of postsynaptic firing rate vs. timing. We tested this possibility for recurrent inhibition (RI) of spinal motoneurons. In in vivo electrophysiological studies of adult Wistar rats anesthetized by isoflurane, we examined repetitive firing of individual lumbosacral motoneurons recorded in current clamp and modulated by synchronous antidromic electrical stimulation of multiple motor axons and their centrally projecting collateral branches. Antidromic stimulation produced recurrent inhibitory postsynaptic potentials (RIPSPs) having properties similar to those detailed in the cat. Although synchronous RI produced marked short-term modulation of motoneuron spike timing and instantaneous firing rate, there was little or no suppression of average firing rate. The bias in firing modulation of timing over average rate was observed even for high-frequency RI stimulation (100 Hz), perhaps because of the brevity of RIPSPs, which were more than twofold shorter during motoneuron firing compared with rest. These findings demonstrate that RI in the mammalian spinal cord has the capacity to support and not impede heightened motor pool activity, possibly during rapid, forceful movements.

Is the human masticatory system devoid of recurrent inhibition?

The aim of the present study was to investigate the existence or otherwise of a functional recurrent inhibitory system (Renshaw cell system) in the motoneurons that innervate human masticatory muscles. In a previous study, L: -acetylcarnitine (L: -Ac), a substance known to potentiate recurrent inhibition in humans was found to alter, in a specific way, the discharge variability, and the synchronous activity of motor units depending on the presence or absence of recurrent inhibition in the corresponding motoneuron pool. Using a similar paradigm, we have recorded the tonic discharge activity of motor unit pairs from the masseter muscle during voluntary isometric contraction while subjects were undergoing continuous intravenous saline (SAL, NaCl 0.9%) perfusion. Following a brief baseline-recording period, the subjects were given a test injection of either L: -Ac or isotonic saline (SAL) in a double blind manner. The variability, synchronization, and coherence between the motor unit discharges were analysed during three successive periods: pre-injection, during injection, and post-injection, each lasting 2-3 min. Neither L: -Ac nor SAL injection induced a significant change in the inter-spike interval (ISI) or the coefficient of variation of the ISIs in the motor units tested. There were also no significant changes in the pattern of synchronous activity or in the coherence, which reflects the common frequency content of the unit discharges. Reminiscent of what had been observed previously with motoneurons without recurrent inhibition in the Abductor Digitorum Minimi muscle, the lack of effects of L: -Ac injection on the firing behaviour of masseter motoneurons may suggest that classical Renshaw cell inhibition is lacking in this motoneuron pool.

Recurrent inhibition of wrist extensor motoneurones: a single unit study on a deafferented patient

In order to document the effects of recurrent inhibition on the firing times of human alpha-motoneurones during natural motor behaviour, a case study was performed on a deafferented patient. The fact that this subject had completely lost the large-diameter sensory afferents provided us with a unique opportunity of selectively stimulating the motor axons in the nerves. The tonic activity of single motor units (n = 21) was recorded in the extensor carpi radialis muscles while applying randomly timed antidromic electrical stimuli to the radial nerve. The peristimulus time histogram analysis showed the presence of biphasic inhibitory effects, including an early, short-lasting component followed by a longer-lasting component occurring 20-40 ms later. The interspike interval (ISI) during which the stimulation occurred was generally lengthened as compared to the previous ISIs. The stimulation was most effective when delivered early (20-30 ms) after a spike. It was also effective, although less so, when delivered at the end of the ISI (70-100 ms after a spike). The lengthening effect sometimes extended over one or two of the subsequent ISIs. The lengthening effect of the motor axon stimulation was followed by an excitatory-like effect, which took the form of a shortening that affected up to five ISIs after the stimulation. The biphasic inhibitory effects and the subsequent facilitatory effects are discussed in terms of the dual nature of the synaptic processes involved in the recurrent inhibitory network, the postactivation facilitation/depression processes and the mutual inhibition occurring between Renshaw cells.

Pharmacologically induced enhancement of recurrent inhibition in humans: effects on motoneurone discharge patterns

The aim of the present study was to investigate the effects of spinal recurrent inhibition on human motoneurone discharge patterns. The tonic discharge activity of motor unit pairs was recorded in the extensor carpi radialis (ECR) and abductor digiti minimi (ADM) muscles during voluntary isometric contraction. While undergoing continuous intravenous saline (NaCl 0.9 %) perfusion, the subjects were given a short lasting injection of L-acetylcarnitine (L-Ac), which has been found to potentiate recurrent inhibition in humans. The variability, synchronization and coherence of the motor unit discharges were analysed during four successive test periods (lasting 2-3 min each). A significant decrease in the inter-spike interval (ISI) coefficient of variation was observed in the discharge patterns of the motor units tested in the ECR and not in the ADM, which were not accompanied by any consistent changes in the mean ISIs of the motor unit activity in either muscle. The L-Ac injection also led to a significant increase in the synchronization in half of the motor unit pairs tested in the ECR muscle (n = 29), whereas no consistent changes were observed with the ADM motor units (n = 25). However, coherence analysis failed to reveal any consistent differences in the incidence of significant values of coherence spectrum between the pre-injection and injection periods among the motor unit pairs tested with either saline or L-Ac injections, in either the ECR or ADM muscles. The contrasting effects on the variability and the synchronization of the motor unit discharges observed with ECR motoneurones known to undergo recurrent inhibition and with ADM motoneurones known to lack recurrent inhibition suggest that the drug may have specific effects which are mediated by an enhancement of the Renshaw cell activity. The decrease in the ISI variability is in line with the hypothesis that recurrent inhibition may contribute along with the post-spike after-hyperpolarization to limiting the influence of the synaptic noise on the firing times of steadily discharging motoneurones. The present data, which suggest that recurrent inhibition plays a synchronizing rather than a desynchronizing role, are in keeping with the fact that the Renshaw cells may provide an important source of common inhibitory inputs.

Identification of an interneuronal population that mediates recurrent inhibition of motoneurons in the developing chick spinal cord

Studies on the development of synaptic specificity, embryonic activity, and neuronal specification in the spinal cord have all been limited by the absence of a functionally identified interneuron class (defined by its unique set of connections). Here, we identify an interneuron population in the embryonic chick spinal cord that appears to be the avian equivalent of the mammalian Renshaw cell (R-interneurons). These cells receive monosynaptic nicotinic, cholinergic input from motoneuron recurrent collaterals. They make predominately GABAergic connections back onto motoneurons and to other R-interneurons but project rarely to other spinal interneurons. The similarity between the connections of the developing R-interneuron, shortly after circuit formation, and the mature mammalian Renshaw cell raises the possibility that R-interneuronal connections are formed precisely from the onset. Using a newly developed optical approach, we identified the location of R-interneurons in a column, dorsomedial to the motor nucleus. Functional characterization of the R-interneuron population provides the basis for analyses that have so far only been possible for motoneurons.

Recurrent inhibition in humans

Methods have been developed to investigate recurrent inhibition (RI) in humans. A conditioning reflex discharge is used to evoke in motoneurones (MNs) supplying homonymous and synergistic muscles, an inhibition the characteristics of which are consistent with RI: it appears and increases with the conditioning motor discharge, has a short latency and a long duration, and is enhanced by an agonist of acetylcholine. As in the cat, homonymous RI exists in all explored motor nuclei of the limbs except those of the digits and the pattern of distribution of heteronymous RI closely matches that of monosynaptic Ia excitation. However, striking inter-species differences exist concerning the distribution of heteronymous RI since it is much more widely extended in the human lower limb than in the cat hindlimb, whereas it is more restricted in the upper limb than in the cat forelimb. Changes in transmission in the recurrent pathway have been investigated during various voluntary or postural contractions involving different (homonymous, synergistic, antagonistic) muscles and it has been found that the activation of Renshaw cells (RCs) by the voluntary motor discharge via recurrent collaterals was powerfully controlled by descending tracts: for example, during homonymous contraction, RI evoked by a given conditioning reflex discharge is much smaller during strong than during weak contraction, which suggests that the descending control of RCs might contribute to the regulation of muscle force. The finding that RC inhibition is more marked during phasic than during tonic contraction of similar force of the homonymous muscle is discussed in relation with the projections of RCs to Ia interneurones mediating reciprocal inhibition. Only in patients with progressive paraparesis is there evidence for decreased RI at rest which may contribute to the exaggeration of the passively-induced stretch reflex underlying spasticity. However, despite the seemingly normal RI at rest in most patients, the control of RCs during voluntary movements is disturbed in these patients, which probably contributes to their motor disability.

Discharge patterns of intramural mechanoreceptive afferents during selective distension of the cat's rectum

The afferent input from the rectum to the central nervous system (CNS) has yet to be thoroughly characterized. The characteristics of mechanoreceptive rectal afferents have been studied in unanaesthetized decerebrate cats. Following lumbo-sacral laminectomy, single-unit activity (occasionally multi-unit activity) was recorded from centrally cut filaments of the sacral dorsal roots (predominantly S2), while a balloon was inflated in the rectum. Starting from their background activities (mean 15.1 imp sec-1, SD 7.6 imp sec-1), afferent discharge rate increased with increasing balloon pressure (mean threshold 6.3 mmHg, SD 3.6 mmHg). The dependence of firing rate on intrarectal pressure began to flatten out at 25 mmHg (mean; SD 10 mmHg). For 22 out of 29 units (76%) complete saturation occurred at 35 mmHg (mean; SD 15 mmHg) with a maximum discharge rate of 31 imp sec-1 (mean; SD 12.6 imp sec-1). In a number of recording sessions, cyclical rectal contractions were observed. In these cases, changes in firing of the units were closely related to changes in intrarectal pressure. Pressure-related afferent activity could be enhanced by parasympathomimetic drugs which augmented rectal contractions. We conclude that sacral dorsal roots contain afferents from low-threshold mechanoreceptors located in the rectal wall, and that these afferents monitor the filling state and contraction level of the rectum.

Temporal changes of pyramidal tract activities after decision of movement: a study using transcranial magnetic stimulation of the motor cortex in humans

To elucidate the effects of the decision to move on the pyramidal tract in humans, we examined the changes in the motor evoked potentials (MEP) of the forearm muscles following transcranial magnetic cortical stimulation (TMS) of the hand area during a go/no-go hand-movement task in 10 normal subjects. The subjects performed an extension of the right wrist according to the go, no-go and control signals, one of which was randomly presented on a TV. A single TMS was applied to the primary hand motor area in the left hemisphere 0-300 ms after each signal. The MEPs recorded from the wrist extensor and flexor muscles changed in amplitude after both go and no-go signals. In comparison with the control, the MEPs were significantly facilitated in the agonistic muscles (wrist extensor muscles) and attenuated in the antagonistic muscles (wrist flexor muscles), at the latencies of 100-200 ms after the go signal (P < 0.02). In contrast, the MEPs of both the extensor and flexor muscles were significantly attenuated during the period of 100-200 ms after the no-go signal (P < 0.001). We speculate that there is strong inhibition on the pyramidal tract after the no-go signal and that the inhibitory effect is non-specific to the target muscles. This inhibition differs from the reciprocal inhibition of the MEP observed in antagonistic muscles after the go signal, and it is probably related to the movement decision originating in the prefrontal cortex.

Renshaw cell responses to intra-arterial injection of muscle metabolites into cat calf muscles

Metabolites released during fatiguing muscle contractions excite group III IV muscle afferents which might inhibit skeleto-motoneuron firing, hypothetically via Renshaw cells. This was tested, in decerebrated, spinalized cats, by recording changes in Renshaw cell spontaneous discharges and responses to antidromic electrical stimulation of motor axons when small-diameter calf muscle afferents were excited by intra-arterially injected bradykinin, serotonin, lactic acid and KCI. Whenever such injections had an effect, it transiently raised or lowered the spontaneous firing rate and almost always decreased the antidromic response to motor axon stimulation. Injection of bradykinin and serotonin commonly decreased the blood pressure and concomitantly the spinal blood flow (as measured using laser Doppler flowmetry), which could have indirectly influenced Renshaw cell firing. But in general, blood pressure and flow changed after the Renshaw cell discharge did, which thus, appears to be modulated independently by group III-IV afferents. These results suggest that the Renshaw cell-mediated effects of neurochemically excited afferents would predominantly disinhibit rather than inhibit motoneurons.

On the role of recurrent inhibitory feedback in motor control

This article reviews presumed roles of recurrent inhibition in motor control, that have been proposed over the past five decades. The discussion is structured in an order of increasing complexity. It starts out with the simplest and earliest circuit, that is recurrent self-inhibition of skeleto-motoneurons, and related functions. It soon becomes clear that in order to understand recurrent inhibition, we must look beyond the simple self-inhibitory CNS circuit. First, recurrent inhibition must be seen in the context of other neural circuits. Second, some quantitative features appear to be correlated with features of the neuromusculo-skeletal periphery. Third, the aspect of lateral inhibition between different members of a motoneuron pool as well as between different motoneuron pools points to the essential multiple input-multiple output structure of recurrent inhibition that again can be understood only by correlating it with features of the neuromusculo-skeletal periphery. Another extension results from the discovery that recurrent inhibition affects not only skeleto-motoneurons, but also gamma-motoneurons, Ia inhibitory interneurons mediating reciprocal inhibition between antagonist motoneurons, other Renshaw cells and cells of origin of the ventral spinocerebellar tract (VSCT). Then the view broadens again, investigating the potential role that recurrent inhibition plays in two far-ranging theories of motor control, the inverse-dynamics approach and the equilibrium-point hypothesis. Finally, the present author tries to formulate, in broad strokes, a personal functional interpretation of recurrent inhibition. All the functional considerations, right or wrong, should yield ideas for new experiments, and this then is the last objective of this review.

Waveform parameters of recurrent inhibitory postsynaptic potentials in cat motoneurons during time-varying activation patterns

A considerable number of theoretical and experimental studies have been undertaken to establish quantitative relationships between the time course of postsynaptic potentials in a neuron and the change in firing probability thereby induced. Depending on background synaptic noise level, the time course of the postsynaptic potential per se as well as its time derivative are both of importance in varying proportion. We have recently begun to study recurrent inhibitory potentials in cat hindlimb motoneurons during rhythmically varying rates of stimulation of motor axons. The amplitude-rate relationship exhibits hysteresis in that amplitudes are usually larger during augmenting than decrementing rates in the cycle. We here report results on the other important variable, that is the slope of recurrent inhibitory potential development, which need not a priori be correlated with amplitude. We found that the slope has a relation to stimulus rate similar to amplitude, so that both parameters are correlated. In pentobarbitone anaesthetized or decerebrate cats, intracellular recordings were obtained from hindlimb skeleto-motoneurons. Various hindlimb muscle nerves were prepared for electrical stimulation to elicit recurrent inhibitory potentials, with dorsal roots cut. Test stimulus patterns consisted of repetitive pulse trains whose rates varied, at modulation frequencies between 0.1 and 1.0 Hz, in one of two waveforms: triangular or sinusoidal. Modulation depths were either "full", with rates varying between a minimum of less than 10 and a maximum of around 50 pulses per s. Or they were about "half" this depth, with mean rates shifted into a "low", "medium" or "high" rate region. Recurrent inhibitory potentials were averaged with respect to stimuli occurring during different phases of the stimulation cycle. Most often when, throughout the cycle, the amplitude changed in a consistent way, so did the slopes of the inhibitory potentials. That is, when the amplitudes rhythmically declined with increasing and recovered with decreasing stimulus rate, the rate of hyperpolarization followed the same pattern. With prominent hysteresis in amplitude, a corresponding hysteresis appeared in slopes. Hence, amplitude and slopes were correlated, occasionally showing a hysteresis among themselves. To a certain extent, these results can be explained by Renshaw cell behaviour, the contribution of the Renshaw cell-motoneuron synapse being unknown and difficult to assess experimentally. For the inhibitory effect of Renshaw cells on motoneurons (and reciprocal Ia inhibitory interneurons), both its magnitude and its time course probably play an important role in determining the efficacy of counteracting local excitatory inputs. The change in slope of inhibitory potentials, and likely its underlying conductance, during cyclic motoneuron activation can be presumed to significantly contribute to the temporal pattern of discharge of motoneurons, in particular in relation to the prevention of synchronization leading to enhanced tremor.

Early and late components in cat Renshaw cell responses to random stimulation of motor axons: their differential sensitivity to preceding activation

Lumbosacral Renshaw cells were activated by random stimulation of motor axons in muscle nerves or ventral roots. The stimulus patterns had mean rates of 9.5-13 or 20-23 pulses per second. The Renshaw cell responses were evaluated by two kinds of peristimulus-time histograms. "Conventional" peristimulus-time histograms were calculated by averaging the cell discharge with respect to all the stimuli in a train. "Conditional" peristimulus-time histograms were determined by averaging the cell discharge with respect to the second ("test") stimulus in pairs of stimuli which were separated by varied intervals. The effects of the conditioning stimuli were evaluated after correcting for the effect of linear superposition of the conditioning and test stimuli. The conventional peristimulus-time histograms showed an excitatory response which often consisted of two distinct components: a narrow and high "early" peak and a broad and low "late" elevation of firing probability. The early and late excitatory components were conditioned in different ways. Whereas the late component was virtually always depressed, the early component showed three patterns: (1) uniform depression; (2) uniform facilitation; (3) a mixture of depression and facilitation. Frequency responses (coherence and gain estimates) were also calculated separately for the cell discharges underlying either the early or the late components. The estimates for the "late spikes" showed a stronger decline with increasing frequency than those for the "early spikes". The origin of the different conditioning effects probably lies in a combination of pre and postsynaptic factors. They may play a role in tremor mechanisms.

Distribution of recurrent inhibition within a motor nucleus. I. Contribution from slow and fast motor units to the excitation of Renshaw cells

The relation between the size of a monosynaptic reflex (MSR) to triceps surae and the resulting Renshaw cell discharge was used to evaluate the contribution from slow and fast motor units to the excitation of Renshaw cells. It is, however, difficult to interpret these results in terms of excitation contributed by slow and fast motor units because of the following reasons. First, the size of the MSR recorded in ventral roots is not linearly related to the number of recruited motor units, since larger motor axons contribute more to the size of the MSR than smaller ones. Second, the number of spikes evoked in a Renshaw cell burst is not linearly related to the excitatory input because Renshaw cell discharge saturates in the case of large responses. The contribution of small, early-recruited motoneurones to Renshaw cell excitation is consequently overestimated. Procedures were introduced to deal with these problems. It is concluded that the last-recruited motor units (probably 'fast twitch, fast fatiguing') on average contribute four times as much excitation to Renshaw cells as the first recruited ('slow twitch') motor units.

Frequency response of spinal Renshaw cells activated by stochastic motor axon stimulation

In anaesthetized or decerebrate cats, motor axons in lumbosacral ventral roots or hindlimb muscle nerves were stimulated with random trains of brief electrical pulses, and Renshaw cell spike sequences were recorded. Spectral analysis was used to determine the range of linear operation of Renshaw cells, via coherence computations, and to calculate their frequency-dependent gains and phases. The analysis showed that the dynamic behaviour of Renshaw cells was different for different strengths of their synaptic input from motor axons and for different mean stimulus rates. In general, the changes in dynamics associated with variation of these two input parameters followed a common trend. This can be related to the average response of Renshaw cells per stimulus, as assessed by peri-stimulus time histograms. For axons having a strong excitatory effect on a Renshaw cell (as judged from the size of early peri-stimulus time histogram peaks), and for low mean stimulus rates (10-23 pulses per second), the linear range of signal transmission (assessed by coherence computation) was usually very broad (from zero sometimes up to over 100 Hz, but mostly up to 50-100 Hz). Following an initial elevation in the range 2-15 Hz, the gain showed first a rapid decrease with frequency, down to a value which at 30-50 Hz could be a tenth of the gain at lower frequencies (2-15 Hz); it then continued to decline slowly. Otherwise the linear range was narrower and/or the coherence was generally lower; the gain was lower and showed little decline with frequency. The phase curves of Renshaw cells generally showed a low-frequency phase lead (up to roughly 10 Hz) and an increasing phase lag thereabove that was generated in part by the conduction delay. The results show that Renshaw cells can follow, particularly sensitively, inputs in a frequency range encompassing the steady firing rates of many alpha-motoneurons. This range of high gain also covers that of a component of physiological tremor (ca. 6-12 Hz), a basic mechanism of which is probably related to unfused contractions of newly recruited motor units firing in this range. It can therefore be expected that recurrent inhibition via Renshaw cells is especially powerful in this physiologically important range of alpha-motoneuron firing.

Inhibitory connections of ipsilateral semicircular canal afferents onto Renshaw cells in the lumbar spinal cord of the cat

1. In intercollicularly decerebrate cats, the excitability of lumbar spinal Renshaw cells (tested by single shocks to ventral roots and deafferented muscle nerves) decreased for 600-1000 ms after conditioning electrical stimulation of ipsilateral semicircular canal nerves. 2. Conditioning stimulation of posterior canal afferents and combined stimulation of anterior and lateral canal afferents were equally effective in causing inhibition of Renshaw cells. No significant differences were observed for Renshaw cells excitable from hind-limb flexor or extensor nerves. 3. Inhibition appeared when one to five stimuli were applied to the canal afferents and arrived at the spinal segmental level 11-15 ms after the onset of conditioning stimulation. 4. Evidence is adduced to suggest that the inhibitory effects on Renshaw cells following stimulation of semicircular canal afferents were mediated directly, i.e. they were not caused by alterations of motoneurone activity. 5. Excitation of Renshaw cells due to stimulation of the canal afferents was rarely observed; it could not be excluded that it was secondary to motoneurone discharges. 6. It is suggested that vestibular inhibition of Renshaw cells ensure a high gain of hind-limb alpha-motoneurones during postural adjustments following a massive disturbance of body equilibrium.

Recurrent inhibitory connexions among neck motoneurones in the cat

1. Intracellular recordings were made from motoneurones innervating neck muscles in the cat. Dorsal roots were cut and muscle nerves electrically stimulated to activate alpha motor axons. 2. Recurrent inhibitory post-synaptic potentials (i.p.s.p.s) evoked by antidromic volleys in homonymous or heteronymous nerves were found in the majority of motoneurones studied, including those to dorsal neck muscles (biventer cervicis, splenius and complexus) as well as to occipitoscapularis and levator scapulae ventralis. 3. Central latencies of the recurrent i.p.s.p.s indicate disynaptic transmission. Amplitudes ranged from 100 microV (criterion level) to 2.2 mV. Average amplitudes were less than 0.6 mV. 4. The recurrent i.p.s.p.s were distributed to non-synergistic as well as to synergistic motoneurones. Analysis of relative strength of recurrent inhibition indicates influence of proximity of motoneurone pools, functional relatedness of muscles, as well as other factors. Variation in intrinsic motoneuronal properties probably underlies positive correlations (independent of variation in resting potential) between recurrent i.p.s.p.s evoked from different sources in motoneurones of a single pool. 5. Recordings (mainly extracellular) were also made from interneurones (Renshaw cells), located in the C3 and C4 segments of the spinal cord, that were excited by antidromic volleys in muscle nerves. The response varied from a single action potential to a burst of up to nineteen action potentials. Central latencies to the first response indicate monosynaptic transmission. Many Renshaw cells were excited by antidromic volleys in several muscle nerves, though this was restricted to nerves of the same segmental level as the Renshaw cell. All the muscle nerves studied were effective in activating Renshaw cells. 6. The results indicate that in many ways the recurrent i.p.s.p.s and the responses of Renshaw cells recorded in the neck segments resemble those in the hind-limb segments. Thus, the basic organization of recurrent inhibition in the neck segments resembles that occurring elsewhere in the spinal cord. A difference is the tendency for recurrent i.p.s.p.s in neck motoneurones to be relatively small in amplitude and Renshaw cell responses to be less strong than those recorded in the hind-limb segments. It is suggested that this is related to the segmentation of neck muscles and their motoneurone pools. 7. It is concluded that recurrent inhibition is a prominent feature of spinal organization governing neck muscles. It can therefore be expected to participate in control of head movements.

Changes in activity of motor units preceding a rapid voluntary movement

Motor unit activity was studied during slightly sustained contraction of the erector spinal muscle before a rapid trunk extension from a moderately bent position, with special reference to a change of the surface EMG to a rhythmic slower wave with a periodicity of 20-30 msec preceding the movement. Synchronous discharge of a few motor units appeared at regular intervals of 20-30 msec during the slower wave. EMG peaks in the slower wave corresponded to synchronous discharges at the regular intervals.

Spinal inhibition in man: depression of the soleus H reflex by stimulation of the nerve to the antagonist muscle

A period of inhibition of the soleus H reflex, called D1 inhibition, follows stimulation of the nerve to the antagonist muscles, the peroneal nerve, in normal man. The results confirm earlier descriptions of D1 and show an intracord latency of approximately 4-5 msec and a reduction of the H reflex by about 60%. D1 reaches its maximum 15-20 msec after stimulation and its duration is more than 30 msec. Effective stimuli were volleys of two to five pulses given as 10 msec duration bursts at a strength of 1.2 motor thresholds. Cutaneous afferents were ineffective and the possibility that D1 is mediated by low threshold muscle afferents is discussed. D1 produces the same proportions of inhibition of the H reflex when the H reflex is facilitated by plantarflexion or depressed by dorsiflexion.

Recurrent actions on gamma-motoneurones mediated via large and small ventral root fibres in the cat

Effects on single lumbar gamma-motoneurones, mediated via fibres running in the ventral roots, were studied by micro-electrode recording in cats anaesthetized with chloralose. Graded electrical stimulation of ventral roots or of peripheral nerves was used. The cells were identified as gamma-motoneurones by antidromic stimulation and by measurement of their axonal conduction velocity. Some of the cells were classified as static or dynamic. The findings confirm the previously demonstrated existence of low-threshold, presumed recurrent, inhibition of both static and dynamic gamma-motoneurones. Strong evidence for the occurrence of high-threshold recurrent inhibition of gamma-motoneurones is also presented. In addition, excitatory effects on gamma-cells, also mediated via fibres in the ventral roots, are described. The low-threshold effects from ventral root fibres are attributed to recurrent alpha-collateral activity and the high-threshold effects to gamma-collateral activity. The significance of recurrent inhibition of gamma-motoneurones is discussed in relation to the 'gain regulator' concept proposed by Hultborn, Lindström & Wigström (1979).

Dynamic properties of Renshaw cells: equivalence of responses to step changes in recruitment and discharge frequency of motor axons

In decerebrate cats, the dynamic responses of Renshaw cells to step changes in input were determined separately both for changes in the number of alpha-axons excited and for changes in the frequency at which they were stimulated. Together, these two input variables to the Renshaw cells describe the level of activity in the motor output from the spinal cord. In either case, the dynamic responses of the interneurons depend only on their static activity before and after an input step occurs, but are otherwise indistinguishable. This favors the interpretation that the two input variables are equivalent under dynamic conditions, i.e., Renshaw cells respond to total motor output.

Topical anesthesia: H-reflex recovery changes by desensitization of the skin

The H-reflex recovery curve of the soleus muscle was studied in 53 normal subjects before and after the application of topical anesthesia (20% benzocaine) to various skin areas and dermatomes of the lower limb. Skin areas were those overlying the calf, tibialis anterior, quadriceps and hamstrings muscles, and the dermatomes were L2, L3, L4, L5, S1, S2. In 42 subjects the H-reflex initial recovery decreased by 10-80 msec with a mean of 20 +/- 17 msec 30 min after application of topical anesthesia. Also, the test reflex (the second sequential H-reflex) had a faster recovery with less inhibition than that before anesthesia. It was also noticed that the longer the inhibition period before anesthesia, the greater the decrease in recovery time. These observations had no correlation with the skin area or dermatome desensitized by the anesthesia. furthermore, these observations were not noted when a placebo was sprayed on the skin. The results from this study indicate that input from cutaneous afferents may have an important inhibitory effect on the alpha-motoneurons of the soleus muscle. This effect is possibly more profound on the large phasic motoneurons. These results are discussed in relation to clinical interpretations of the H-reflex recovery curves.

Selective effects of (-)-baclofen on spinal synaptic transmission in the cat

When ejected microelectrophoretically near spinal interneurones of cats anaesthetised with pentobarbitone and under conditions where postsynaptic excitability was maintained artificially at a constant level, (-), but not (+), -baclofen selectively reduced monosynaptic excitation by impulses in low threshold muscle (Ia and Ib) and cutaneous (Aalpha) afferents. Polysynaptic excitation of interneurones and Renshaw cells by impulses in higher threshold afferents was less affected, and baclofen had little or no effect on the cholinergic monosynaptic excitation of Renshaw cells. Glycinergic and gabergic inhibitions of spinal neurones were relatively insensitive to baclofen. These stereospecific actions of baclofen, produced by either a reduction in the release of excitatory transmitter or postsynaptic antagonism, suggest that Ia, Ib, and Aalpha afferents may release the same excitatory transmitter which differs from that of spinal excitatory interneurones. Microelectrophoretic (-), but not (+), -baclofen also reduced primary afferent depolarization of ventral horn Ia extensor afferent terminations produced by impulses in low threshold flexor afferents, without altering either the electrical excitability of the terminations or their depolarization by electrophoretic GABA or L-glutamate. This stereospecific action of baclofen is interpreted as a reduction in the release of GABA at depolarizing axo-axonic synapses on Ia terminals.

Changes in recurrent inhibition during voluntary soleus contractions in man studied by an H-reflex technique

1. The recurrent inhibition, brought about by a conditioning H-reflex discharge, was estimated in human subjects by the amplitude of a test H-reflex involving only the soleus motoneurones which fired in response to the conditioning volley. The modifications of the recurrent inhibition during contraction were evaluated by comparing the amplitude of the test H-reflex to a reference H-reflex. Both reflexes experienced the excitation underlying the voluntary contraction, but only the test H-reflex was subjected to the recurrent inhibition evoked by the conditioning H-reflex discharge. 2. Distinct differences were observed between the modifications of the test reflex and those of the reference H-reflex during both tonic and phasic voluntary contractions. Evidence is presented that these differences were due to changes in the amount of recurrent inhibition elicited by the conditioning discharge. 3. The changes in recurrent inhibition were studied while the subjects performed voluntary tonic contractions of various forces. The weakest contractions were accompanied by a decrease in the size of the test reflex. With greater contraction forces, there was no longer an inhibition of the test reflex, but instead a facilitation which grew continuously with increased contraction forces. The test reflex could eventually exceed the reference H-reflex amplitude at the strongest contractions. This is taken to indicate that the recurrent inhibition following the conditioning discharge was progressively decreasing, as the contraction force increased. 4. During ramp contractions, whatever the contraction velocity, the time courses of the variations of the test and reference H reflexes were almost inverse. Evidence is presented that these differential time courses were due to changes in the amount of recurrent inhibition elicited by the conditioning discharge. 5. The possibility of occlusion in the recurrent pathway ;as considered and it was concluded that the decrease in the recurrent inhibition elicited by the conditioning discharge was essentially due to an inhibitory control (spinal and/or suprasegmental) acting on Renshaw cells. This inhibition of Renshaw cells eventually counteracts the increasing excitatory inputs (resulting from the voluntary motor discharge) which they receive via motor axon collaterals during increasing tonic contractions and throughout ramp contractions. 6. The functional significance of the Renshaw cell inhibition during voluntary contraction is discussed in relation to the inhibition exerted through the recurrent pathway on both motoneurones and Ia inhibitory interneurones. It is suggested that the depression of Renshaw cell activity could play an important role during voluntary movements by favouring reciprocal Ia inhibition.

Effect of oxotremorine on the response of antidromically activated Renshaw cells in decerebrate cats

In intercollicular decerebrate cats, some of which were made spinal in addition, the effect of oxotremorine (intravenous injection, 10-30 mug/kg) was tested on antidromically activated Renshaw cells. Methylatropine premedication prevented the otherwise often fatal drop in blood pressure; ipsilateral dorsal roots L6-S1 and contralateral hindlimb nerves were cut to exclude segmental receptor interference. During supramaximal stimulation of ventral root L7 or the gastrocnemius nerves, an increase of activity ranging from 10-110% was observed. The drug occasionally evoked spontaneous discharges in Renshaw cells, or enhanced activity already present. Alpha motoneuron activity decreased in most cases. The interaction of oxotremorine with atropine and eserine was also investigated on Renshaw cells. Our results suggest that one of the effects of oxotremorine may be a disbalance between motor output and recurrent inhibition.

Reflex responses of gamma motoneurones to vibration of the muscle they innervate

1. High frequency vibration was applied to the tendon of the non-contracting triceps surae muscle while recording the background discharges of single gamma fibres only small nerve bundles were cut, leaving most of the nerve supply to the triceps intact. 2. 22% out of a total of sixty-three gamma efferents were tonically inhibited by vibration. The inhibition appeared between 25 and 50mum peak-to-peak amplitude of vibration and increased to a plateau for amplitudes of about 100mum. The dependence of the tonic vibration reflex of alpha-efferents on the amplitude of vibration was found to be similar. Increasing the frequency of vibration from 150 to 300 Hz increased the degree of inhibition. 3. 33% of the fusimotor neurones investigated responded to muscle vibration with an increase in discharge rate. The threshold amplitudes of this reflex ranged from 20 to 50mum. Some features of the reflex, in particular the parallel post-vibratory facilitation found in alpha and gamma efferents, pointed to a polysynaptic pathway organized in an alpha-gamma linkage. 4. All gamma efferents inhibited by vibration showed inhibitory responses to antidromic stimulation of the parent ventral root, and most of them were inhibited by ramp stretch of the triceps. The gamma motoneurones facilitated by vibration, however, were excited by muscle stretch and were less susceptible to antidromic inhibition, some lacking it completely. 5. Cutting the nerves to triceps abolished the inhibitory as well as the excitatory responses of gamma efferents to muscle vibration. Both fusimotor reflexes were preserved after spinal section and subsequent administration of L-DOPA. 6. It is concluded that both of the fusimotor reflex effects of vibration are caused by excitation of primary spindle endings within the triceps. The inhibition of fusimotor neurones is thought to be mediated by Renshaw cells activated during vibration. The significance of positive feed-back on to gamma motoneurones as a result of autogenetic facilitation by Ia afferents is discussed in connexion with stability in the stretch reflex loop.