Journal of Physiology (1990), 431, pp. 365-377 With 5 figures Printed in Great Britain

365

CONVERGENCE OF Ia FIBRES FROM SYNERGISTIC AND ANTAGONISTIC MUSCLES ONTO INTERNEURONES INHIBITORY TO SOLEUS IN HUMANS BY MARCO SCHIEPPATI, CARLO ROMANO AND IVANA GRITTI From the Istituto di Fisiologia Umana II, Universita degli Studi di Milano, Via Mangiagalli 32, I-20133 Milano, Italy

(Received 9 March 1990) SUMMARY

1. The possibility that Ia afferent fibres from the gastrocnemius medialis (GM) and from the tibialis anterior (TA) muscle could converge on to a single interneuronal pool inhibitory to the soleus motoneurones was investigated. 2. The soleus H reflex, evoked by tibial nerve stimulation in the popliteal fossa, was conditioned by separate or combined stimulation of the nerves to the GM or TA muscles. Stimulus intensity was below the motor threshold (MTh), and the conditioning-test intervals were such as to evoke short-latency inhibition of the soleus H reflex. Care was taken to avoid current spread and artifacts connected with the closeness in time and space of the conditioning and test stimuli. 3. Separate stimulation of both GM and TA nerves was able to induce significant inhibitory effects on the H reflex amplitude at stimulus strengths larger than 0-75 x MTh, on the average. Combined stimulation of the two nerves was able to reduce the H reflex at lower stimulus strengths, at which either nerve was ineffective alone. 4. Conditioning stimulus strengths close to the MTh reduced the H reflex to approximately 80 % of the control value, both on single and combined stimulation, i.e. saturation of the inhibitory effect was found. 5. By extrapolating the regression line through the normalized data from all subjects, it was assumed that the smallest stimulus strength necessary to drive the inhibitory interneurones to threshold was, on the average, 0.5 and 0-6 x MTh, on combined and separate nerve stimulation, respectively. 6. Tonic voluntary activation of the soleus abolished the inhibitory effects of both separate and combined stimulations. This was tested on the H reflex, on the rectified and averaged EMG, and on the peristimulus histogram of single motor unit

discharge. 7. The findings strongly suggest the existence of spatial summation of the effects from GM and TA muscle at the level of a single interneuronal pool. Most probably, the responsible afferent fibres are group I spindle afferents, and the interneurones those mediating the reciprocal inhibition. The data do not support the notion of parallel pathways, exclusive to each nerve. MS 8337

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M. SCHIEPPATI, C. ROMANO AND I. GRITTI INTRODUCTION

Conditioning stimulation of the nerve to the gastrocnemius medialis (GM) muscle results in a short-latency and short-lasting period of inhibition of the soleus H reflex (Bouaziz, Bouaziz & Hugon, 1971). The same stimulation can also produce a decrease in the amplitude of the tonic EMG (Bouaziz, Bouaziz & Hugon, 1975) or in the discharge of single motor units (Mao, Ashby, Wang & McCrea, 1984). These effects were attributed to the combined action of Ib fibres from the tendon organs, and to Renshaw recurrent inhibition. Soleus motoneurones are also inhibited by foot dorsiflexion or triceps stretch (Mark, Coquery & Paillard, 1968; Gerilovsky, Gydikov & Radicheva, 1977; Gottlieb & Agarwal, 1978; Burke, Gandevia & McKeon, 1983; Romano & Schieppati, 1987; see Schieppati, 1987). This has been explained by, among other things, the action of group II fibres (Robinson, McComas & Belanger, 1982) or by peripheral mechanical effects (Gerilovsky, Tsvetinov & Trenkova, 1989). We have recently provided evidence in favour of the hypothesis that the inhibition of the soleus H reflex induced by stimulation of the nerve to the GM muscle is mediated, at least in part, by activation of GM spindle primary afferent fibres (Gritti & Schieppati, 1989). This conclusion was based on the following findings: (1) stimulus strength well below the motor threshold was sufficient for inducing the effect, (2) the effect had a very short latency, (3) long-duration vibration of the triceps abolished the inhibition, owing to the increase in the threshold to electrical stimulation of the GM Ia fibres (Coppin, Jack & MacLennan, 1970; Hayward, Nielsen, Heckman & Hutton, 1986), and (4) slight selective contraction of the GM was able to reproduce soleus H reflex inhibition. The above hypothesis would imply the possibility that the two muscles are not necessarily synergistic under all conditions, but can be functionally antagonistic. This view is supported by the different insertions of soleus (monoarticular and plantarflexor) and GM (biarticular, plantarflexor and leg flexor), and by the difference in their relative contribution to posture maintenance and correction (Nardone, Corra & Schieppati, 1990), and to various types of movement (Duchateau, Le Bozec & Hainaut, 1986; Nardone & Schieppati, 1988a, b). In this connection, it is interesting to note that voluntary co-contraction of tibialis anterior (TA) muscle and GM (but not soleus) is easily achieved by all subjects asked to do so, the leg becoming flexed at the knee and the foot dorsiflexed. Therefore, there being no doubt about the existence of a Ia-mediated disynaptic inhibition from the antagonistic TA muscle onto the soleus (see Crone, Hultborn, Jespersen & Nielsen, 1987), we investigated the possibility that both GM and TA Ia fibres converge onto the same inhibitory interneurones. The existence of such a neural circuit in humans would be of some interest, in the light of other differences in the spinal reflex pathways between carnivores and primates, already postulated (Hongo, Lundberg, Phillips & Thompson, 1984). Preliminary data have been briefly reported (Schieppati, Romano & Gritti, 1989). METHODS

Twelve subjects between 21 and 41 years of age participated in this study, which was approved by the local Ethical Committee. All gave informed consent to the experimental procedure. They

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were seated on an examination chair, with the knee and ankle joint angles at 110 and 90 deg, respectively, and the trunk and head in upright position. Surface electrodes, set 2 cm apart, recorded the electromyographic activity (EMG) of the soleus muscle (posteriorly on the lower third of the calf), gastrocnemius medialis (GM) and lateralis (GL) (on the upper third of the calf in a posteromedial and posterolateral position) and tibialis anterior (TA) (anterolaterally).

Stimulation The H reflex of the soleus muscle was evoked through stimulation of the posterior tibial nerve by electrodes placed in the popliteal fossa (cathode) and on the knee. Care was taken to avoid spread of current to the GM, GL or peroneal nerves (1) by using low stimulus strength (soleus H reflex never exceeded 10-20 % of the maximum motor response) and duration (0 5 ms) (Panizza, Nilsson & Hallett, 1989), (2) by inspecting any direct mechanical or EMG activity (M response) in the GM, GL or TA muscle, and (3) by checking the absence of an H reflex in the pretibial muscles at rest and during tonic or phasic voluntary dorsiflexion of the foot. An H reflex in the GM muscle was present in some experiments. Stimulation of the GM nerve was performed by means of a bipolar electrode (distance between leads 2 cm), placed on the course of the nerve identified as the line joining the points from which a direct motor response in the muscle was obtained by minimum current strength. The electrode was spaced 7-12 cm (mean 10-6) distally from the electrode in the popliteal fossa. As a test that nerve stimulation affected Ia fibres, the appearance of an H reflex in the GM on voluntary tonic or phasic contraction of the gastrocnemii was carefully inspected. Care was taken to avoid spread of current to the posterior tibial nerve by inspecting any appearance of the M response in the soleus muscle, and by checking the absence of the soleus H reflex on tonic or phasic plantarflexion of the foot. Stimulation to the GM nerve was delivered at a conditioning-test interval of -0-6 ms (the soleus stimulus preceded the GM stimulus). This interval takes into account the higher conduction velocity of the GM Ia fibres; however, it is not the most effective for inducing the largest possible inhibition. The advantage is that it avoids superimposition of the two stimuli, a condition that might lead to artifacts connected with closeness in time of the conditioning and test stimuli (Gritti & Schieppati, 1989). This was verified in four subjects by observing the total absence of effects on a 'conditioned' M response in the soleus muscle, purposely evoked on augmenting the test stimulus strength. The intensity of the stimulus (duration 0-5 ms) ranged usually between 0-5 and 1 times the motor threshold ( x MTh). For the stimulation of the peroneal nerve, another bipolar electrode was positioned at the head of the fibula, and moved some short distance in order to identify a point where the threshold for the M response in the TA muscle was lower than in the peroneal muscles, as judged by tactile examination of the stress of the TA tendon. To minimize and check current spread, procedures similar to those used for GM nerve were followed. The conditioning-test interval was 2-5 ms. Stimulus strength (duration 0-5 ms) ranged between 0-5 and 1 x MTh. The same GM and TA conditioning stimulations were delivered in three subjects during slight voluntary tonic contraction of the soleus muscle (10-20 % of the maximum voluntary contraction, MVC), the test H stimulus to the posterior tibial nerve being of such a strength as to yield H reflexes of an amplitude comparable to that obtained at rest. The subjects were provided with both the signals of force and of rectified and integrated gastrocnemius medialis EMG (time constant 100 ms) on an oscilloscope screen, in order to obtain tonic contractions of constant amplitude. The subjects relaxed their muscles for a short period every 30-60 s to avoid muscle fatigue. In two subjects, effects of the stimulations were investigated in the rectified (non-integrated) and averaged EMG of the soleus muscle. In two other subjects, peristimulus time histograms (PSTH, bin width 0-5 ms) were computed from the discharge of single, tonically activated soleus motor units and recorded by means of fine wires inserted into the muscle (Nardone, Romano & Schieppati, 1989), that were conditioned by the same stimulations as above. Stimulus frequency was 2 Hz, 100 and 2000 periods were computed, for the rectified EMG and the PSTH, respectively; each period lasted 100 ms. Stimulation of regions of the leg surface spaced some 5 cm away from the TA and GM nerve stimulation points was also performed, in order to identify any effect on the soleus H reflex connected with the activation of cutaneous fibres. The intensity used was the same that elicited a threshold M response when applied to the muscle nerve. This intensity was then expressed in multiples of the perception threshold for each subject.

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Procedure The conditioning stimuli were delivered both in isolation and simultaneously. At the beginning, each conditioning stimulus was delivered separately at its maximum strength. The intensities were then diminished to the maximum value that evoked no effect on the H reflex. We assumed that at this intensity the inhibitory interneurones, possible site of convergence from the two inputs, were facilitated subthreshold by each separate input: consequently the soleus motor pool would not be affected. Then the two stimuli were delivered together: under this condition, inhibition of the soleus H reflex was interpreted as a sign of spatial summation of converging effects from the two muscles onto the same interneurones. Stimulus strength was then reduced further, up to a value at which no inhibition of soleus H reflex was seen even on combined stimulation. In eight cases the intensities (x MTh) at which the minimum significant inhibition was obtained on combined stimulation were equal for both nerves, in the remaining four the two intensities were different by 005 or 0-1 x MTh. In each experimental run, soleus H reflexes were evoked at a fixed frequency (ranging in different sessions from 0-1 to 0-15 Hz) throughout the session. The conditioning stimulation was presented in a pseudorandom manner, and twenty to thirty unconditioned and conditioned H reflexes were collected for statistical analysis under each condition. The differences between the peak-to-peak amplitudes of conditioned and control reflexes were evaluated by means of the Student's t test (P < 005). Conditioning of soleus on-going activity was performed using the same protocol, the stimulus intensities ranging between 0-8 and 1 x MTh, the delay between the conditioning stimulations of the GM and TA nerves being 3 ms (as when the H reflex was used). RESULTS

Resting conditions Data were obtained from twelve subjects, some of them tested more than once on different days for reliability of results. Separate stimulation of GM and TA nerves elicited a statistically significant inhibition of the soleus H reflex in only ten cases, even at the maximum value used. However, in all the subjects the combined stimulation of both nerves, at this and at lesser intensities, evoked a significant inhibition. Figure 1 shows the findings obtained in one subject by systematically changing the intensity of both conditioning stimuli. At 08 x MTh and below, no significant inhibition was evident on the separate stimulations. However, when both stimuli were delivered together, the inhibitory effect was present. This effect became insignificant at stimulus strengths below 07 x MTh. At the maximum stimulus strengths used (1 x MTh), the inhibition ofthe soleus H reflex on combined stimulation was largest albeit not equal to the sum of the two separate effects. Figure 2 shows the average values obtained from all subjects, including those in which separate stimulation of the nerves induced no significant inhibition. In order to show the effects of the convergence, we have chosen to assemble the data from the various subjects on the basis of the threshold intensity for the effect, rather than on the basis of the stimulus strength ( x MTh), because equal intensities evoked different effects from subject to subject. This is not unexpected since the effect of the stimulating current might be influenced by the distribution of the motor and sensory fibres within the nerves, and by possible differences in the actual excitability of the relevant interneurones. In the graph of Fig. 2 only three stimulus intensities are reported for simplicity: (1) that at which the effect of convergence just induced a significant decrease of the test H reflex; at this value separate stimulations were without effect in all subjects (centre histograms), (2) the largest possible intensity at which no significant inhibitory effects were induced even on combined stimulation

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370

(left histograms), and (3) that at which the maximum inhibition was observed on separate, and also on combined stimulation (right histograms). The values reported in the abscissa are the grand averages of the intensities at which the above conditions were fulfilled in each subject. When the two stimuli were delivered together at the 10-

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maximum intensity, the levels of inhibition induced by each separate stimulus were not significantly different from each other, or from the value obtained on combined stimulation (analysis of variance, P > 0 05). Figure 3 is the result of an attempt at identifying the stimulus intensities at which the inhibitory interneurones, site of the converging effects, were brought to threshold, on the assumption that the stimulus strength eliciting a statistically significant inhibition of the H reflex was necessarily larger than those just sufficient to discharge the interneurones (due to the t test procedure, the discrepancy between the two intensities would be larger, the smaller the number of repetitions on which the mean values are computed). The minimum intensity at which, in every subject, a significant inhibition of the H reflex was induced on combined TA and GM stimulation is plotted in correspondence to 100 % on the abscissa. This value is made equal to the 'threshold for effect' for each subject: it corresponds, on the average, to an inhibition of 9.3 + 0-7 % (mean + s.E.M.) and to an intensity of 0-75 x MTh. The other values found on the abscissa are the other stimulus strengths used, expressed for each subject as a percentage of the above defined 'threshold for effect' (both

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ms Fig. 4. Effect of voluntary soleus contraction. A, absence of significant inhibition of soleus H reflex during slight voluntary soleus contraction, on either single or combined stimulation (grand means+pooled S.D. from three subjects). The same conditioning stimulus intensity (0-9 x MTh) was effective at rest in all conditioning sets. B, soleus rectified and averaged EMG. C, soleus motor unit activity reported as a PSTH. The upper trace (bin width 1 ms) was obtained by stimulating the posterior tibial (TP) nerve at an ms

371

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M. SCHIEPPATI, C. ROMANO AND I. GRITTI

conditioning stimuli were changed together by the same fraction). The values on the ordinate are the corresponding inhibitions observed. The regression line fitted to the data (y = 16-73 - 025x, R2 = 0-5) meets the line y = 0 (no effect) at 67 % on the abscissa. (Strictly speaking the regression lines should be the extrapolation of the linear part of a sigmoidal function, since (1) at low stimulus strengths no effects whatsoever must be induced, given the existence of a threshold in the interneurones, and (2) at high stimulus strength the effect shows a plateau.) From this representation of the population data, the inference is made that, on average, the intensity just sufficient to bring to threshold the presumed common interneurones on arrival of the convergent I a fibre volleys from the two nerves, would be 0 50 x MTh (i.e. 0-67 x 0 75). A similar procedure has been followed for inferring the values of stimulus intensity ( x MTh) sufficient to drive to threshold the interneurones when stimuli are applied separately to TA or GM nerve. The two corresponding regression lines were strikingly similar, and similar in turn to that obtained by using the data of the convergence (TA, y = 17-01 -0 26x, R2 = 0-41; GM, y = 17-09-0725x, R2 = 0 47). In these cases, the 100% values, i.e. the intensity at which the stimulation of either nerve induced a just significant inhibitory effect was on the average 0-87 and 0-89 x MTh for GM and TA respectively. The corresponding intensities necessary to bring to threshold the interneurones were then, on the average, 0-60 and 0-58 x MTh, i.e. about 0.1 x MTh larger than on combined stimulation.

Voluntary innervation During a slight isometric contraction limited to the soleus muscle, no inhibitions of the H reflex were obvious in our hands, on conditioning stimulation of either or both nerves between 0-8 and 1 x MTh. Figure 4A compares the inhibitions obtained on combined stimulation at the same stimulus strength on the H reflex at rest and on an H reflex of the same amplitude evoked during voluntary activation of the soleus (average of the data from three subjects). Figure 4B and C shows that no inhibitions were detectable on inspection of the rectified and averaged soleus EMG (B) or on the PSTH of a single unit of the soleus muscle (C). It should be noted that in the two subjects from which the data in B and C of Fig. 4 were obtained, the H reflex at rest was inhibited to 75-7 and 86-7 % by the combined stimulation at the same intensity used in the case of the averaged EMG and PSTH. As a test of the reliability of the methods, the uppermost traces of B and C show the effects induced by posterior tibial nerve stimulation at an intensity equal to 0'8 times that necessary to induce a threshold H reflex at rest. Skin stimulation The cutaneous stimulation was without effect. Figure 5 shows the average values of the H reflex of three subjects, in which the intensity of the two conditioning stimuli was, on the average, 1-33 + 0-35 (mean + s.E.M.) times the threshold for perception (see Methods). It should be noted that the effects were absent on (1) separate intensity below that necessary to induce an H reflex at rest. Second and third traces show the effects of separate nerve stimulation (TA, common peroneal nerve; GM, gastrocnemius medialis nerve); in the lowermost trace the effects of combined stimulation are reported (Conv). Conditioning stimulus strength was 09 x MTh and bin width 05 ms.

CONIVERGENCE ON INHIBITOR Y INWTERNEURONES

373

stimulation of the skin overlying either TA or GM, on (2) combined stimulation of these two points, and on (3) any combinations of 'skin' and muscle nerve (i.e. skin overlying TA plus nerve to GM muscle, and the opposite). DISCUSSION

In this investigation we have reproduced, in a larger group of subjects than in a prev,ious work (Gritti & Schieppati. 1989), an inhibition of the soleus H reflex on 120

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stimulation of the nerve to its synergist muscle G1M. For reasons discussed in the quoted paper and briefly recalled in the Introduction, the inhibition was attributed to the activation of I a spindle afferent fibres. It is shown here that this inhibition disappears during voluntary soleus contraction: in fact (1) the amplitude of the H reflex is not significantly decreased by the conditioning stimulation during plantarflexing effort, and (2) the effects of the conditioning stimulation are undetectable on the tonic EMG and on the discharge of single motor units. This discrepancy with the data in the literature (Bouaziz et al. 1975; Kudina & Pantseva, 1988) is due to the stimulus intensities used here, which never exceeded the threshold for a direct motor response, in this way eliminating any antidromic discharge of GM motoneurones (or even soleus, if the stimulation had spread to the soleus nerve), which might have an effect on the H reflex of the active soleus motor pool through the recurrent inhibition (Miles. Le & Tiirker, 1989). The other result of this work is the demonstration that the inhibitory effects induced by GM nerve stimulation add to those induced by stimulation of the TA nerve. The latter effect is most probably due to activation of I a fibres from TA spindles impinging on the interneurones of the reciprocal inhibition, because of (1) the loW stimulus intensity used, (2) the delay for the inhibition, (3) the extent of the

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same, and (4) its abolition by soleus voluntary contraction (see also Crone et al. 1987; Rossi & Mazzocchio, 1988). On this assumption, the interneurones mediating the inhibition from the I a fibres from the antagonist TA to the soleus muscle would be common to the same type of fibres from the synergic GM. That the same type of lowthreshold fibres are involved is supported by the fact that the strength of the stimulus sufficient for inducing an inhibition of the soleus H reflex was similar for both nerves; when both nerves were stimulated together, significant convergent inhibitions were evoked in some subjects by intensities of 06 x MTh. This value increases to 075 x MTh if the average value from all the subjects is considered: it is noticeable that this inhibition on combined stimulation was always present, while single stimulation of GM and TA nerves (in the present group) or of the TA nerve (Crone et al. 1987) was not able to induce significant inhibition in all the subjects tested. Interestingly enough, an estimate of the intensity necessary for bringing the inhibitory interneurones to threshold on combined stimulation, obtained by extrapolation of the data from all subjects, gave an approximate value as low as 0 5 x MTh, while the intensity necessary to yield the same effect when the stimulus was delivered to the single nerves was about 0-6 x MTh. Whether the interneuronal site of this convergence is composed of all the inhibitory interneurones interposed along the Ia reflex pathway, or whether more interneurones are present, activated by either nerve only, cannot be answered with certainty. But it is interesting to note that, at the highest stimulus intensities giving maximal inhibition from either conditioning stimulation, the inhibition on combined stimulation was not larger than that on separate stimulation of each nerve. This saturation effect points to a great percentage of cells of a single inhibitory interneuronal pool being facilitated by both nerves. For that matter, this effect appears to be just a particular case of a more general situation. The relationships between the intensity of stimulation and the inhibitory effect induced are indistinguishable for the two nerves. These are in turn similar to that between the combined stimulation and the convergent inhibitory effect. If the abscissa were expressed in x MTh, the 'convergence' line would be shifted to the left of those of the single nerves by approximately 0-1 x MTh, which is approximately the extra intensity that must be delivered to either nerve in order to obtain the same inhibition as that induced by the combined stimulation. The equal slopes of the three regression lines would indicate that the gain of the inhibitory effects from the two inputs is similar, and that each effect adds linearly to the other. These findings are in accord with the view that spatial summation takes place at the level of the same interneuronal pool. Support for the notion of a single interneuronal pool is given by the absence of inhibitory effects during slight voluntary innervation of the soleus: under this condition, the inhibition obtained at rest by stimulation of either or both nerves disappears. The descending command for plantarflexion would at the same time increase the excitability of the soleus motoneurones and of their associated Ia inhibitory interneurones. The latter would in turn inhibit the Ia interneurones presumably associated with the 'TA-GM' motor pool. On the other hand, contraction of GM is able to induce an inhibitory effect on the soleus H reflex (Gritti & Schieppati, 1989) similar to that induced by contraction of TA (Shindo, Harayama, Kondo &

375 CONVERGENCE ON INHIBITORY INTERNEURONES Tanaka, 1984; Crone, Hultborn & Jespersen, 1985; Iles, 1986; Iles & Roberts, 1987; Crone & Nielsen, 1989). In this case, the descending command would possibly activate the same 'TA-GM' Ia-interneuronal pool, inhibitory to the soleus. An alternative possibility would be that two separate interneuronal pools exist, one receiving GM fibres and the other receiving TA fibres, and both subjected to the same descending and segmental control. But this explanation does not comply with the convergence existing at rest, leaving as the most parsimonious interpretation a single interneuronal pool receiving both TA and GM Ia fibres. The control experiments in which the skin was stimulated did not disclose any convergence of cutaneous and muscle afferents. This is true within the experimental conditions employed, and does not exclude the existence of such a circuit, since skin afferents with a slightly lower conduction velocity would impinge on the interneurones at a time when the I a effect disappears. However, it has to be considered that (1) the intensity used corresponded to, on average, just 1-33 times the threshold for perception, and (2) it has been reported that skin afferents from areas other than the foot are without effect on the reciprocal inhibition from TA to soleus (Rossi & Mazzocchio, 1988). To our knowledge, the circuit hypothesized above has not been proposed previously in humans. In cats, the heteronymous Ia facilitation from the GM to the soleus motor pool is distributed to all motoneurones (Eccles, Eccles & Lundberg, 1957); in the baboon the affected neurones are less than half the pool (Hongo et al. 1984); in man, no heteronymous facilitation has been demonstrated (Bouaziz et al. 1975; Pierrot-Deseilligny, Morin, Bergego & Tankov, 1981; Mao et al. 1984). On the contrary, inhibition was found (Gritti & Schieppati, 1989). The present results would suggest that the interneurones mediating the heteronymous inhibition from GM to soleus are the same that mediate the reciprocal inhibition from the antagonist TA. This envisages a situation whereby the same type of afferents from two muscles commonly considered antagonistic share a common inhibitory interneuronal pool. This is not peculiar, since (1) it has been shown in the cat that several synergistic muscles evoke a Ia inhibition on the motor pool of a common antagonistic muscle through the same interneurones (Hultborn & Udo, 1972), and (2) TA and GM can be considered synergistic muscles acting in opposition to the antagonistic soleus muscle (see Introduction). This situation could result from the changes in the nervous system accompanying the evolution of the upright stance and bipedal locomotion (for a discussion of this topic see Hongo et al. 1984; Mao et al. 1984; Forssberg, 1985; Pierrot-Deseilligny, 1985; Bayoumi & Ashby, 1989). This hypothesis is certainly less likely if one considers the smallness of the inhibitory effects observed, which reached, at most, an amplitude of about 20% of the control reflex. However, if the role postulated for this inhibitory convergence was indeed to support locomotor synergies of the lower limb, it is plausible that the interneurones would be weakly activated at rest but strongly modulated during gait, very much as occurs in the cat (Feldman & Orlovsky, 1975). For example, late in the swing phase of walking the GM is stretched by the extension of the knee (Nilsson, Thorstensson & Halbertsma, 1985), while the TA shows a prominent EMG burst (Capaday & Stein, 1986): both phenomena should involve a strong activation of Ia interneurones inhibitory to the soleus motor pool, just when any plantarflexion would be inappropriate.

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M. SCHIEPPATI, C. ROMANO AND I. GRITTI

This work was supported by grants from the Italian MPI, Regione Lombardia, and CARIPLO. Dr Steven Forman scrutinized the English. REFERENCES

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