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Recurrent inhibition of firing motoneurones in man
Electroencephalograph); and clinical Neurophysiologv, 1988, 69:179-185 Elsevier Scientific Publishers Ireland, Ltd.
179
EEG 01935
Recurrent inhibition of firing motoneurones in man L.P. Kudina and R.E. Pantseva Institute for Problems of Information Transmission, U.S.S.R. Academy of Sciences, Moscow ( U.S. S. R.) (Accepted for publication: 14 June 1987)
Summary The effects of antidromic stimulation of large motoneurones on firing small motoneurones of soleus muscle have been studied. Against the background of rhythmic firing of small motor units (MUs) activated during weak voluntary muscle contraction, thick efferents of the tibialis posterior nerve were selectively stimulated and an M response was evoked in which small MUs were not involved. This provided a means of avoiding antidromic stimulation of the motoneurone under study and, thus, analysing the effect of stimulation without its summation with after-hyperpolarization. The background firing rate of MUs was 4.5-9.2/sec. PSTHs revealed a distinct inhibitory effect with a latency of 35-40 msec (slightly exceeding the latency of monosynaptic reflex) and duration 5-30 msec. It was concluded that the short-latency inhibition could be identified as recurrent inhibition. The effectiveness of recurrent inhibition evaluated by the lengthening of the interspike interval was shown to depend on the arrival time of the volley in the interval and on the background firing rate of the motoneurone. When the inhibitory volley arrived at the beginning of the interspike interval it was ineffective. This indicates that in the investigated range of firing rates the motoneurone is unable to exert an inhibitory effect on its own firing via recurrent collaterals. The inhibitory volley became highly effective at the end of an interspike interval, when the membrane potential approached threshold. The lengthening of interspike interval was more marked at a lower firing rate of the motoneurone. An increase in the background firing rate reduced the extent of recurrent inhibition (at a rate above 10/sec up to its complete ineffectiveness). Various methods for evaluating recurrent inhibition in a firing motoneurone are compared. Key words: Spinal cord; Human motoneurones; Recurrent inhibition
R e c u r r e n t i n h i b i t i o n o f m o t o n e u r o n e s has b e e n the s u b j e c t o f m a n y a n i m a l studies. In m a n this i n h i b i t i o n was first d e s c r i b e d b y V e a l e a n d cow o r k e r s ( V e a l e a n d R e e s 1973; V e a l e et al. 1973). T h e a u t h o r s assessed r e c u r r e n t i n h i b i t i o n b y a r e d u c t i o n in t h e a m p l i t u d e of a t e s t i n g m o n o s y n a p t i c H reflex of the soleus m u s c l e a f t e r a c o n d i t i o n i n g s t i m u l u s a c t i v a t i n g R e n s h a w cells, e i t h e r b y a n t i d r o m i c s t i m u l a t i o n of ' l a r g e ' m o t o n e u r o n e s (an M r e s p o n s e w a s e v o k e d ) o r b y o r t h o d r o m i c s t i m u l a t i o n via Ia a f f e r e n t s o f ' s m a l l ' m o t o n e u r o n e s (an H reflex was e v o k e d ) . I n t h e first c a s e c o n d i t i o n i n g a n d t e s t i n g s t i m u l i w e r e a d d r e s s e d to d i f f e r e n t m o t o n e u r o n e s , large a n d small, r e s p e c tively. T h u s , the e f f e c t o f c o n d i t i o n i n g s t i m u l a t i o n Correspondence to: L.P. Kudina, Institute for Problems of Information Transmission, U.S.S.R. Academy of Sciences, U1. Yermolovoy 19, GSP-4, 101 447 Moscow (U.S.S.R.).
uncomplicated by after-hyperpolarization was a n a l y s e d . I n the s e c o n d case b o t h s t i m u l i w e r e a d d r e s s e d to the s a m e p o p u l a t i o n of s m a l l m o t o n e u r o n e s . It s h o u l d be n o t e d t h a t a f t e r - h y p e r p o l a r i z a t i o n , w h i c h d e v e l o p s in a m o t o n e u r o n e f o l l o w i n g its d i s c h a r g e a n d a d d s u p to the i n h i b i tory influence of stimulation, hampers interpret a t i o n o f t h e results. R e c u r r e n t i n h i b i t i o n o f m o t o n e u r o n e s in m a n w a s d e a l t w i t h in a series o f studies by P i e r r o t - D e s e i l l i g n y et al. ( P i e r r o t - D e s e i l l i g n y a n d Bussel 1975: H u l t b o r n a n d P i e r r o t - D e s e i l l i g n y 1979: P i e r r o t - D e s e i l l i g n y a n d M a z i r r e s 1984). R e c u r r e n t i n h i b i t i o n w a s e v a l u a t e d b y the d e c r e a s e in a m p l i t u d e o f a t e s t i n g H reflex of t h a t m o t o n e u r o n e p o p u l a t i o n to w h i c h the c o n d i t i o n i n g s t i m u l u s was a d d r e s s e d . It was f o u n d t h a t d u r i n g w e a k v o l u n t a r y c o n t r a c t i o n o f the soleus m u s c l e the i n t e n s i t y o f r e c u r r e n t i n h i b i t i o n was g r e a t e r t h a n t h a t at
0013-4649/88/$03.50 c©1988 Elsevier Scientific Publishers Ireland, Ltd.
180 rest, while with increase in the strength of voluntary contraction the recurrent inhibition abated (Hultborn and Pierrot-Deseilligny 1979). Apart from the investigations of recurrent inhibition on a population of motoneurones there have been studies on single motoneurones in man under conditions of their natural activation (voluntary muscle contraction). Datta and Stephens (1979) evaluated inhibition by the lengthening of the interspike interval of a motor unit (MU) after its reflex response. However, using such methods it is impossible to determine the latency of the inhibitory effect and hence its origin, since, due to the long duration of the interspike interval (nearly 100 msec) both short-latency and longlatency inhibitory volleys induced by stimulation may arrive within the interval following the reflex response of the MU. To ascertain recurrent inhibition as one of the components of a 'silent period' after an M response, Person and Kozhina (1978a, b) employed the method of peristimulus time histograms (PSTHs) of single MUs, which provided a means for assessing the inhibition latency. The use of antidromic stimulation of large motoneurones and investigation of stimulation effects on small motoneurones enabled the authors to avoid the complicating effect of after-hyperpolarization. The authors failed to detect recurrent inhibition in hand muscles. A similar method has been used for investigating the effectiveness of recurrent inhibition on single firing MUs of the human soleus muscle in the present study. A preliminary report was published earlier (Kudina and Pantseva 1984).
Methods Using a visual feedback of M U potentials, volunteer subjects maintained steady firing of single MUs by weak voluntary isometric contraction of the soleus muscle. M U potentials were picked up by bipolar needle electrodes. The summated E M G was recorded by surface electrodes. During weak muscle contraction the lowest threshold small motoneurones (with thin axons) were recruited. Animal experiments show that recurrent inhibition is generally potent on small
L.P. KUD1NA. R.E. PANTSEVA motoneurones. To study the recurrent inhibition of small motoneurones induced by antidromic activation of large motoneurones, the tibialis posterior nerve was stimulated and an M response was evoked. Bipolar surface stimulating electrodes were placed at the popliteal fossa. The stimulus duration was 0.5-1 msec. The intervals between single stimuli were at least 5 - 6 sec. In order to study recurrent inhibition uncomplicated by after-hyperpolarization, it was necessary to eliminate the orthodromic response of small motoneurones to stimulation of Ia afferents and their antidromic response to that of efferents. To exclude Ia afferent stimulation we explored the popliteal fossa with stimulating electrodes and found an area in which stimuli yielded small M responses without any H reflex, i.e., the thickest efferents were stimulated. The amplitude of the M response did not exceed 10% of the maximum. Small motoneurones with thin axons (high threshold with respect to electrical stimulation) did not participate in such M responses. The absence of H reflex and M response of MUs under study was reliably controlled by plotting PSTHs in each experiment. M U potentials were identified visually. PSTHs of single MUs were plotted, based on 100-360 trials. The bin width was 5 msec, i.e., the precision of M U potential timing was not less than 5 msec. The deviation was regarded as significant ( P = 0.95) when the number of potentials in the bin went beyond ~_+ 2 S.D., where ~ is the mean number of potentials in the bin of the prestimulus part of PSTH (200 msec prior to a stimulus). Apart from plotting PSTHs, in each trial the duration of the interspike interval in which the inhibitory volley arrived (to be called the interspike interval tested) was compared with the mean of 5 - 7 intervals before each stimulus. In each trial the stimulus was applied at a random moment relative to the motoneurone discharges, and hence in different trials the inhibitory volley arrived at different moments of the interspike interval tested. Thus, the effectiveness of an inhibitory volley throughout the interspike interval of the motoneurone was tested. To determine the arrival time of the inhibitory volley in the interspike interval, the time between the preceding regular MU dis-
RECURRENT INHIBITION OF MOTONEURONES
181
charge and the stimulation time was measured in each trial and then the correction was made for the latency of the inhibitory effect derived from the P S T H of each MU. The duration of an interspike interval tested and the arrival time of an inhibitory volley within this interval were expressed as a percentage of the mean b a c k g r o u n d interspike interval, which made it possible to compare data obtained on different MUs. Spike trains of 28 M U s from 8 experiments involving 5 subjects were analysed.
Resuffs The b a c k g r o u n d firing rate of single M U s ranged from 4.5 to 9.2/sec. As a result of stimulation in a n u m b e r of trials lengthening of the interspike interval tested (i.e., the inhibitory effect) was observed (Fig. 1). However, this effect was inconstant both for different M U s in one trial and for one and the same M U in different trials. In order to evaluate the stimulating effect statistically and determine its latency P S T H s for single M U s were plotted. P S T H s for all 28 analysed M U s showed a significant decrease in firing probability with a latency of 3 5 - 4 0 msec (Fig. 2). This was slightly longer than the H reflex latency of these M U s (30-35 msec).
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Fig. 2. Detection of recurrent inhibition of a firing motoneurone by means of peristimulus time histograms (PSTHs). a-c: 4 MUs with different mean firing rates. Horizontal bracket beneath PSTHs indicates a period of inhibition a~ a significant decrease in MU firing probability.
The duration of inhibition for various M U s ranged from 5 to 30 msec. Comparison of PSTHs for M U s with low and higher background firing rates revealed a difference in the duration of the inhibitory effect. For M U s with a firing rate of 4 . 5 - 6 . 5 / s e c the duration of inhibition was 10 30 msec (Fig. 2a, b), while for M U s with a firing rate of 6 . 8 - 9 . 2 / s e c it was only 5 - 1 5 msec (Fig. 2c, d). Seven of 28 PSTHs showed that inhibition was followed by a significant increase in MU firing probability with a latency of 55 70 msec and duration of 15-45 msec (Fig. 2b). The increase in firing probability cannot be fully accounted for by resumption of M U firing after a period of inhibition, as it was more marked and lasted longer than inhibition. In addition, it was accompanied by shortening of the interspike intervals of firing M U s and by recruitment of new M U s (silent before stimulus). All this shows that during stimulation in some cases inhibition is followed by an excitatory effect (apparently from cutaneous afferents). To analyse the effect of an inhibitory volley on m o t o n e u r o n e rhythmic firing in more detail, the YSTH of each M U was c o m p a r e d with the results of analysis of the interspike intervals tested. When the P S T H s showed only inhibition without a subsequent excitatory effect (Fig. 3a), m a n y interspike intervals tested were longer as c o m p a r e d
182
L.P. K U D I N A , R.E. PANTSEVA
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Fig. 3. Detection of recurrent inhibition of a firing motoneurone by means of analysis of interspike intervals. Single MU (mean firing rate is 7.6/sec). a: PSTH; b: change in the effectiveness of inhibitory volley dependent on the time of its arrival in interspike interval. Abscissa, arrival time of inhibitory volley, as percentage of duration of mean interval of background firing. Ordinate, duration of interspike interval in which inhibitory volley arrived, as percentage of duration of mean interval of background firing. Each point is the result of one trial. Arrows show the scatter of background firing intervals.
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Fig. 4. Detection of recurrent inhibition of a firing motoneurone by analysis of interspike intervals under the action of inhibitory and subsequent excitatory volleys. Single MU (background mean firing rate is 7.3/sec). Legends as for Fig. 3.
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with the mean interval of background activity (Fig. 3b). However, some of the interspike intervals tested, despite the arrival of the inhibitory volley, showed no lengthening. When the PSTHs showed an excitatory effect following the inhibition (Fig. 4a), many interspike intervals tested were shorter than the mean background interval (Fig. 4b), i.e, the presence of inhibition was 'concealed' by the excitatory volley. The effect of an inhibitory volley depended on the time of its arrival in the interspike interval. When the inhibitory volley arrived in the first half of the interspike interval tested, its duration either did not go beyond the scatter of background firing intervals (Fig. 3b) or even became shorter under the action of a subsequent excitatory volley (Fig. 4b); in other words, the inhibitory volley was ineffective. But when it arrived in the second half or at the end of the interspike interval tested the latter became longer, i.e, the inhibitory volley
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Fig. 5. Dependence of the effectiveness of recurrent inhibition on the arrival time within the interspike interval at different durations of mean interval of background motoneurone firing. Single MU. a: same as Fig. 3b, but in msec. Horizontal broken lines, limits of scatter in background firing intervals. Filled circles, trials in which mean interspike interval of background firing is less than 125 msec. Open circles, mean interval greater than 126 msec. b: PSTH: 1, all trials under study: 2, selected trials with mean interspike interval of background firing less than 125 msec: 3, selected trials with interspike interval greater than 126 msec. Hatching indicates bins in which those tested interspike intervals ended, whose durations went beyond the scatter limits of the background firing intervals.
RECURRENT INHIBITION OF MOTONEURONES became effective (Figs. 3b and Fig. 4b). The effectiveness of inhibition was compared for the same MU firing at low and higher rates. At a low background firing rate the mean lengthening of the interspike interval tested was more pronounced than at higher background firing rates. This depended mainly on those changes in the duration of the interspike interval tested which occurred when the inhibitory volley arrived at the end of it (Fig. 5a). The effectiveness of inhibition evaluated by the duration of the inhibitory effect
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in PSTH was also greater at a low firing rate of the motoneurone (Figs. 5b and 6). At the same time, the duration of inhibition in the summated PSTH was considerably shorter than the lengthening of the interspike intervals tested (Fig. 5a, b). The analysis of these quantitative differences showed that the duration of inhibition in the summated PSTH depended on the trials with higher background firing in which the lengthening of the interspike interval tested was less pronounced than in trials with lower background firing. In these latter the interspike intervals tested were the longest (exceeding the scatter of background firing intervals, as can be seen in the right part of Fig. 5a) but they had no effect on the duration of the inhibition in the summated PSTH (Fig. 5b). At a high background firing rate of a motoneurone the inhibition might be ineffective. This can be demonstrated by selecting trials with especially high background firing rate from the summated PSTH (Fig. 6).
Discussion
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Fig. 6. Peristimulus time histograms of a single MU potential at different background mean firing rates, a: all trials under study; b: selected trials with higher background firing rate: c: selected trials with lower background firing rate. Other legends same as in Fig. 2.
The present results show that antidromic stimulation of large motoneurones led to short-latency inhibition of the rhythmic firing of small motoneurones. According to the experimental conditions, small MUs were not activated during stimulation, ruling out after-hyperpolarization as a cause of the effect revealed. By analogy with animal experiments it may be assumed that such shortlatency inhibition of motoneurone firing was induced by Renshaw cells activated by antidromic stimulation. This assumption is supported by the fact that the latency of the inhibition approximates to that of the H reflex. However, it is necessary to consider a possible participation of other inhibitory mechanisms associated with stimulation of afferents (Ib, I1 and cutaneous groups) and with muscle contraction in the analysed effect. In our experiments selective stimulation of muscle nerve efferents without activation of Ia afferents was performed. Under these conditions it may be assumed that Ib afferents (the thresholds of which are statistically higher than those of Ia afferents) as well as afferents of
184
group II were even less likely to be activated. As for cutaneous afferents, some of them may be activated by percutaneous stimuli, but the latency of inhibition in this case is longer (Shahani and Young 1973; Person and Kozhina 1978a, b). The latency of inhibitory effects resultant from evoked muscle contraction is also longer than that of the inhibition revealed. Proceeding from these facts, we assume that the mechanism underlying inhibition of the rhythmic firing of small motoneurones is recurrent inhibition. According to Eccles et al. (1961), the duration of IPSPs induced by synchronous activation of Renshaw cells in a non-firing motoneurone was about 40 msec. Therefore, the duration of the inhibitory volley arriving at the motoneurone firing at a rate of 4 - 1 1 / s e c (interspike interval of 90 250 msec) proves to be considerably shorter than that of the interspike interval. According to our data, the effectiveness of recurrent inhibition evoked by a short inhibitory volley for a firing motoneurone depended on 2 factors: the arrival time of a volley in the interspike interval and the background firing rate of the motoneurone. When the inhibitory volley arrived at the beginning of the interspike interval it produced no effect on its duration. This result provided evidence that, in the investigated range of firing rates, the motoneurone cannot exert an inhibitory influence on its own firing via recurrent collaterals since the volleys from Renshaw cells, having a short latency, will always arrive at the beginning of the interspike interval when it is ineffective. As indicated above, Datta and Stephens (1979) evaluated recurrent inhibition by the lengthening of the interspike interval of an MU after its reflex response. Present results suggest also that a volley from Renshaw cells arriving at the beginning of the interval is not likely to be the cause of its lengthening. The inhibitory volley ineffective at the beginning of the interspike interval became more effective as it approached the end of the interval. A similar increase in the effectiveness of an inhibitory volley arriving at the end of the interspike interval of a firing human motoneurone has been demonstrated in the case of reciprocal inhibition
L.P. KLIDINA. R.E. PANTSEVA
(Kudina 1978, 1980). It is likely that these results reflect peculiarities of interaction of the short inhibitory volley with the membrane potential in the interspike interval at a low firing rate of the motoneurone. The increase in the effectiveness of the inhibitory volley at the end of the interspike interval can probably be explained by potentiation of IPSPs. This agrees with the data of Fetz and Gustafsson (1983) who showed that the size of Ia ISPSs in cat motoneurones increased substantially as the motoneurone membrane potential approached threshold. The inhibitory volley timed at the end of the interspike interval caused its greatest lengthening when the background firing rate of the motoneurone was low. If the effectiveness of recurrent inhibition was evaluated by the duration of a significant reduction in firing probability in the PSTH, the higher effectivenesss of inhibition was also observed at a low background firing rate of the motoneurone. This is consistent with the data obtained by the PSTH method in studies of both recurrent inhibition (Ellaway and Murphy 1980) and other types of inhibition (Person and Kozhina 1978a, b). At the same time our data show that the summated PSTH does not give the complete characteristics of the inhibitory volley effect since it does not reveal the most effective influence in the trials with the lower background firing and in some cases even 'conceals' the ineffectiveness of inhibiton in the trials with a higher background firing rate. More complete information on the effectiveness of inhibition timing in a firing motoneurone can be obtained by analysing PSTHs together with data on changes in the durations of interspike intervals, dependent on the arrival time of the inhibitory volley in the interval. It can be assumed that such an approach to evaluating the effectiveness of inhibitory (and excitatory) volleys on a human firing motoneurone may be useful for understanding the principles underlying the interaction of different inputs to the motoneurone during natural muscle contraction.
The authors express their gratitude to Dr. R.S. Person for discussion.
R E C U R R E N T I N H I B I T I O N OF M O T O N E U R O N E S
References Datta, A.K. and Stephens, J.A. The stimulus locked interval histogram: a method that may allow investigation of Renshaw inhibition in man. J. Physiol. (Lond.), 1979, 293: P16 P17. Eccles, J.C., Eccles, R., Iggo, A. and Ito, M. Distribution of recurrent inhibition among motoneurones. J. Physiol. (Lond.), 1961, 159: 479-499. Ellaway, P.H. and Murphy, P.R. A comparison of the recurrent inhibition of a- and 7-motoneurones in the cat. J. Physiol. (Lond.), 1980, 315: 43-58. Fetz. E.E. and Gustafsson, B. Relation between shapes of postsynaptic potentials and changes in firing probability of cat motoneurones. J. Physiol. (Lond.), 1983, 341: 387-410. Hultborn, H. and Pierrot-Deseilligny, E. Changes in recurrent inhibition during voluntary soleus contraction in m a n studied by an H-reflex technique. J. Physiol. (Lond.), 1979, 297:229 251. Kudina, L.P. Study of the reciprocal inhibition on discharging h u m a n motor units. Neurofiziologiya (Kiev), 1978, 10: 626-635. Kudina, L.P. Reflex effects of muscle afferents on antagonist studied on single firing motor units in man. Electroenceph. din. Neurophysiol., 1980, 50: 214-221. Kudina, L.P. and Pantseva, R.E. Revealing and analysis of
185 recurrent inhibition of firing motoneurones in man. Neurofiziologiya (Kiev), 1984, 16: 88-96. Person, R.S. and Kozhina, G.V. Study of the silent period by means of post-stimulus histograms. Neurofiziologiya (Kiev), 1978a, 10: 177-184. Person, R.S. and Kozhina, G.V. Study of orthodromic and antidromic effects of nerve stimulation on single motoneurones of h u m a n hand muscles. EMG clin. Neurophysiol., 1978b, 18: 437-456. Pierrot-Deseilligny, E. and Bussel, B. Evidence for recurrent inhibition by motoneurons in h u m a n subjects. Brain Res., 1975, 88: 105-108. Pierrot-Deseilligny, E. et Mazi+res, L. Circuits r~flexes de la moelle ~pini6re chez l'homme. Contr61e au cours du mouvement et r61e fonctionnel (2 pattie). Re','. neurol., 1984, 140: 681-694. Shahani, B.T. and Young, R.R. Studies of the normal h u m a n silent period. In: J.E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, Vol. 3. Karger, Basel, 1973: 589-602. Veale. J.k. and Rees, S. Renshaw cell activity in man. J. Neurol. Neurosurg. Psychiat., 1973, 36: 674-683. Veale, J.L., Rees, S. and Mark, R.F. Renshaw cell activity in normal and spastic man. In: J.E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, Vol. 3. Karger, Basel, 1973: 523-537.
179
EEG 01935
Recurrent inhibition of firing motoneurones in man L.P. Kudina and R.E. Pantseva Institute for Problems of Information Transmission, U.S.S.R. Academy of Sciences, Moscow ( U.S. S. R.) (Accepted for publication: 14 June 1987)
Summary The effects of antidromic stimulation of large motoneurones on firing small motoneurones of soleus muscle have been studied. Against the background of rhythmic firing of small motor units (MUs) activated during weak voluntary muscle contraction, thick efferents of the tibialis posterior nerve were selectively stimulated and an M response was evoked in which small MUs were not involved. This provided a means of avoiding antidromic stimulation of the motoneurone under study and, thus, analysing the effect of stimulation without its summation with after-hyperpolarization. The background firing rate of MUs was 4.5-9.2/sec. PSTHs revealed a distinct inhibitory effect with a latency of 35-40 msec (slightly exceeding the latency of monosynaptic reflex) and duration 5-30 msec. It was concluded that the short-latency inhibition could be identified as recurrent inhibition. The effectiveness of recurrent inhibition evaluated by the lengthening of the interspike interval was shown to depend on the arrival time of the volley in the interval and on the background firing rate of the motoneurone. When the inhibitory volley arrived at the beginning of the interspike interval it was ineffective. This indicates that in the investigated range of firing rates the motoneurone is unable to exert an inhibitory effect on its own firing via recurrent collaterals. The inhibitory volley became highly effective at the end of an interspike interval, when the membrane potential approached threshold. The lengthening of interspike interval was more marked at a lower firing rate of the motoneurone. An increase in the background firing rate reduced the extent of recurrent inhibition (at a rate above 10/sec up to its complete ineffectiveness). Various methods for evaluating recurrent inhibition in a firing motoneurone are compared. Key words: Spinal cord; Human motoneurones; Recurrent inhibition
R e c u r r e n t i n h i b i t i o n o f m o t o n e u r o n e s has b e e n the s u b j e c t o f m a n y a n i m a l studies. In m a n this i n h i b i t i o n was first d e s c r i b e d b y V e a l e a n d cow o r k e r s ( V e a l e a n d R e e s 1973; V e a l e et al. 1973). T h e a u t h o r s assessed r e c u r r e n t i n h i b i t i o n b y a r e d u c t i o n in t h e a m p l i t u d e of a t e s t i n g m o n o s y n a p t i c H reflex of the soleus m u s c l e a f t e r a c o n d i t i o n i n g s t i m u l u s a c t i v a t i n g R e n s h a w cells, e i t h e r b y a n t i d r o m i c s t i m u l a t i o n of ' l a r g e ' m o t o n e u r o n e s (an M r e s p o n s e w a s e v o k e d ) o r b y o r t h o d r o m i c s t i m u l a t i o n via Ia a f f e r e n t s o f ' s m a l l ' m o t o n e u r o n e s (an H reflex was e v o k e d ) . I n t h e first c a s e c o n d i t i o n i n g a n d t e s t i n g s t i m u l i w e r e a d d r e s s e d to d i f f e r e n t m o t o n e u r o n e s , large a n d small, r e s p e c tively. T h u s , the e f f e c t o f c o n d i t i o n i n g s t i m u l a t i o n Correspondence to: L.P. Kudina, Institute for Problems of Information Transmission, U.S.S.R. Academy of Sciences, U1. Yermolovoy 19, GSP-4, 101 447 Moscow (U.S.S.R.).
uncomplicated by after-hyperpolarization was a n a l y s e d . I n the s e c o n d case b o t h s t i m u l i w e r e a d d r e s s e d to the s a m e p o p u l a t i o n of s m a l l m o t o n e u r o n e s . It s h o u l d be n o t e d t h a t a f t e r - h y p e r p o l a r i z a t i o n , w h i c h d e v e l o p s in a m o t o n e u r o n e f o l l o w i n g its d i s c h a r g e a n d a d d s u p to the i n h i b i tory influence of stimulation, hampers interpret a t i o n o f t h e results. R e c u r r e n t i n h i b i t i o n o f m o t o n e u r o n e s in m a n w a s d e a l t w i t h in a series o f studies by P i e r r o t - D e s e i l l i g n y et al. ( P i e r r o t - D e s e i l l i g n y a n d Bussel 1975: H u l t b o r n a n d P i e r r o t - D e s e i l l i g n y 1979: P i e r r o t - D e s e i l l i g n y a n d M a z i r r e s 1984). R e c u r r e n t i n h i b i t i o n w a s e v a l u a t e d b y the d e c r e a s e in a m p l i t u d e o f a t e s t i n g H reflex of t h a t m o t o n e u r o n e p o p u l a t i o n to w h i c h the c o n d i t i o n i n g s t i m u l u s was a d d r e s s e d . It was f o u n d t h a t d u r i n g w e a k v o l u n t a r y c o n t r a c t i o n o f the soleus m u s c l e the i n t e n s i t y o f r e c u r r e n t i n h i b i t i o n was g r e a t e r t h a n t h a t at
0013-4649/88/$03.50 c©1988 Elsevier Scientific Publishers Ireland, Ltd.
180 rest, while with increase in the strength of voluntary contraction the recurrent inhibition abated (Hultborn and Pierrot-Deseilligny 1979). Apart from the investigations of recurrent inhibition on a population of motoneurones there have been studies on single motoneurones in man under conditions of their natural activation (voluntary muscle contraction). Datta and Stephens (1979) evaluated inhibition by the lengthening of the interspike interval of a motor unit (MU) after its reflex response. However, using such methods it is impossible to determine the latency of the inhibitory effect and hence its origin, since, due to the long duration of the interspike interval (nearly 100 msec) both short-latency and longlatency inhibitory volleys induced by stimulation may arrive within the interval following the reflex response of the MU. To ascertain recurrent inhibition as one of the components of a 'silent period' after an M response, Person and Kozhina (1978a, b) employed the method of peristimulus time histograms (PSTHs) of single MUs, which provided a means for assessing the inhibition latency. The use of antidromic stimulation of large motoneurones and investigation of stimulation effects on small motoneurones enabled the authors to avoid the complicating effect of after-hyperpolarization. The authors failed to detect recurrent inhibition in hand muscles. A similar method has been used for investigating the effectiveness of recurrent inhibition on single firing MUs of the human soleus muscle in the present study. A preliminary report was published earlier (Kudina and Pantseva 1984).
Methods Using a visual feedback of M U potentials, volunteer subjects maintained steady firing of single MUs by weak voluntary isometric contraction of the soleus muscle. M U potentials were picked up by bipolar needle electrodes. The summated E M G was recorded by surface electrodes. During weak muscle contraction the lowest threshold small motoneurones (with thin axons) were recruited. Animal experiments show that recurrent inhibition is generally potent on small
L.P. KUD1NA. R.E. PANTSEVA motoneurones. To study the recurrent inhibition of small motoneurones induced by antidromic activation of large motoneurones, the tibialis posterior nerve was stimulated and an M response was evoked. Bipolar surface stimulating electrodes were placed at the popliteal fossa. The stimulus duration was 0.5-1 msec. The intervals between single stimuli were at least 5 - 6 sec. In order to study recurrent inhibition uncomplicated by after-hyperpolarization, it was necessary to eliminate the orthodromic response of small motoneurones to stimulation of Ia afferents and their antidromic response to that of efferents. To exclude Ia afferent stimulation we explored the popliteal fossa with stimulating electrodes and found an area in which stimuli yielded small M responses without any H reflex, i.e., the thickest efferents were stimulated. The amplitude of the M response did not exceed 10% of the maximum. Small motoneurones with thin axons (high threshold with respect to electrical stimulation) did not participate in such M responses. The absence of H reflex and M response of MUs under study was reliably controlled by plotting PSTHs in each experiment. M U potentials were identified visually. PSTHs of single MUs were plotted, based on 100-360 trials. The bin width was 5 msec, i.e., the precision of M U potential timing was not less than 5 msec. The deviation was regarded as significant ( P = 0.95) when the number of potentials in the bin went beyond ~_+ 2 S.D., where ~ is the mean number of potentials in the bin of the prestimulus part of PSTH (200 msec prior to a stimulus). Apart from plotting PSTHs, in each trial the duration of the interspike interval in which the inhibitory volley arrived (to be called the interspike interval tested) was compared with the mean of 5 - 7 intervals before each stimulus. In each trial the stimulus was applied at a random moment relative to the motoneurone discharges, and hence in different trials the inhibitory volley arrived at different moments of the interspike interval tested. Thus, the effectiveness of an inhibitory volley throughout the interspike interval of the motoneurone was tested. To determine the arrival time of the inhibitory volley in the interspike interval, the time between the preceding regular MU dis-
RECURRENT INHIBITION OF MOTONEURONES
181
charge and the stimulation time was measured in each trial and then the correction was made for the latency of the inhibitory effect derived from the P S T H of each MU. The duration of an interspike interval tested and the arrival time of an inhibitory volley within this interval were expressed as a percentage of the mean b a c k g r o u n d interspike interval, which made it possible to compare data obtained on different MUs. Spike trains of 28 M U s from 8 experiments involving 5 subjects were analysed.
Resuffs The b a c k g r o u n d firing rate of single M U s ranged from 4.5 to 9.2/sec. As a result of stimulation in a n u m b e r of trials lengthening of the interspike interval tested (i.e., the inhibitory effect) was observed (Fig. 1). However, this effect was inconstant both for different M U s in one trial and for one and the same M U in different trials. In order to evaluate the stimulating effect statistically and determine its latency P S T H s for single M U s were plotted. P S T H s for all 28 analysed M U s showed a significant decrease in firing probability with a latency of 3 5 - 4 0 msec (Fig. 2). This was slightly longer than the H reflex latency of these M U s (30-35 msec).
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The duration of inhibition for various M U s ranged from 5 to 30 msec. Comparison of PSTHs for M U s with low and higher background firing rates revealed a difference in the duration of the inhibitory effect. For M U s with a firing rate of 4 . 5 - 6 . 5 / s e c the duration of inhibition was 10 30 msec (Fig. 2a, b), while for M U s with a firing rate of 6 . 8 - 9 . 2 / s e c it was only 5 - 1 5 msec (Fig. 2c, d). Seven of 28 PSTHs showed that inhibition was followed by a significant increase in MU firing probability with a latency of 55 70 msec and duration of 15-45 msec (Fig. 2b). The increase in firing probability cannot be fully accounted for by resumption of M U firing after a period of inhibition, as it was more marked and lasted longer than inhibition. In addition, it was accompanied by shortening of the interspike intervals of firing M U s and by recruitment of new M U s (silent before stimulus). All this shows that during stimulation in some cases inhibition is followed by an excitatory effect (apparently from cutaneous afferents). To analyse the effect of an inhibitory volley on m o t o n e u r o n e rhythmic firing in more detail, the YSTH of each M U was c o m p a r e d with the results of analysis of the interspike intervals tested. When the P S T H s showed only inhibition without a subsequent excitatory effect (Fig. 3a), m a n y interspike intervals tested were longer as c o m p a r e d
182
L.P. K U D I N A , R.E. PANTSEVA
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Fig. 3. Detection of recurrent inhibition of a firing motoneurone by means of analysis of interspike intervals. Single MU (mean firing rate is 7.6/sec). a: PSTH; b: change in the effectiveness of inhibitory volley dependent on the time of its arrival in interspike interval. Abscissa, arrival time of inhibitory volley, as percentage of duration of mean interval of background firing. Ordinate, duration of interspike interval in which inhibitory volley arrived, as percentage of duration of mean interval of background firing. Each point is the result of one trial. Arrows show the scatter of background firing intervals.
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with the mean interval of background activity (Fig. 3b). However, some of the interspike intervals tested, despite the arrival of the inhibitory volley, showed no lengthening. When the PSTHs showed an excitatory effect following the inhibition (Fig. 4a), many interspike intervals tested were shorter than the mean background interval (Fig. 4b), i.e, the presence of inhibition was 'concealed' by the excitatory volley. The effect of an inhibitory volley depended on the time of its arrival in the interspike interval. When the inhibitory volley arrived in the first half of the interspike interval tested, its duration either did not go beyond the scatter of background firing intervals (Fig. 3b) or even became shorter under the action of a subsequent excitatory volley (Fig. 4b); in other words, the inhibitory volley was ineffective. But when it arrived in the second half or at the end of the interspike interval tested the latter became longer, i.e, the inhibitory volley
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RECURRENT INHIBITION OF MOTONEURONES became effective (Figs. 3b and Fig. 4b). The effectiveness of inhibition was compared for the same MU firing at low and higher rates. At a low background firing rate the mean lengthening of the interspike interval tested was more pronounced than at higher background firing rates. This depended mainly on those changes in the duration of the interspike interval tested which occurred when the inhibitory volley arrived at the end of it (Fig. 5a). The effectiveness of inhibition evaluated by the duration of the inhibitory effect
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in PSTH was also greater at a low firing rate of the motoneurone (Figs. 5b and 6). At the same time, the duration of inhibition in the summated PSTH was considerably shorter than the lengthening of the interspike intervals tested (Fig. 5a, b). The analysis of these quantitative differences showed that the duration of inhibition in the summated PSTH depended on the trials with higher background firing in which the lengthening of the interspike interval tested was less pronounced than in trials with lower background firing. In these latter the interspike intervals tested were the longest (exceeding the scatter of background firing intervals, as can be seen in the right part of Fig. 5a) but they had no effect on the duration of the inhibition in the summated PSTH (Fig. 5b). At a high background firing rate of a motoneurone the inhibition might be ineffective. This can be demonstrated by selecting trials with especially high background firing rate from the summated PSTH (Fig. 6).
Discussion
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The present results show that antidromic stimulation of large motoneurones led to short-latency inhibition of the rhythmic firing of small motoneurones. According to the experimental conditions, small MUs were not activated during stimulation, ruling out after-hyperpolarization as a cause of the effect revealed. By analogy with animal experiments it may be assumed that such shortlatency inhibition of motoneurone firing was induced by Renshaw cells activated by antidromic stimulation. This assumption is supported by the fact that the latency of the inhibition approximates to that of the H reflex. However, it is necessary to consider a possible participation of other inhibitory mechanisms associated with stimulation of afferents (Ib, I1 and cutaneous groups) and with muscle contraction in the analysed effect. In our experiments selective stimulation of muscle nerve efferents without activation of Ia afferents was performed. Under these conditions it may be assumed that Ib afferents (the thresholds of which are statistically higher than those of Ia afferents) as well as afferents of
184
group II were even less likely to be activated. As for cutaneous afferents, some of them may be activated by percutaneous stimuli, but the latency of inhibition in this case is longer (Shahani and Young 1973; Person and Kozhina 1978a, b). The latency of inhibitory effects resultant from evoked muscle contraction is also longer than that of the inhibition revealed. Proceeding from these facts, we assume that the mechanism underlying inhibition of the rhythmic firing of small motoneurones is recurrent inhibition. According to Eccles et al. (1961), the duration of IPSPs induced by synchronous activation of Renshaw cells in a non-firing motoneurone was about 40 msec. Therefore, the duration of the inhibitory volley arriving at the motoneurone firing at a rate of 4 - 1 1 / s e c (interspike interval of 90 250 msec) proves to be considerably shorter than that of the interspike interval. According to our data, the effectiveness of recurrent inhibition evoked by a short inhibitory volley for a firing motoneurone depended on 2 factors: the arrival time of a volley in the interspike interval and the background firing rate of the motoneurone. When the inhibitory volley arrived at the beginning of the interspike interval it produced no effect on its duration. This result provided evidence that, in the investigated range of firing rates, the motoneurone cannot exert an inhibitory influence on its own firing via recurrent collaterals since the volleys from Renshaw cells, having a short latency, will always arrive at the beginning of the interspike interval when it is ineffective. As indicated above, Datta and Stephens (1979) evaluated recurrent inhibition by the lengthening of the interspike interval of an MU after its reflex response. Present results suggest also that a volley from Renshaw cells arriving at the beginning of the interval is not likely to be the cause of its lengthening. The inhibitory volley ineffective at the beginning of the interspike interval became more effective as it approached the end of the interval. A similar increase in the effectiveness of an inhibitory volley arriving at the end of the interspike interval of a firing human motoneurone has been demonstrated in the case of reciprocal inhibition
L.P. KLIDINA. R.E. PANTSEVA
(Kudina 1978, 1980). It is likely that these results reflect peculiarities of interaction of the short inhibitory volley with the membrane potential in the interspike interval at a low firing rate of the motoneurone. The increase in the effectiveness of the inhibitory volley at the end of the interspike interval can probably be explained by potentiation of IPSPs. This agrees with the data of Fetz and Gustafsson (1983) who showed that the size of Ia ISPSs in cat motoneurones increased substantially as the motoneurone membrane potential approached threshold. The inhibitory volley timed at the end of the interspike interval caused its greatest lengthening when the background firing rate of the motoneurone was low. If the effectiveness of recurrent inhibition was evaluated by the duration of a significant reduction in firing probability in the PSTH, the higher effectivenesss of inhibition was also observed at a low background firing rate of the motoneurone. This is consistent with the data obtained by the PSTH method in studies of both recurrent inhibition (Ellaway and Murphy 1980) and other types of inhibition (Person and Kozhina 1978a, b). At the same time our data show that the summated PSTH does not give the complete characteristics of the inhibitory volley effect since it does not reveal the most effective influence in the trials with the lower background firing and in some cases even 'conceals' the ineffectiveness of inhibiton in the trials with a higher background firing rate. More complete information on the effectiveness of inhibition timing in a firing motoneurone can be obtained by analysing PSTHs together with data on changes in the durations of interspike intervals, dependent on the arrival time of the inhibitory volley in the interval. It can be assumed that such an approach to evaluating the effectiveness of inhibitory (and excitatory) volleys on a human firing motoneurone may be useful for understanding the principles underlying the interaction of different inputs to the motoneurone during natural muscle contraction.
The authors express their gratitude to Dr. R.S. Person for discussion.
R E C U R R E N T I N H I B I T I O N OF M O T O N E U R O N E S
References Datta, A.K. and Stephens, J.A. The stimulus locked interval histogram: a method that may allow investigation of Renshaw inhibition in man. J. Physiol. (Lond.), 1979, 293: P16 P17. Eccles, J.C., Eccles, R., Iggo, A. and Ito, M. Distribution of recurrent inhibition among motoneurones. J. Physiol. (Lond.), 1961, 159: 479-499. Ellaway, P.H. and Murphy, P.R. A comparison of the recurrent inhibition of a- and 7-motoneurones in the cat. J. Physiol. (Lond.), 1980, 315: 43-58. Fetz. E.E. and Gustafsson, B. Relation between shapes of postsynaptic potentials and changes in firing probability of cat motoneurones. J. Physiol. (Lond.), 1983, 341: 387-410. Hultborn, H. and Pierrot-Deseilligny, E. Changes in recurrent inhibition during voluntary soleus contraction in m a n studied by an H-reflex technique. J. Physiol. (Lond.), 1979, 297:229 251. Kudina, L.P. Study of the reciprocal inhibition on discharging h u m a n motor units. Neurofiziologiya (Kiev), 1978, 10: 626-635. Kudina, L.P. Reflex effects of muscle afferents on antagonist studied on single firing motor units in man. Electroenceph. din. Neurophysiol., 1980, 50: 214-221. Kudina, L.P. and Pantseva, R.E. Revealing and analysis of
185 recurrent inhibition of firing motoneurones in man. Neurofiziologiya (Kiev), 1984, 16: 88-96. Person, R.S. and Kozhina, G.V. Study of the silent period by means of post-stimulus histograms. Neurofiziologiya (Kiev), 1978a, 10: 177-184. Person, R.S. and Kozhina, G.V. Study of orthodromic and antidromic effects of nerve stimulation on single motoneurones of h u m a n hand muscles. EMG clin. Neurophysiol., 1978b, 18: 437-456. Pierrot-Deseilligny, E. and Bussel, B. Evidence for recurrent inhibition by motoneurons in h u m a n subjects. Brain Res., 1975, 88: 105-108. Pierrot-Deseilligny, E. et Mazi+res, L. Circuits r~flexes de la moelle ~pini6re chez l'homme. Contr61e au cours du mouvement et r61e fonctionnel (2 pattie). Re','. neurol., 1984, 140: 681-694. Shahani, B.T. and Young, R.R. Studies of the normal h u m a n silent period. In: J.E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, Vol. 3. Karger, Basel, 1973: 589-602. Veale. J.k. and Rees, S. Renshaw cell activity in man. J. Neurol. Neurosurg. Psychiat., 1973, 36: 674-683. Veale, J.L., Rees, S. and Mark, R.F. Renshaw cell activity in normal and spastic man. In: J.E. Desmedt (Ed.), New Developments in Electromyography and Clinical Neurophysiology, Vol. 3. Karger, Basel, 1973: 523-537.