Repetitive doublets of human motoneurones: analysis of interspike intervals and recruitment pattern

Electroencephalography and clinical Neurophysiology, 85 (1992) 243-247

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© 1992 Elsevier Scientific Publishers Ireland, Ltd. 0924-980X/92/$05.00

E L M O C O 91579

Repetitive doublets of h u m a n motoneurones: analysis of interspike intervals and recruitment pattern L.P. Kudina and N.L. Alexeeva Institute for Problems of Information Transmission, Russian Academy of Sciences, Moscow (Russia) (Accepted for publication: 24 February 1992)

Summary In order to study the probable mechanisms of repetitive doublets in h u m a n motoneurones, the firing patterns of single motor units (MUs) of the trapezius were analysed during a weak voluntary muscle contraction. The mean frequencies of M U s were 9.4-21.7 i m p / s e c (the m e a n interspike interval ranged from 46.0 to 106.7 msec). Repetitive doublets (up to 28 in succession) were recorded in 21 out of 120 MUs, mostly at the onset of a slow recruitment. These were followed by single discharges. Intradoublet intervals ranged between 2.5 and 20.0 msec. A significant difference between single spike firing and doublet firing was revealed by plotting interspike interval histograms, showing that two distinct mechanisms were involved. T h e analysis of interspike interval successions belonging to several M U s firing simultaneously showed that one of the M U s could start with doublets while the others went on firing single spikes with the regular mean frequency and interspike interval scatter. The results lead us to suggest that the intrinsic properties of motoneurones can he regarded as the main factor in the origin of repetitive doublets. It seems that a descending synaptic drive also contributes to the control of double firing since in a n u m b e r of cases no doublets were produced at the beginning of M U activity. The findings are discussed with regard to the problems of regulating repetitive firing of h u m a n motoneurones by after-potentials. Steady delayed depolarization is assumed to be a possible mechanism of repetitive doublets. Key words: H u m a n m o t o n e u r o n e firing; Repetitive doublets; After-potentials

Repetitive firing of motoneurones evoked by both intracellular depolarization and natural activation during voluntary muscle contraction usually consists of low-frequency single discharges (the interspike intervals are about 50-200 msec). It is controlled by afterhyperpolarization as one of the main factors (Gustafsson 1974, 1984). However, sometimes motoneurones can produce a different type of firing, i.e., "double discharges" or "doublets" (with interspike intervals from 2.5 to 20.0 msec) caused by delayed depolarization. This latter either smoothly decays or forms a " h u m p " which can spontaneously reach the firing level, evoked an additional discharge (Granit et al. 1963; Kernell 1964; Calvin and Schwindt 1972; Calvin 1980). As a rule, each doublet is followed by a longer than usual interspike interval caused by the summation of after-hyperpolarization (Calvin and Schwindt 1972). This type of motoneurone firing has been revealed during intracellular motoneurone depolarization in cat and under voluntary muscle contraction in man. For the latter case two kinds of doublet have been found; occasional doublets (one or two double discharges observed among usual single discharges with a low rate) Correspondence to: L.P. Kudina, Institute for Problems of Information Transmission, Russian Academy of Sciences, UI. Yermolovoy 19, GSP-4, 101447 Moscow (Russia).

and repetitive ones (long spike trains consisting of alternating doublets and lengthened post-doublet interspike intervals). Occasional doublets of human motoneurones have been analysed in detail and delayed depolarization has been accepted to be a mechanism underlying them (Denslow 1948; Kudina 1974; Bawa and Calancie 1983; Kudina and Churikova 1990). In contrast to this, repetitive doublets have been mentioned in some reports (Denslow 1948; Kudina 1974) and analysed in only one study (Bawa and Calancie 1983). From the comparison of short interspike intervals at the onset of ballistic movements and short intradoublet ones, Bawa and Calancie have suggested that these firing patterns originate from separate mechanisms in the spinal cord. The nature of repetitive doublets remains obscure. In particular, it is not quite clear whether doublets reflect the intrinsic motoneurone properties or result from a specific synaptic drive to the motoneurone pool. To clarify the question motoneurone recruitment patterns and interspike intervals of repetitive doublets in man were investigated. Methods

The experiments were carried out on 6 normal subjects who provided informed consent. Based on Denslow's data showing that the doubling was espe-

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cially marked in trunk muscles, in particular, in the trapezius (Denslow 1948), the latter was chosen for our study. Bipolar needle electrodes (usually two) were inserted into trapezius to record the action potentials from single MUs. The subjects were comfortably seated in an arm-chair and provided with visual and audio feedback of the M U discharges. The subjects were asked to recruit one or several MUs by a gentle slowly developing voluntary muscle contraction and to maintain steady firing of MUs. In most experiments, each MU was recruited several times. The M U potentials were identified visually. The interspike interval duration during single spike and doublet firing was measured and interspike interval histograms of single MUs were plotted. The durations of doublet and post-doublet intervals were compared with that of the m e a n interspike interval of single spike firing. For all MUs under study the mean frequency of single spike firing was determined as a reciprocal value of the mean interspike interval calculated over 1 sec. In addition, the duration of interspike intervals of MUs incapable of firing double discharges coinciding with doublets of the other MUs was analysed. For statistical analysis Student's t test was used. Probabilities of less than 0.05 were considered to be significant.

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Results Doublets were recorded in spike trains of 28 out of 120 (about 23%) MUs, in 4 subjects out of 6. Seven out of 28 MUs produced "occasional" doublets only, i.e., one or two doublets interspersed with single discharges. Twenty-one MUs produced occasional doublets as well as long series of repetitive doublets, including up to 28 double discharges in succession. As a rule, each doublet was followed by a lengthened post-doublet interspike interval as compared to the mean interspike interval of M U firing without doublets (background activity). In total, 1243 doublets, including 264 occasional doublets and 137 series of repetitive doublets (979 double discharges), were recorded. In the present study the accent was put on the series of repetitive doublets. Repetitive doublets occurred most frequently at the onset of an MU recruitment after which single discharges followed (Fig. la, top and middle records). In some recruitments the same M U could start with single discharges (Fig. la, bottom records). Fig. lb illustrates a long train of single M U doublets. The M U started with doublets, then doubling was interrupted by 3 single potentials, after which doublet firing was re-

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MOTONEURONE REPETITIVE DOUBLETS sumed. Double discharges also occurred during the cessation of firing. It should be mentioned that the amplitude of the second potential in doublets was often smaller than of the first, especially when the intradoublet interval was short. During single spike firing the mean frequencies of MUs were 9.4-21.7 i m p / s e c (mean interspike interval ranged from 46.0 to 106.7 msec). The duration of doublet interspike intervals for different MUs producing repetitive doublets as well as for MUs with only occasional doublets varied between 2.5 and 20.0 msec, with the majority of the intervals (85% and 74% for repetitive and occasional doublets, respectively) between 2.5 and 7.5 msec. For a single MU it could vary within the same range. Post-doublet interspike intervals went up to 66.0-232.5 msec (143.5-217.9% of the mean interspike interval). For all MUs under study both doublet and post-doublet intervals were significantly different from the mean interspike intervals of MUs during their single firing ( P < 0.01). To characterize further the doublet firing, the distribution of M U interspike intervals was studied. An example of typical interspike interval histograms for a single M U is given in Fig. 2. This M U produced 15 series of doublets (in total 134 doublets) followed by single spike firing. One can see that the histogram in Fig. 2a, including interspike intervals of both single spike and doublet firing, showed an unusual, skewed interval distribution. However, on separating interspike intervals of single spike successions (Fig. 2b) from those of doublet firing (Fig. 2c) it can be demonstrated that in the former case the interspike interval distribution was normal (interspike interval ranged from 40.0 to 120.0 msec) while in the latter it was far from normal. This was the reason why the summed histogram in Fig. 2a had an unusual shape. Doublets formed a separate group of short intervals (for a given MU, from 2.5 to 7.5 msec) which were far outside the statistical scatter of the shortest interspikc intervals for M U single spike firing. On the other hand, post-doublet intervals in most cases were beyond the longest interspike intervals of single firing. Similar results were obtained for all MUs under study. Thus, the same M U showed quite different distribution patterns of interspike intervals during single and doublet firing. In a number of experiments, MUs with doublet firing and those producing only single discharges were recorded simultaneously (Fig. lc). An attempt was made to decide whether M U single firing remained unchanged while the "neighbouring" MUs, fired double discharges by comparing the m e a n interspike interval duration in background with the duration at the m o m e n t when other MUs produced doublets. The comparison (15 MUs were analysed) did not reveal any significant changes of m e a n interspike intervals (for total material, P > 0.05). As can be seen in Fig. 3, one

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Fig. 2. Interspike interval histograms of a single MU with repetitive doublets and single discharges: (a) histogram based on both single and doublet firing (n = 529); (b) selected single firing (n = 261); (c) selected doublet firing (n = 268). Black bin, doublet interspike intervals. Hatched bins, post-doublet intervals. of the MUs started with doublets while the other kept firing with the usual single discharges occurring with the same mean frequency and scatter of interspike intervals as in a background firing. To analyse whether an M U with doublets kept the same firing pattern during repeated recruitments, 118 starts of 21 MUs were examined. Twelve out of 21

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observed: (1) shorter doublet interspike intervals at the beginning of a series (2.5-7.5 msec) tended to lengthen at the end (to 15.0-20.0 msec); (2) the longest postdoublet interspike intervals at the beginning of a series (in particular, after the first doublet) progressively shortened at the end of the series. This is illustrated in Figs. 3 and 4. As to long doublet series (in total, 25 series including 435 doublets were recorded in 4 out of 21 MUs), the MU firing pattern was more complicated. As demonstrated by Fig. 5, doublet and post-doublet interspike intervals changed in a wave-like manner, increasing doublet intervals and decreasing post-doublet intervals alternated with the reverse pattern. Both short and long series of doublets discontinued as soon as doublet and post-doublet interspike intervals approached the mean values of single interspike intervals.

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Fig. 4. Sequences of interspike intervals of a single MU during 4 recruitments. Three times the MU started with a short train of repetitive doublets, once with single discharges. During recruitment with doublets, a number of double discharges and the duration of doublet and post-doublet interspike intervals varied. Symbols as in Fig. 3.

MUs showed doublet series in each of 55 recruitments• Nine MUs began firing with doublets in 24 out of 63 starts, but in 39 recruitments MUs started with single discharges. The typical samples of interspike inte~al successions for a single MU are given in Fig. 4. I n 3 from 4 recruitments, the MU started with a series of doublets, once with single discharges. During repetitive recruitments, MU firing patterns (i.e., the number of double discharges and doublet/ post-doublet interspike intervals)varied. Doublet and post-doublet intervals in short (3-11 doublets) and long (12-28 doublets) series changed in different ways. In 21 MUs with 112 short doublet series, including 544 double discharges, the following characteristics were

Discussion

The results presented show that during gentle voluntary contraction of human trapezius (i.e., under a weak excitatory drive) some MUs may have two patterns of repetitive firing, i.e., single spike and doublet firing. These find!ng are in keeping with earlier results reported by Denslow (1948) and Bawa and Calancie (1983)• In our experiments a clear distinction between these patterns was revealed by plotting interspike interval histograms. The analysis of interval distribution indicates that repetitive doublets are a specific kind of motoneurone firing, quite different from the usual single spike firing. This point of view is supported by the data of Calvin (1980) and Bawa and Calancie (1983). Based on these findings it may be assumed that there are two distinct mechanisms underlying the single and doublet repetitive firing produced by the same motoneurone.

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MOTONEURONE REPETITIVE DOUBLETS

Single firing of motoneurones is believed to be controlled by after-hyperpolarization. The interaction between a suprathreshold excitatory drive and a long-lasting after-hyperpolarization results in single firing at a slow rate. "Occasional" doublets among low-rate single spikes are evidently due to after-depolarization ("delayed depolarization") which spontaneously reaches threshold and causes an additional discharge (Calvin 1980). Apart from spontaneous rises in delayed depolarization up to threshold level, another possible mechanism of doublet generation during voluntary muscle contraction was suggested, i.e., that a weak increase in the excitatory synaptic drive is superimposed on delayed depolarization (Kudina and Churikova 1990). This suggestion was based on monosynaptic testing of the excitability of a firing motoneurone and showed that motoneurones capable of firing doublets had a higher excitability at the beginning of an interspike interval (the first 20 msec) and, therefore, possessed specific transduction properties, as compared to motoneurones incapable of firing double discharges. It is more difficult to explain the occurrence of a long train of repetitive doublets. At least two factors can be responsible for repetitive doublet firing, either the intrinsic properties of the motoneurones or a specific synaptic drive or both. Although the contribution of these factors cannot be tested directly in human experiments, our results suggest some evidence for a certain influence of both mechanisms with a dominating role of motoneurone properties. Our data demonstrating the occurrence of doubling in a single MU firing, while the firing pattern in other MUs remains unchanged, support the hypothesis of motoneurone intrinsic properties as the contributor to repetitive doubling. The alternative explanation, i.e., that doubling is caused only by specific changes in descending or segmental excitatory synaptic drives seems less likely because these input changes would have to be confined to only some motoneurones. At the same time the influence of the synaptic drive cannot be eliminated entirely. Our findings show that the motoneurone capability of firing double discharges is realized in many but not in each start, mostly during a very slow and gentle voluntary muscle contraction. Bawa and Calancie (1983) reported that the subjects could improve their ability to fire repetitive doublets by training. Evidently, a motoneurone is capable of firing with repetitive doublets only when a certain synaptic input

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is activated. An additional condition is that the motoneurone depolarization should reach a threshold very slowly, which agrees with Baldissera's data (1984) on the influence of the rate of the depolarization rise on the firing pattern of a neurone. The question remains of what specific mechanism underlies the repetitive doublet firing in human motoneurones. Our data showing the same range of intradoublet intervals for occasional and repetitive doublets suggest that a common mechanism is responsible for them. However, the occurrence of a prolonged train of double discharges can hardly be attributed to a spontaneous rise of delayed depolarization to a threshold level. Presumably, the steady state of delayed depolarization, probably induced by the activation of a certain descending synaptic input, accounts for repetitive doublet firing. The authors are grateful to L. Kravtsova for scrutinizing the English.

References Baldissera, F. Impulse frequency encoding of the dynamic aspects of excitation. Arch. Ital. Biol., 1984, 122: 43-58. Bawa, P. and Calancie, B. Repetitive doublets in human flexor radialis muscle. J. Physiol. (Lond.), 1983, 339: 123-132. Calvin, W.H. Normal repetitive firing and its pathophysiology. In: I.S, Lockard and A.A. Ward, Jr. (Eds.), Epilepsy: a Window to Brain Mechanisms. Raven Press, New York, 1980: 97-121. Calvin, W.H. and Schwindt, P.C. Steps in production of motoneuron spikes during rhythmic firing. J. Neurophysiol., 1972, 35: 297-310. Denslow, J.S. Double discharges in human motor units. J. Neurophysiol., 1948, 11: 209-215. Granit, R., Kernell, D. and Smith, R.S. Delayed depolarization and the repetitive firing response to intracellular stimulation of mammalian motoneurones. J. Physiol. (Lond.), 1963, 168: 890-910. Gustafsson, B. Afterhyperpolarization and the control of repetitive firing in spinal neurones of the cat. Acta Physiol. Scand., 1974, Suppl. 416, 47 pp. Gustafsson, B. Afterpotentials and transduction properties in different types of central neurones. Arch. Ital. Biol., 1984, 122: 17-30. Kernell, D. The dalayed depolarization in cat and rat motoneurones. In: J.C. Eccles and J.P. Schad6 (Eds.), Progress in Brain Research, Vol. 12. Elsevier Publishing Company, Amsterdam, 1964: 42-52. Kudina, L.P. Double discharges in human motoneurones. Neurofiziologiya (Kiev), 1974, 6: 152-160. Kudina, L.P. and Churikova, L.I. Testing excitability of human motoneurones capable of firing double discharges. Electroenceph. clin. Neurophysiol., 1990, 75: 334-341.