Functional differentiation of motor units in human opponens pollicis muscle

EXPERIMENTAL

NEUROLOGY

Functional

A.

of

36-47 (1976)

Differentiation Opponens

GYDIKOV,

Iustitufe

50,

G.

DIMITROV,

Physioloyg,

Bztlgariatt Received

of Motor Units Pollicis Muscle D.

KOSAROV,

Acadrmy Jamc

AND

of

Scicrtccs,

in Human

bJ. DIMITROVA Sofia,

Bulgaria

21, 1975

Studies were made on the parameters of the single contraction and the size of the summated equivalent potential determined through location of motor units in human opponens pollicis muscles upon electrical stimulation and during voluntary activity. The investigation of the single contraction parameters upon electrical stimulation was performed before and after stimulation with a frequency of 30 impulses/set for 3 min. Three types of motor units were differentiated in the opponens pollicis: fast contracting, resistant to fatigue (FR) ; slow contracting, resistant to fatigue (S) ; fast contracting, fatiguable (FF). The motor units of type FR were the smallest, those of type S were middle in size, those of type FF were the largest. During voluntary activity the lowest-threshold motor units were of types S and FR.

INTRODUCTION There is a distinct functional differentiation of the muscle fibers in mammals (13, 19, 20, 22). Muscles are heterogeneous in that they consist most homogeneous is of fibers which vary in properties, Comparatively the gastrocnemius, in which some authors find only one type of fiber (4, 17) while others establish two types of fibers (20, 22). The motor units are, however, homogeneous, i.e., they are composed of muscle fibers of one and the same type (5, 7, 16). Attempts have recently been made to find some characteristics on the basis of which to make a classification of the motor units without overlapping of the separate types. The best result in this direction was achieved by using the following two criteria: (a) character of the tetanic contraction-the tetanic contraction of some of the motor units at constant frequency of stimulation shows a tendency towards decreasing while in other motor units it does not decrease and even slightly increases ; (b) fatigue in case of continuous high-frequency stimulation-some of the motor units

Copyright All rights

Q 1976 by Academic Press. reproduction in any iorm

oi

MOTOR

UNITS

37

rapidly and considerably change their contractile abilities (the contraction force sharply decreases) while others are more resistant to fatigue. Three types of motor unit have been differentiated on the basis of these criteria (4-6). Motor units of the first type are easy fatiguable, the tetanus decreases at a constant frequency of stimulation. These motor units were found to have a short contraction time and were designated as type FF (fast twitch contracting, fast fatiguable). Motor units of the second type are relatively resistant to fatigue, but also show a decrease in tetanic contraction at a constant frequency of stimulation. Their contraction time is short and they are designated as type FR (fast twitch contracting, resistant to fatigue). Motor units of the third type are most resistant to fatigue, the tetanus does not decrease at a constant frequency of stimulation. Their contraction time is long and they are designated as type S (slow). It is interesting to look for analogous types of motor units in human muscles, and particularly to study the contraction parameters before and after electrical stimulation producing fatigue. Different methods (3, 8, 21, 23) have been proposed for measuring the contraction time of small bundles of muscle fibers and separate motor units upon electrical stimulation. In all these methods, use is made of strain gauges. The tensogram after each impulse of the motor unit studied is to be averaged to obtain the single contraction curve eliminating the noise. The time from the beginning of contraction to the maximum (contraction time), and occasionally the half-relaxation time, too, is measured. It is difficult to study the single contraction of separate motor units by these methods. During voluntary activation, only the low-threshold units might be studied, while upon electrical stimulation there is no certainty that it is just a matter of a single motor unit. According to Buchthal and Schmalbruch (3)) a contraction time shorter than 60 msec testifies to a fast motor unit, while a longer time suggests a slow unit. Sica and McComas (21) consider the contraction time of 78 msec as being the limit to the slow and fast motor units. No data are available for fatigue of human motor units upon electrical stimulation. METHODS Motor units were studied in opponens pollicis muscle of 11 clinically healthy men and women aged 19 to 45 who showed a normal EMG. The contractions and faiigue of 41 motor units upon electrical stimulation, and the contraction of nine motor units during voluntary activity were investigated. In addition, all units tested were located, and the summated equivalent potential showing the motor unit size (15) was determined.

38

GYDIKOV

ET

AL.

FIG. 1. Multi-electrode for investigating motor units in human opponens pollicis muscle upon electrical stimulation and during voluntary activity. Electrode leading off area 1 mm’.

The potentials were led off by means of surface electrodes of a small leading off area (9) and a three-channel Disa electromyograph. We used the multi-electrode shown in Fig. 1. This multi-electrode consisted of four monopolar electrodes for location which we oriented transversely to the muscle fibers by means of vectorelectromyogram (VEMG), the latter being recorded from two bipolar electrodes (14). When the position of the multi-electrode was correct, one of the electrodes used for location, and two other monopolar electrodes fell on a line parallel to the muscle fibers. Two of the channels of the electromyograph were used for recording the electrical impulses of the motor units studied, as we switched on the electrodes of the multi-electrode by means of an input switchbox. The third channel was used for recording the contraction of the motor units. To this end, the signal of a strain gauge transmitting the mechanical effect of motor unit contraction was fed to the d-c amplifier of the mean voltage unit. Furthermore, the mechanical activity and the electrical impulses led off from one of the electrodes were recorded on a Hewlet Packard tape recorder. During the experiment the subject’s left arm was positioned freely on a hard support. The last phalanx of the thumb touched an elastic plate with strain gauge. The subject was instructed to relax the muscles as completely as possible. Upon electrical stiniulatioii of separate tnotor units, a stilnulating electrode was placed on median nerve in the distal part of the forearm. The stimulating impulse had a frequency of 1 impulse/set and duration of

0.05 msec. We tried to find a suitable position of the stimulating electrode in order to stimulate only one of the axons inervating opponens pollicis. Upon threshold stimulation of the motor nerves, the evoked potential might be elicited from one or more motor units. To discriminate between the two cases,we made use of the criteria proposed by Bergmans (1, 2) which

we partly

nlodified

and supplemented:

(a)

The

potential

from

a

MOTOR

39

UNlTS

single motor unit appears and disappears according to the “all-or-none” principle upon changing the stimulus intensity below and above the threshold. (b) It is approximately within the range of the potentials from separate motor units during voluntary activity. The latter criterion in our experiments was modified, as we took not the maximally measured potential but the summated equivalent potential determined upon location (15). (c) The potential from one motor unit keeps its shape when the stimulating impulse amplitude is varied slightly which might result in variations in the latency. It is assumed that the variations in the latency are determined by stimulation of the axon through different Ranvier nodes. It is not very likely that two or more axons could give simultaneous variations in the latency, and if the variations are independent the shape of the evoked potential will change. (d) Another criterion is the potential similarity upon the so-called repetitive firing. This phenomenon is most easily obtainable at a frequency from nearly 6-8 impulses/set. It is due to an activation on the initial segment of the alpha motoneuron produced by the antidromic impulse. It is most unlikely that this phenomenon always occurs simultaneously in two or more neurons. (e) These criteria were supplemented with another one-the changes in the potential configuration at different points as regards the motor endplate area and the line of maximal potentials (linea maxima) must correspond to the changes in a single motor unit (Fig. 2). In case of more than one motor unit one might observe two lineae maximae. Although each of these criteria is necessary but not sufficient, taken together they give a high certainty in distinguishing potentials from a single motor unit. While complying with the criteria, we assumed that it was a matter of stimulation of a single motor unit. We located it, determined its size, and recorded repeatedly its contraction. In some motor units, the single conmm

A

B

FIG. 2. Changes in the shape of the potentials from one motor unit depending on the distance to the motor endplate area (A) and linea maxima (B) The distances arc given in millimeters; 30 msec duration of every record.

40

GYDIKOV

ET

At.

C

FIG. 3. Mechanogram of three motor units in human opponens pollicis before and after stimulation with a frequency of 30 impulse/set for 3 min. (A) Fast contracting, fatiguable motor unit. (B) Slow contracting unit resistant to fatigue. (C) Fast conbefore -.-.-after stimulation with a tracting unit resistant to fatigue; ___ frequency of 30 impulse/set for 3 min. The curves are obtained through averaging 50 contractions by ART 1000. Normalized amplitude.

traction curves were defined; while in others, noise from various sources was superimposed upon registration. To obtain the actual curve we averaged 15 to 50 single contractions (Fig. 3). This was done by an ART1000 analyzer at sampling time of 256 msec and 1-msec steps. The averaging was triggered by the motor unit impulse. From the averaged curves we measured the contraction time as the time from the beginning of contraction to its maximum, the half-relaxation time, and the contraction amplitude. To study the motor unit fatigue we raised the stimulation frequency to 30 impulses/set. After 3 min stimulation with this frequency we began recording again the mechanical effect of the motor ullit contraction at a frequency of 1 impulse/set. These records were processed in the same way.

MOTOR

UNITS

41

To study the single motor unit contraction during voluntary activity we trained some of the subjects to maintain only one unit (the lowest threshold unit at the moment) active for a long time. We recorded also the electrical and mechanical’ responses at a firing frequency of 8 to 12 impulses/set. Under these conditions the mechanical response was strongly affected by other mechanical oscillations. For this reason we averaged 65 single contractions. The averaging was triggered by the electrical impulses of the motor units. In a series, only impulses having an odd consecutive number were used for triggering by a simple logic circuit. We measured the contraction time and the contraction amplitude on the averaged curve providing it contained only one maximum. RESULTS Figure 3 shows the single contraction curves of three motor units from the opponens pollicis upon electrical stimulation of the median nerve before and after 3 min stimulation with a frequency of 30 impulses/set. It is evident that contraction time and half-relaxation time of the different units before high-frequency stimulation are different. Two types of motor units, fast and slow, are observed depending on the length of contraction time and half-relaxation time. The slow motor units are relatively resistant to fatigue. The changes in contraction and half-relaxation times are minimal after 3 min stimulation with a frequency of 30 impulses/set. The mean values before stimulation are: 74 msec for contraction time and 59 msec for half-relaxation time, while after the stimulation they are 75 msec for contraction time and 58 for half-relaxation time. Two groups of fast motor units were established. Those of the first group are comparatively resistant to fatigue. No appreciable changes on contraction and half-relaxation times were found after continuous highfrequency stimulation-the mean values before the stimulation were 57 msec for the former and 37 msec for the latter; the mean values after the stimulation were 57 msec for the former and 39 msec for the latter. The fast motor units of the second group were easily fatiguable. The changes in the mean values of contraction and half-relaxation times under the effect of 3 min high-frequency stimulation were considerable; from 54 to 71 mrec for contraction time and from 34 to 83 msec for half-relaxation time. These changes were statistically significant at P < 0.01. On the basis of these results, the motor units from opponens pollicis might be divided into three groups : slow resistant to fatigue ; fast resistant to fatigue; and fast fatiguable. Of the 41 motor units studied, 16 proved

42

CYDIKOV

ET

1----1h 50

4

b

0

AL.

100

150

tmr

100

10

tms

C

FIG. 4. Mechanograms of motor units in human opponens pollicis during voluntary activity. (A) Fast contracting units. (B) Slow contracting units. (C) Mechanogram with two maxima showing a tetanic frequency of firing. Such mechanograms are discarded in the present paper. The curves are obtained through averaging 65 single contractions by ART 1000. Normalized amplitude.

be slow resistant to fatigue; l-k, fast resistant to fatigue, and 11, fast fatiguable. The single contraction amplitude did not correlate with the motor unit type. However, the single contraction curve of one and the same unit might have different amplitudes depending on both the tension of the other muscles (flexors, extensors, abductors, and adactors of the thumb) and the changes in the arm position as this did not affect appreciably the length of contraction and half-relaxation times. The properties of the different motor units correlated with the size of the summated equivalent potential determined through location. The data for the different types of motor units are summarized in Table 1. During voluntary activity, the single contraction curves after averaging were different and slow and fast motor units were differentiated (Fig. 4). In some cases, we observed two maxima which suggested tetanic to

Fast resistant Slow resistant Fast fatiguable

Type

to fatigue to fatigue

0.28-1.10 0.60-l .95 2.38-5.30

Range

Summated equivalent potential hV)

CHARACTERISTIC~ALUES

0.62 1.38 3.40

Mean

OFTHE

TABLE

48-60 69-83 51-60

Range

Contraction time (msec)

PARAMETERS

1

57 74 54

Mean

OFTHE

26-45 51-81 24-52

Range

Half-relax time (msec)

37 59 34

Mean

DIFFERENTTYPES

48-63 67-81 61-81

Range

-

57 75 71

Mean

Contraction time after stim. (msec)

OF MOTORUNITS

28-4.5 52-78 SO-113

Range

Half-relax time after stim. (msec)

39 58 83

Mean

z

2 cn

is s C td c!

44

C~DIKOV

ET

AL.

contraction (Fig. 4). These cases have been excluded. From the remaining nine motor units, six were slow with contraction times of 64-76 msec, and three were fast with contraction times of 38-55 msec. Though a second maximum was lacking, it might be assumed that some interimpulse intervals had been shorter than the single contraction time. That is why the half-relaxation time was not measured. The single contraction amplitude of one and the same motor unit varied depending on the same external factors acting upon electrical stimulation too, and for this reason it will not be interpreted. The summated equivalent potential was within the range of 026-2.00 mV; 0.26-1.00 mV for the fast units, and 0.552.05 mV for the slow units. Thus it might be assumed that it is a matter of motor units of the types fast resistant to fatigue and slow resistant to fatigue. DISCUSSION In the present experiment, the criteria of Bergmans (1, 2) for differentiation of potentials from separate motor units in human muscles upon nerve stimulation have been supplemented. A new criterion has been introduced in differentiating threshold potentials from one or more motor units, namely, the shape of the volume conducted potential at a different distance from the motor endplate area and linea maxima. Furthermore, instead of comparing the evoked potential size at a given point on the surface of the muscle with the size of separate units, we compared the summated equivalent potentials obtained through location, and this proved to be a more precise evaluation. By these modified criteria, the threshold potentials have, in some cases, been considered as resulting from the simultaneous stimulation of more than one motor unit, though according to the criteria of Bergmans they should have been viewed as potentials from a single motor unit. The contraction time and the half-relaxation time obtained in our experiments upon electrical stimulation and during voluntary activity are in keeping with the data of other authors (3, 18, 21, 23). The data obtained upon electrical stimulation provided conclusive evidence about the presence of slow and fast motor units, and thus corroborated the data of Sica and McComas (21) . During voluntary activity, reliable parameters might be obtained providing the firing frequency of the motor units is subtetanic. Milner, Brown, Stein, and Yemm (18) employed a special method which allowed averaging of the mechanical effects, only if the intervals between the consecutive impulses were such that tetanus had not existed. In our experiments, the distortion provoked by tetanus was eliminated in another way. The averaging of the mechanogram was triggered, not by every motor unit impulse, but by the alternative impulses. In case of tetanus,

MOTOR

UNITS

45

the averaged curve showed two maxima. In nine motor units the second maximum was lacking in the mechanogram and only these are described in the present paper. The firing frequency of these nine units was subtetanic. We established that just as in experimental animals, the motor units might be divided in two large groups, fatiguable and resistant to fatigue, according to the changes in the averaged single contraction curve after continuous high-frequency stimulation. Through combination of both measures, contraction velocity and fatiguability, three groups of motor units were established: slow and fast resistant to fatigue; and fast fatiguable. This finding is in keeping with the data of Burke, Levine, Zajac, Tsaris, and Engel (5) for rat gastrocnemius, and thus we could adopt the designations S, FR, and FF. However, in the paper cited, basic criteria for differentiating the three types of motor unit are not fatigue and the parameters of the single contraction, but fatigue and the pattern of the tetanic contraction. Furthermore, in gastrocnemius of rat, the smallest are the motor units of type S, larger are the units of type FR, and the largest are those of Type FF (6) ; while in the human opponens pollicis the smallest are the units of type FR, larger are those of type S, and the largest are those of type FF. A difference might appear because of differences of the muscles studied as well as species differences. The method used in our experiments provided only an approximate evaluation of the motor unit size. The summated equivalent potentials might depend not only on the unit size, i.e., on the number of muscle fibers in the unit, but on other factors, too, e.g., the thickness of muscle fibers. However, the differences in the summated equivalent potentials are considerable, and the inaccuracies of the method could not change the general conclusion that the motor units of type FR in human opponens pollicis are smaller than those of type S. The measurements of the single contraction amplitude gave unambiguous results ; different amplitudes were observed with the same motor units depending on the conditions as this did not influence appreciably the time parameters of the single contraction. Using an analogous method, the evaluation of single contraction amplitude, Sica and McComas (21)) in experiments with extensor hallucis breves of man, found no correlation between the motor unit size and the contraction time. Obviously, this is due to the influence of other factors on the amplitude measured. The most important factor is the tension of other muscles attached to the same link. The fact that the changed tension does not essentially influence the time parameters suggests that these muscles act mainly as a variable elasticity overcome by the contracting motor unit.

46

GYDIKOV

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AL.

We observed a clear-cut correlation between the size of motor unit (evaluated by the summated equivalent potential upon location) and its We. The experiments during voluntary activity showed that the lowestthreshold motor units in human opponens pollicis are of types S and FR. This might be expected, bearing in mind the size of the different type of units and the fact that the low threshold motor units have a small summated equivalent potential (12). Regardless of the small number of units tested, it could be suggested that those of type S predominate since the method escludes with a higher probability the analysis of motor units of this type because they attain tetanic contraction at low frequency. Bearing in mind the size of the summated equivalent potential, we assume that the fast motor units observed during low voluntary activity can only be of type FR and not of type FF. However, the results obtained from opponens pollicis do not justify general conclusions for all human muscles. Two types of motor units in the human muscles, tonic and phasic, were differentiated in previous works of ours (10, 11). This was done on the basis of studies on the correlation between the firing frequency and the level of steady-state isometric muscle tension. When investigating the pattern of firing and the potential changes during fatigue, we foulid that the tonic motor units are resistant to fatigue, while the phasic ones are fatiguable. It was also established that the tonic motor units have a low threshold and small summated equivalent potential, while the phasic ones are large and have a high threshold. Fatigue in the present studies was judged from the changes in the mechanical contraction. Yet, it is obvious that the units of types FR and S correspond to the tonic motor units, those of type FF correspond to the phasic motor units. Taking into account the fact that when the muscle effort increases, the motor units are recruited depending on their size; it might be assumed that first to be recruited are the tonic units of types FR and S. The phasic units of type FF are recruited at high muscle tension only. REFERENCES 1. BERGMANS,

J.

1970.

“The

physiology

of

single

human

nerve

fibres.”

Vander,

Louvain. J. 1973. Physiological observations on single human nerve fibres, Vol. 2, pp. 89-127. In “New developments in electromyography and neurophysiology.” J. E. Desmedt [Ed.]. Karger, Basel. 3. BUCHTHAL, F., and H. SCHMALBRUCH. 1970. Contraction times and fibre types in intact human muscle. Acta Physiol. Scaml. 79: 435-452. 4. BURKE, R. E., D. N. LEVINE, M. SALCMAN, and P. TSAIRIS. 1974. Motor units in cat soleus muscle : Physiological, histochemical and morphological characteristics. J. Pkysiol. 238: 503-514.

2. BERGMANS,

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R. E., D. N. LEVINE, F. E. ZAJAC, P. TSAIRIS, and W. K. ENGEL. 1971. Mammalian motor units. Physiological-histochemical correlation in 3 types in cat gastrocnemius. Science 174: 709. 6. BURKE, R. E., P. TSAIRIS, D. N. LEVINE, F. E. ZAJAC, and W. K. ENGEL. 1973. Direct correlation of physiological and histochemical characteristics in motor units of cat triceps surae muscle. In “New developments in electromyography and clinical neurophysiology.” J. E. Desmedt [Ed.]. Vol. I, pp. 23-30. 7. EDSTRBM, L., and E. KUGELBERG. 1968. Histochemical composition distribution of fibers and fatigability of single motor units. J. Ncnrol. Ncnro.rurg. Psychiat. 31: 424-433. S. GATEV, V., and I. IVANOV. 1972. Excitation-contraction latency in human muscles. Agressology 13D : 7-12. 9. GYDIKOV, A., and D. KOSAROV. 1971. Vectorelectromyographic investigations of impulses from separate motor units. Communication I : Vectorelectromyographic stape of the impulses from motor units led from longitudinal lines. Electromyography 11: 429-451. 10. GYDII~OV, A., and D. KOSAROV. 1973. Physiological characteristics of the tonic and phasic motor units in human muscles, pp. 75-94. In “Motor control.” Gydikov et al. [Eds.]. Plenum Press, New York. 11. GYDIKOV, A., and D. KOSAROV. 1974. Some features of different motor units in human biceps brachii. Pj%gers Arch. 347: 75-88. 12. GYDXOV, A., D. KOSAROV, and N. TANKOV. 1972. Studying the alpha motoneuron activity by investigating motor units of various sizes. Electromyography 12: 10-28. 13. HENNEMAN, E., and C. B. OLSON. 1965. Relation between structure and function in the design of skeletal muscle. J. Ncuropltysiol. 28: 581-598. 14. KOSAROV, D., A. GYDIKOV, and N. TANKOV. 1972. Vectorelectromyographic shape of the impulses from mo:or units led from transverse lines and from longitudinal and transverse planes. Electromyograjhy 12 : 29-51. 15. KOSAROV, D., A. GYDIKOV, and N. TANKOV. 1974. Improvement of the method of motor units location. Electromyogr. Clin. Neurofihysiol. 14 : 97-107. 16. KUGELBERG, E., and L. EDSTR~M. 1968. Differential histochemical effects of muscle contraction on phosphorylase and glycogen in various types of fibres, relation to fatigue. I. Neural. Neurosurg. Psychiat. 31: 41.5-423. 17. MCPHEDRAN, A. M., R. B. WUERKER, and E. HENNEMAN. 1965. Properties of motor units in a homogeneous red muscle (soleus) of the cat. 1. NeurophysioZ. 28: 71-84. 1% MILNER-BRAWN, H. S., R. B. STEIN, and R. JEMM. 1973. The contractive properties of human motor units during voluntary isometric contractions. J. Ph~~iol. 228 : 285-306. 10. NACHMIAS, V. T., and H. A. PADYKULA. 1958. A histochemical study of normal and denervated red and white muscles of the rat. J. Biophys. Biochem. Cytol. 4: 47-54. 20. OGATA, T., and M. MORI. 1964. Histochemical study of oxidative enzymes in vertebrate muscle. Z. Histochem Cytochcm. 12 : 171-182. 21. SICA, R. E. P., and A. J. MCCOMAS. 1972. Fast and slow twitch units in a human muscle. J. Neuvol. Neurosurg. Psychiat. 34: 113-120. 22. STEIN, J. M., and H. A. PADYKULA. 1962. Histochemical classification of individual skeletal muscle fibers of the rat. Amer. J. Anat. 110: 103-124. 23. STEIN, R. B., A. S. FRENCH, and R. JOMM. 1972. New methods for analyzing motor functions in man and animals. Brain Res. 40: 187-192. 5. BURICE,