Comparison of Static Fusimotor Innervation in Cat Peroneus Tertius and Longus Muscles

Françoise Emonet-Dénand, Yves Laporte, and Julien Petit

Laboratoire de Neurophysiologie, Collège de France, 75231 Paris Cedex 05, France

    ABSTRACT
Abstract
Introduction
Methods
Results
Discussion
References

Emonet-Dénand, Françoise, Yves Laporte, and Julien Petit. Comparison of static fusimotor innervation in cat peroneus tertius and longus muscles. J. Neurophysiol. 80: 249-254, 1998. Static fusimotor innervation was compared in cat peroneus longus and tertius muscles because the gamma  to spindle ratio is considerably higher in the longus (~60 gamma  axons for 17 spindles) than in the tertius (~24 gamma  axons for 14 spindles). Single gamma  axons were identified as static (gamma s) by their typical effects on the response of primary ending to ramp stretch. The intrafusal muscle fibers that single gamma s axons activated in the spindles they supplied were identified by the features of cross-correlograms between Ia impulses and stimuli, at 100 Hz, and by those of primary ending responses during stimulation at 30 Hz. In each experiment, a large proportion of the gamma  population was tested on about nine spindles. A statistical analysis was used to estimate the number of spindles supplied by single gamma s axons and the proportion of gamma s axons that supply only one spindle among those the stimulation of which had activated either bag2 or chain fibers alone in a single spindle. In peroneus longus, nearly all gamma s axons supply one or two spindles, whereas in peroneus tertius, the majority of gamma s axons supply from three to six spindles. The proportion of nonspecifically distributed gamma s axons, i.e., of axons that supply both bag2 fibers and chain fibers either in the same or in different spindles, is much lower (56%) in the longus than in the tertius (83%) as previously observed on a population of gamma s axons that supplied from three to six spindles. Correspondingly, the proportion of specific axons is much higher in the longus (44%) than in the tertius (17%). In none of the two muscles was a strict relationship observed between the conduction velocity of gamma s axons and their intrafusal distribution (specific bag2, specific chain fibers, nonspecific). However, gamma s supplying bag2 fibers either specifically or in combination with chain fibers tended to have faster conduction velocities, which suggests that, in various motor acts, the proportion of activated bag2 and chain fibers may be related to the proportions of activated fast and slow gamma s axons.

    INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References

It is well established that the static fusimotor control of spindle activity is exerted through two very different types of intrafusal muscle fiber, the slow contracting nuclear bag2 fibers and the fast contracting nuclear chain fibers, but uncertainty persists regarding the way static gamma  axons (gamma s) are distributed to these fibers, possibly because most studies on this question have been carried out on a relatively small number of axons and in few muscles (for review, see Banks 1994).

Recently a quantitative analysis of the intrafusal distribution of static gamma  axons was made in cat peroneus tertius muscle because, in this small muscle, it was possible to observe the actions of a large number of single gamma s axons on about three-quarters of the spindle population. Using the method of Celichowski et al. (1994) for identifying the type of static effectors activated by single static gamma  axons, all three possible patterns of innervation of individual spindles were observed: bag2 fibers alone, chain fibers alone, and bag2 and chain fibers together. In most cases, these patterns varied among the spindles supplied by each static gamma  axon, showing that, in this muscle, the great majority of static gamma  axons are nonspecifically distributed to bag2 and chain fibers.

On average a peroneus tertius muscle contains 14 spindles (Scott and Young 1987) supplied by 24 gamma  axons (Horcholle-Bossavit et al. 1988). Because about three-quarters of these axons are static, the ratio of the number of static gamma  axons to the number of spindles is only slightly larger than 1 (1.2). In peroneus longus muscles, which on average contain 17 spindles (Scott and Young 1987) supplied by 60 gamma  axons (Horcholle-Bossavit et al. 1988), the gamma  to spindle ratio is markedly larger (2.6). This difference led us to compare the static gamma  innervations in the two muscles with the aims, first, of determining whether or not peroneus longus gamma s axons were all specifically distributed (some to bag2, the others to chain fibers) and, second, if it was not the case and specifically and nonspecifically distributed gamma s axons coexisted in peroneus longus, whether their proportions would compare with those observed in peroneus tertius. The number of spindles supplied by single static gamma  axons and the conduction velocities of different sorts of gamma s axons also were compared.

    METHODS
Abstract
Introduction
Methods
Results
Discussion
References

Experiments were carried out on adult cats (2-2.8 kg) anesthetized with pentobarbital sodium (Nembutal, 35 mg/kg) supplemented intravenously as required. Most of the techniques used in this study have been described fully in previous papers, especially in one on the distribution of peroneus tertius static gamma  axons (Celichowski et al. 1994). Static gamma  axons (gamma s) were identified by the typical changes their repetitive stimulation elicited in the response of primary endings to ramp stretch (Crowe and Matthews 1964; Emonet-Dénand et al. 1977). Static innervation was more difficult to study in peroneus longus than in peroneus tertius because, first, most peroneus longus gamma  axons supply a much smaller number of spindles than peroneus tertius gamma  axons (Brown and Butler 1975; Gioux and Petit 1993; Petit et al. 1983) and, second, because it was generally not practicable to study in each of the five experiments the actions exerted by more than one-half of the gamma population (~30 axons) on about one-half of the spindle population (9 spindles). Consequently, a statistical analysis was developed for estimating the frequencies of occurrence of gamma  axons supplying different numbers of spindles (see further text).

Identification of intrafusal muscle fibers activated by gamma s axons

This was made with the Celichowski et al. (1994)'s method, which rests on cross-correlograms between stimuli at 100 Hz and Ia afferents impulses and on the features of primary ending responses during stimulation at 30 Hz.

At 100 Hz, the contraction of the very fast contracting chain fibers is not completely fused. Therefore when chain fibers are activated in the studied spindle (either alone or with bag2 fibers), each periodical increase in chain fiber unfused tetanic contraction increases the probability of a Ia impulse occurring after a relatively constant delay; this results in a significant peak in cross-correlograms between stimuli and Ia impulses. When only the much slower contracting bag2 fibers are activated at 100 Hz, no such peaks are observed because the contraction of these fibers is completely fused.

At 30 Hz, the stimulation of chain fibers elicits either a one-to-one driving of the primary ending discharge (1 afferent impulse being elicited with a constant delay after each gamma  impulse) or a very irregular increase in the discharge of the ending with minimal instantaneous frequency that is close to that of the stimulation. On the other hand, when bag2 fibers alone are activated at that frequency, their nearly fused contraction elicits a sustained and moderate increase in the instantaneous frequency. Coactivation of chain and bag2 fibers elicits an irregular discharge the minimal instantaneous frequency of which is well above that of the stimulation (see Fig. 2 in Celichowski et al. 1994).


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FIG. 2. Intrafusal effects elicited by static gamma  axons of different conduction velocities observed in 1 experiment on peroneus tertius muscle (top) and in 1 experiment on peroneus longus muscle (bottom). Classes of 2.5 m/s. Each box indicates the type of effect elicited by a single gamma  in 1 of the sampled spindles. black-square, square , and , effects due to chain fibers alone, to bag2 fibers alone, or to chain and bag2 fibers, respectively. A line joining several boxes indicates that the same axon supplied several spindles. In the peroneus tertius experiment, 37 effects elicited by 14 gamma s axons on 11 spindles were identified. In the peroneus longus experiment, 39 effects elicited by 28 gamma s axons on 9 spindles were identified. Dynamic axons (not illustrated) also were observed: 5 in the peroneus tertius (conduction velocities 22, 23, 27, 31, and 39 ms) and 4 in the peroneus longus (conduction velocities: 21, 25, 27, and 30 m/s).

Statistical analysis

The statistical analysis described below was developed by one of us (Petit) to estimate, from collected experimental data, the number of spindles supplied by individual gamma  axons and the proportions among certain axons (see RESULTS) of gamma s axons that supplied only one spindle.

During an experiment, among the N spindles of the muscle, a sample of ne spindles (Ia fibers) was prepared. Then the numbers ni of single gamma  axons that activated i spindles in the sample were determined; i could vary from 0 to imax. (It was found that imax = 4 for the peroneus longus muscle and imax = 6 for the peroneus tertius muscle.) The sets of numbers ni obtained in the different experiments were compared using a nonparametric test (Kruskal-Wallis) and a parametric test (LSD, least significant differences). The sets that could be considered to belong to a single population with a 95% confidence level were pooled, giving a single set of numbers ni for the muscle under study (1 experiment on peroneus tertius had to be discarded because in this experiment most gamma  axons activated none or only 1 spindle). The frequency that single gamma  axons activated i spindles in the observed sample was then
<IT>f<SUB>i</SUB></IT> = <FR><NU><IT>n<SUB>i</SUB></IT></NU><DE><IT>Ng</IT></DE></FR>
where Ng was the total number of gamma  axons studied in all the experiments. f0 was the frequency of "inactive" gamma  axons that did not activate any spindle in the sample.

The inactive axons were observed randomly during the experiment and not particularly toward the end that would have suggested fusimotor failure. The stimulation of nearly all inactive axons was seen to increase the spontaneous afferent discharges, which were monitored continuously in the muscle nerve. As the number of Ia fibers in these thin nerves is small, the activation and/or the increase in activity of only one or two Ia afferent fibers was detected readily. In the few instances in which an increase was not obvious, it was assumed that these axons had a weak action on one or two spindles. However, it cannot be excluded that fusimotor failure, as reported by Brown et al. (1969), was responsible for the apparent ineffectiveness of some axons but the probability of such an occurrence seemed low.

Those inactive gamma  axons presumably included a small proportion of dynamic gamma  axons; this was probably very small for two reasons: the proportion of dynamic axons in our experiments was small (13% for the peroneus longus) and the probability that a dynamic gamma axon did not activate a spindle in the sample was lower than that for static axons because dynamic gamma  axons usually supply more spindles than static gamma  axons. For these reasons, it was considered that errors due to dynamic axons were inside the confidence interval (see further).

The frequencies fi were used to calculate confidence intervals for the probabilities pai. (see Walpole et al. 1998). The probability that a gamma  axon activated i spindles in the sample was pai and the probability that a gamma  axon did not activate i spindles was (1 - pai). As the mean value of the frequency was pai, as the standard deviation for pai was <RAD><RCD><IT>p<SUB>ai</SUB></IT>(1 − <IT>p<SUB>ai</SUB></IT>)</RCD></RAD>, and as Ng was large enough, we could write
probability &cjs0362;‖<IT>f<SUB>i</SUB> − p<SUB>ai</SUB></IT>‖ < <FR><NU><IT>h</IT><RAD><RCD><IT>p<SUB>ai</SUB></IT>(1 − <IT>p<SUB>ai</SUB></IT>)</RCD></RAD></NU><DE><RAD><RCD><IT>Ng</IT></RCD></RAD></DE></FR>&cjs0363; = <FR><NU>1</NU><DE><RAD><RCD>2π</RCD></RAD></DE></FR><LIM><OP>∫</OP></LIM><SUP><IT>h</IT></SUP><SUB>−<IT>h</IT></SUB><IT>e</IT><SUP>−<IT>s<SUP>2</SUP></IT></SUP><SUP>/2</SUP>d<IT>s</IT>
This probability could be calculated as a function of h and the inequality could be rewritten (fi - pai)2 < (h2/Ng)pai(1 - pai). Therefore pai had the above probability, function of h, to be inside the ellipse (fi - pai)2 = pai(1 - pai)h2/Ng.

Using this equation, for each value of fi, two values of pai could be calculated that were the upper and lower limits of the confidence interval. h = 2 for a 95% confidence level, and h = 1 for a 85% confidence level. It should be noted that for a frequency fi = 0, the probability pai could be different from zero. imax was the maximal number of spindles in the sample activated by a single gamma  axon. We assumed that the maximal number of spindles in the muscle supplied by a single gamma  axon was m = imax + 1. Therefore the frequency fm = 0, but we could write the relation
<IT>p<SUB>am</SUB> = p<SUB>sm</SUB>p<SUP>m</SUP></IT>
pam probability that a gamma  axon activated m spindles in the sample. psm probability that a gamma  axon supplied m spindles in the whole muscle. p = ne/N probability that a spindle was in the sample (pm was the probability that the m spindles were in the sample).

When a gamma  axon activated (m - 1) spindles in the sample, either the gamma  axon supplied (m - 1) spindles in the whole muscle and the (m - 1) spindles were in the sample or the gamma  axon supplied m spindles but one spindle was not in the sample. For the probabilities, the relation could be written
<IT>p<SUB>a<UP>(</UP>m<UP>−1)</UP></SUB></IT> = <IT>p<SUB>s<UP>(</UP>m<UP>−1)</UP></SUB>p<SUP>m<UP>−1</UP></SUP></IT> + &cjs0358;<AR><R><C><IT>m</IT></C></R><R><C><IT>m</IT> − 1</C></R></AR>&cjs0359;<IT>p<SUB>sm</SUB>p<SUP>m<UP>−1</UP></SUP></IT>(1 −<IT>p</IT>)
pa(m-1) probability that a gamma  axon activated (m - 1) spindles in the sample. ps(m-1) probability that a gamma  axon supplied (m - 1) spindles in the whole muscle. (1 - p) probability that a spindle was not in the sample. More generally
<IT>p<SUB>ai</SUB></IT> = <IT>p<SUP>i</SUP></IT><LIM><OP>∑</OP><LL><SUB><IT>k=</IT>i</SUB></LL><UL><SUP><IT>m</IT></SUP></UL></LIM>&cjs0358;<AR><R><C><IT>k</IT></C></R><R><C><IT>i</IT></C></R></AR>&cjs0359;<IT>p<SUB>sk</SUB></IT>(1 − <IT>p</IT>)<SUP><IT>k−i</IT></SUP>
pai probability that a gamma  axon activated i spindles in the sample. psk probability that a gamma  axon supplied k spindles in the whole muscle. i could vary from 0 to m.

It was assumed that a gamma  axon supplied at least one spindle in the whole muscle. Therefore ps0 = 0
&cjs0358;<AR><R><C><IT>k</IT></C></R><R><C><IT>i</IT></C></R></AR>&cjs0359; = <FR><NU><IT>k</IT>!</NU><DE><IT>i</IT>!(<IT>k − i</IT>)!</DE></FR>
pai and psi were calculated solving the system of equations. Several conditions had to be fulfilled: pai had to be inside the confidence intervals determined with the experimental frequencies fi. (The fi were not a unique solution); psi >=  0; and Sigma mi=0 pai. It could be verified that if this condition is true Sigma mi=1 psi = 1 is also true.

The number of solutions for the system depended on the confidence intervals for the pai. the value of h was decreased to decrease the width of the confidence intervals and consequently to decrease the number of solutions. When the number of solutions was low enough (<50) the mean values of psi were calculated. These mean values are displayed on the histograms as well as the standard deviations of psi values.

The shape of the histogram depended on the number N of spindles in the whole muscle. N was relatively constant in our experiments because the frequencies pi could be considered to belong to a same population. We used for N the mean values determined in histological works.

    RESULTS
Abstract
Introduction
Methods
Results
Discussion
References

Number of spindles innervated by single static gamma  axons

In peroneus longus, of 153 single gamma  axons prepared in five experiments, 92 were identified as static and 14 as dynamic. The action of 47 axons could not be identified because their stimulation accelerated none of the discharges of the nine Ia fibers prepared in each experiment (inactive axons).

Among the 92 static gamma  axons, 64 were observed to activate only one spindle of the studied sample, 24 activated two spindles, 3 activated three spindles, and 1 activated four spindles. The frequencies of occurrence of static gamma  axons that supplied from one to five spindles in this muscle (Fig. 1, top, black-square) were estimated by a statistical analysis (see METHODS) from the frequencies of occurrence of axons observed to activate either zero, one, two, three, or four spindles (). In this analysis on 139 gamma  axons (92 gamma s + 47 inactive axons), the inactive axons were treated as if they were all static for two reasons: the proportion of identified dynamic gamma  axons observed in this study was low (13%) and the probability of finding inactive dynamic axons is lower than that of finding inactive static axons because dynamic gamma  axons generally supply a larger number of spindles than most static axons. The errors resulting from that simplification were considered as being within the confidence intervals (see METHODS).


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FIG. 1. Estimated frequency of static gamma  axons supplying a various number of spindles in peroneus longus and in peroneus tertius muscles (black-square). Frequency of occurrence, fi, (see METHODS) of gamma s axons that were observed to activate a various number of spindles (from 0 to 4 in peroneus longus and from 0 to 6 in peroneus tertius; ). Vertical bars in , confidence intervals (see METHODS); vertical bars in black-square, SD.

In peroneus tertius of 90 single gamma  axons prepared in six experiments (from Celichowski et al. 1994), 59 were identified as static, 22 as dynamic, and only 9 were inactive. Among the 59 static axons, 15 were observed to activate one spindle, 9 activated two spindles, 19 activated three spindles, 9 activated four spindles, 6 activated five spindles, and 1 activated six spindles. The estimated frequencies of occurrence of single static gamma  axons supplying from one to seven spindles are indicated in Fig. 1, bottom, black-square. In marked contrast with peroneus longus, in which nearly all gamma  static axons supply either one or two spindles, the majority of peroneus tertius static gamma  axons supply from three to five spindles but there is also a noticeable proportion of axons that supply only one spindle.

Intrafusal distribution of static gamma  axons

The distribution of single gamma s axons to intrafusal muscle fibers (bag2 and chain fibers) was determined in each of the spindles they activated. All three possible patterns of innervation were observed in individual spindles: bag2 fiber alone, chain fibers alone, and bag2 and chain fibers together. In this last case, there were large variations in the proportions of the actions exerted by the two types of fibers as previously reported (Celichowski et al. 1994).

In peroneus longus, among the 64 gamma s axons that were observed to activate only one spindle in the examined sample, 46 supplied only one type of intrafusal muscle fiber, either chain fibers (37) or bag2 fibers (9). Calculated probabilities (see METHODS) indicated that, of these 46 axons, 22 axons did supply only one spindle in the muscle, whereas the 24 others supplied more than one spindle. Therefore 22 axons were maintained in the group of axons that could be classified because of their known distribution in that single spindle, whereas 24 axons were not considered further because there was no way of determining their complete distribution.

The original group of 92 gamma s axons thus was reduced to 68. Of these 68 axons, 38 (56%) were classified as nonspecifically distributed and 30 (44%) as specifically distributed. Among these axons, eight were observed to activate the same type of intrafusal muscle fibers in two spindles (bag2 in 2 instances, chain fibers in 6 instances) and were classified as specifically distributed because, in peroneus longus, gamma  axons that supply more than two spindles are extremely rare. The 22 other axons were classified as specifically distributed, either to bag2 fibers (4 axons) or to chain fibers (18 axons) because either type of fibers alone were activated in the single spindles these axons were estimated to supply.

Thus in peroneus longus the proportions of nonspecific axons (56%) and of specific axons (44%)---including 9% bag2 and 35% chains---are very different from those observed in peroneus tertius. Of 42 peroneus tertius gamma s axons that supplied from three to six spindles, 35 (83%) were found to be distributed nonspecifically and only 7 (17%) to be distributed specifically (Celichowski et al. 1994).

Conduction velocities of static gamma  axons

No strict relationship was found between the type of distribution of static axons (either to bag2 only, to chain fibers only, or to bag2 and chain fibers together) and their conduction velocity. However, it was observed in both muscles that the conduction velocity of gamma s axons that supplied bag2 fibers, either specifically or in combination with chain fibers, tended to be faster than the conduction velocity of the other axons.

In some experiments this tendency was particularly clear as illustrated by the histograms of conduction velocities of 14 gamma s axons studied in an experiment in peroneus tertius (Fig. 2, top) and of 28 gamma s axons studied in an experiment in peroneus longus (Fig. 2, bottom).

Both histograms show that bag2 effects are elicited predominantly by fast-conducting axons, although slow axons also may elicit bag2 effects. Conversely chain effects are elicited predominantly by the slowest axons.

In Fig. 3, the conduction velocities of 12 peroneus tertius gamma  axons that supplied bag2 fibers in at least three spindles either alone or with chain fibers (2 and 10, respectively) are compared with those of 30 other axons that include specific chain axons and nonspecific axons with occasional bag2 fibers.


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FIG. 3. Conduction velocities of peroneus tertius static gamma  axons. Classes of 2.5 m/s. Top: axons innervating bag2 fibers in >= 3 spindles, either alone (2 axons) or with chain fibers (10 axons). Bottom: other axons (see text).

The conduction velocities of the axons of the first group ranged from 29 to 44 m/s (mean conduction velocity 35 ± 4.4 SD), whereas those of the axons of the second group ranged from 22 to 40 m/s (mean conduction velocity: 29 ± 4.4 SD). The variances for the two groups are not significantly different (F test), but the two means are significantly different (t-test, 95% confidence level).

In peroneus longus, when all gamma s axons are considered together, this tendency is blurred because of large individual variations in the conduction velocity ranges of these axons and in the numbers of active axons prepared in each experiment. Moreover, although the statistical analysis (see preceding text) allowed the inclusion in the group of classifiable axons of nearly one-half of the axons that activated either bag2 or chain fibers alone in a single spindle, they could not be individually identified; therefore no conduction velocities could be ascribed to these axons.

For these reasons, conduction velocities of all active gamma s axons observed in each of the five experiments are presented in the histograms of Fig. 4. It can be seen that, in spite of the individual differences, axons that supply bag2 fibers either alone (square ) or with chain fibers () tend in each experiment to be faster than those that supply only chain fibers (black-square).


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FIG. 4. Conduction velocities of peroneus longus static gamma  axons observed in 5 experiments (A-E). Classes of 2.5 m/s. black-square, square , and , axons that were observed to activate either chain fibers alone, bag2 fibers alone or chain, and bag2 fibers together respectively, either in only 1 of the sampled spindles (boxes with no dots) or in >1 spindle (boxes with a dot).

    DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References

The intrafusal distribution of static gamma  axons was compared in cat peroneus longus and tertius muscles because there are almost three times as many gamma  axons in the longus than in the tertius although the spindle contents of the two muscles are comparable. Large differences were observed. First, nearly all static gamma  axons in peroneus longus supply one or two spindles, whereas most tertius static axons supply from three to six spindles. Second, the proportion of specifically distributed axons (that is, axons supplying either bag2 or chain fibers alone) was much higher in the longus (44%) than in the tertius (17%), and correspondingly the proportion of nonspecifically distributed axons was lower (56%) in the longus than in the tertius (83%). The peroneus tertius figures are based on the axons supplying from three to six spindles studied by Celichowski et al. (1994). If peroneus tertius gamma  axons supplying one or two spindles had been included in this study, the percentage of specific axons would have been only slightly larger because these axons represent ~20% of the gamma s population, and the conclusion that the great majority of gamma  static axons in the tertius are non specifically distributed would not have been significantly affected. In neither muscle were static gamma  axons found to be distributed in only two groups predominantly supplying either bag2 or chain fibers respectively as proposed by Boyd (1986).

The much larger gamma  to spindles ratio found in the longus does not allow by itself to predict the distribution of gamma s axons because, theoretically, static gamma  axons, whatever their number, could all be distributed either specifically or nonspecifically. However, the fact that the proportion of specifically distributed axons is much higher in the longus than in the tertius raises the possibility that the degree of specificity is determined probabilistically, at least in part. The gamma  to spindles ratios in longus and tertius are among the largest and the smallest, respectively, of all such ratios so far measured (Boyd and Davey 1968). If in several muscles with ratios falling between these extreme values, the proportions of specifically distributed axons consistently were found to be related to the ratios, the probabilistic nature of the gamma s distribution then would be strongly supported. However, it is likely that other factors contribute to the distribution. This is suggested, for instance, by the slight tendency toward segregated innervation of bag2 and chain fibers that was previously observed in individual spindles in the tertius (Celichowski et al. 1994) and that also was found in the longus (unpublished observations). This segregation is not in itself an evidence for the existence of specifically distributed axons because, in the tertius for example, most static gamma  axons were observed to be distributed randomly to one or other fiber types or both among the several spindles (from 3 to 6) individual axons supplied (Celichowski et al. 1994).

The flexion-abduction of the foot elicited by the longus would seem to necessitate a more precise control than the extension-abduction of the fifth digit elicited by the tertius. The features of the static innervation of the longus spindles should contribute to this finer control because a large number of static gamma  axons and a high proportion of specifically distributed axons suggest a finer control of the sensory endings discharges than in tertius spindles. However, other factors should be considered such as the beta  innervation, which is particularly developed in tertius spindles (Jami et al. 1982) and perhaps could compensate for the comparatively low number of gamma  axons.

No strict relationship was found between the conduction velocities of axons and their distribution to bag2 fibers only, chain fibers only, or bag2 and chain fibers together. However, in both, muscles gamma s axons that supplied bag2 fibers, either alone or in combination with chain fibers, tended to have faster conduction velocities (i.e., a larger axonal diameter) than other axons, in agreement with previous reports (Banks 1991; Emonet-Dénand and Gladden 1993) and with observations in tibialis posterior showing that slow gamma  axons preferentially supply chain fibers (Brown et al. 1965). If such a tendency exists in all muscles, the proportion of bag2 and chain fibers activated in various motor acts might be determined by the proportion of activated fast and slow gamma  axons, whether the recruitment of their respective motoneurons is related or not to the size of these neurons.

The proportion of activated bag2 and chain fibers of course can be modified by the recruitment in various proportions of specific gamma  motoneurons, but it should not be overlooked that chain action also could be substantially enhanced by a large increase, even temporary, in the frequency of discharge of the many gamma  motoneurons that supply both bag2 and chain fibers. The contraction of the relatively slow bag2 fibers will not increase beyond 50-60 Hz, whereas that of the much faster twitch chain fibers will continue to increase up to frequencies as high as 150-180 Hz.

The complementarity of the actions exerted by bag2 and chain fibers recently was shown in peroneus tertius spindles during sinusoidal muscle stretch of linearly increasing frequency (Emonet-Dénand et al. 1997), but most of the functional consequences of this coactivation remain to be elucidated.

    ACKNOWLEDGEMENTS

  The authors thank Drs. R. Banks and L. Jami for critical reading of the manuscript and S. de Saint Font for preparing it.

  This work was supported by the Association Française contre les Myopathies and the Fondation pour la Recherche Médicale.

    FOOTNOTES

  Address reprint requests to Y. Laporte.

  Received 15 January 1998; accepted in final form 2 April 1998.

    REFERENCES
Abstract
Introduction
Methods
Results
Discussion
References

0022-3077/98 $5.00 Copyright ©1998 The American Physiological Society