The Journal of Neurophysiology Vol. 77 No. 3 March 1997, pp. 1425-1431
Copyright ©1997 by the American Physiological Society

Functional Consequences of Bag₂ and Chain Fiber Coactivation by Static -Axons in Cat Spindles

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. Functional consequences of bag₂ and chain fiber coactivation by static -axons in cat spindles. J. Neurophysiol. 77: 1425-1431, 1997. A study of the distribution in cat peroneus tertius spindles of 42 single static -axons was recently carried out with a physiological method for identifying the intrafusal muscle fibers supplied by single -axons. It was found that 35 axons (83%) supplied both slow-contracting bag₂ fibers and fast-contracting chain fibers. The distribution of these axons generally varied from one spindle to another among all the spindles that each of them supplied (bag₂ and chain fibers together, bag₂ alone, chains alone). To find some functional consequences of this coactivation, responses of primary endings to sinusoidal stretch of the muscle (amplitude 0.5-1 mm, frequency linearly increasing from 0.6 to 8-9 Hz in 12 s) were recorded at different average muscle lengths (0.5, 1.0, and 1.5 mm shorter than maximal physiological length) in nembutalized cats during repetitive stimulation at 10, 20, and 30 Hz of single -axons previously shown to supply bag₂ and chain fibers in the spindles bearing the primary endings. These responses were compared with responses elicited in passive spindles and during activation of either bag₂ fibers or chain fibers alone. Several records of discharge frequency were averaged. During stimulation at 30 Hz of -axons coactivating bag₂ and chain fibers, the averaged discharge of primary endings became continuous (that is, without interruption during each shortening phase as occurs in passive spindles) over the whole range of stretch frequencies. The modulation of the discharge was roughly sinusoidal, with an amplitude that increased with the stretch frequency. Stimulation at 30 Hz of -axons activating bag₂ fibers alone elicited a modulation of comparable shape and amplitude but only in the range of sinusoidal stretch from 0.6 to 3-4 Hz. Stimulation at 30 Hz of -axons activating chain fibers alone elicited for each cycle in the range of 0.6 to 5-6 Hz a distorted modulation of large amplitude with a minimal frequency close to that of the stimulation. The average muscle length did not significantly influence these various responses. In summary, the coactivation of bag₂ and chain fibers, at presumed physiological frequencies, enables primary endings to continuously signal changes of length over a large range of stretch velocities independently of the average muscle length.

	INTRODUCTION

Abstract Introduction Methods Results Discussion References

In cat spindles the intrafusal distribution of static -axons to nuclear bag₂ fibers and nuclear chain fibers generally varies from one spindle to another. The three possible distributions (to bag₂ and chain fibers together, to bag₂ fibers alone, and to chain fibers alone) are combined in different ways among all the spindles that individual static -axons generally supply. This was observed in tenuissimus muscle by Barker et al. (1973) and by Banks (1991) and in peroneus tertius muscle by Celichowski et al. (1994). In that recent study, 35 of 42 static -axons were found to be nonspecifically distributed and only 7 were found to be specifically distributed (5 to chain fibers and 2 to bag₂ fibers). Therefore, when the ensemble of the nonspecifically distributed static -axons is considered, with all their individual differences, it appears that they control the activity of spindle sensory endings through the coactivation of bag₂ and chain fibers. These intrafusal muscle fibers differ in their morphology and structure [see recent review by Barker and Banks (1994)]; in their contraction, which is much faster in chain than in bag₂ fibers (Bessou and Pagès 1975; Bessou et al. 1968; Boyd 1976); and in their mode of activation, generally by action potentials that produce twitches in chain fibers and by local potentials which produce focal contraction in bag₂ fibers (Barker et al. 1978; Bessou and Pagès 1972).

Primary ending discharges elicited either by the coactivation or by the selective activation of bag₂ and chain fibers have been recorded during ramp stimulation of -axons and during repetitive stimulation of -axons with ramp-and-hold muscle stretch (Boyd and Ward 1982; Boyd et al. 1985a,b; Celichowski et al. 1994; Dickson et al. 1993). The responses obtained in these particular experimental conditions are clearly very different, but they give few clues as to the functional consequences that coactivation of bag₂ and chain may have.

In the present study the spindle bearing muscles (cat peroneus tertius) were submitted to a sinusoidal stretch whose frequency linearly increased from 0.6 to 8-9 Hz. It was assumed that some functional consequences of coactivating such different muscle effectors would be more easily disclosed by analyzing primary ending responses to a stretch with a large range of velocities. Responses observed during coactivation of bag₂ and chain fibers were compared with those observed in passive spindles (that is, without activation of intrafusal muscle fibers by their motor supply). They were also compared with those observed during selective activation of either bag₂ or chain fibers so as to determine the contribution of each kind of muscle fiber in the response observed during their coactivation. Preliminary reports of this work have been published in abstract form (Emonet-Dénand et al. 1995a,b).

	METHODS

Abstract Introduction Methods Results Discussion References

Preparation

Eight experiments were carried out on peroneus tertius spindles of adult cats (2-2.5 kg) anesthetized with 35 mg/kg ip Nembutal (pentobarbital sodium, Sanofi Laboratories, Libourne, France) supplemented intravenously as determined to be required by monitoring the shape of the pupils, the arterial blood pressure, and the pinna reflexe. In each of the cats, after an extensive hindlimb denervation, several functionally single Ia fibers (up to 10) were prepared by splitting L₇-S₁ dorsal roots in filaments so as to record the instantaneous frequency of discharge of each one's primary ending. Single -axons were then prepared by splitting L₇-S₁ ventral roots in filaments whose stimulation elicited all-or-none action potentials in the nerve to the peroneus tertius. Most of these axons were readily identified as static by the very strong acceleration (or by the driving) of primary ending discharge elicited by repetitive stimulation at 100 Hz. The characteristic alterations of ramp-and-hold responses elicited by stimulation at that frequency were also observed.

The identification of the type(s) of intrafusal muscle fiber supplied in a particular spindle by a single static -axon was carried out with the method of Celichowski et al. (1994). It rests on the features of primary ending responses to a constant stimulation at 30 Hz and on cross correlograms between stimuli and Ia impulses calculated on-line during constant stimulation at 100 Hz. These frequencies were chosen to take advantage of the different contractile properties of bag₂ and chain fibers. At 30 Hz, the contraction of bag₂ fibers is nearly completely fused, whereas that of the much faster chain fibers still presents large oscillations. The 100-Hz frequency is far beyond the fusion frequency of bag₂ contraction, but it is still lower than that of chain fibers, with the consequence that each oscillation in their contraction may activate the primary ending and generate an afferent Ia impulse after a nearly constant delay. Figure 1 shows examples of responses observed when either bag₂ fibers alone (left), chain fibers alone (middle), or bag₂ and chain fibers together (right) are activated. At 30 Hz, selective bag₂ activation elicits a sustained and generally regular increase in the primary ending discharge frequency, whereas selective activation of chain fibers elicits a very irregular increase in discharge frequency that periodically falls to a value close to that of the stimulation or a 1:1 driving of the discharge (not illustrated). Coactivation of bag₂ and chain fibers gives a compound response consisting of a very irregular discharge frequency superimposed on a constant level whose frequency is well above that of the stimulation. Cross correlograms also show clear differences. When bag₂ fibers alone are activated, different values of time intervals between stimuli and Ia impulses are evenly represented, whereas when chain fibers alone are activated, time intervals around only one value are found, as indicated by a large peak (this peak is sometimes followed by a smaller and broader peak because a very strong twitch in chain fibers may initiate 2 afferent impulses, an early impulse in the rising phase of the twitch and a 2nd impulse occurring at a later and less constant time interval). When bag₂ and chain fibers are coactivated, time intervals of different values are observed as well as a smaller but significant peak. The responses due to the coactivation of the two types of intrafusal muscle fibers vary quantitatively with the relative strength of the chain and bag₂ fibers activated by single -axons (see Fig. 5 in Celichowski et al. 1994). In the present study only those axons giving responses clearly indicative of a strong activation of both types of fibers were used and served as a model of coactivation.

View larger version (13K): [in this window] [in a new window]   FIG. 1. Identification of intrafusal muscle fibers supplied by single static -axons in individual spindles. Responses to -stimulation at 30 Hz (middle) and correlograms of time intervals between stimuli and Ia impulses calculated during -stimulation at 100 Hz (bottom). Left: activation of bag2 fibers only. Middle: activation of chain fibers only. Right: coactivation of bag2 and chain fibers.

View larger version (51K): [in this window] [in a new window]   FIG. 5. Comparison, in the same spindle, of the modifications of a primary ending response elicited by bag2 and chain fiber coactivation with the responses elicited by separate activation of either bag2 or chain fibers. A: passive spindle (2 records averaged). B-D: stimulation at 30 Hz. E-G: stimulation at 10 Hz (in each case, 4 records were averaged). B and E: stimulation of an axon activating only bag2 fibers (b2). C and F: stimulation of another axon activating only chain fibers (ch). D and G: costimulation of the 2 axons (b2 + ch). In each display, the facilitated response to the 1st cycle of the sinusoidal stretch (see Fig. 3) is not shown.

Sinusoidal stretch

The tendon of the peroneus tertius muscle was attached to a servo-controlled electromagnetic puller. Sinusoidal stretch 0.5 mm (sometimes 1 mm) in amplitude was superimposed over one of three muscle lengths [2.5, 1.5, and 0.5 mm shorter than maximal physiological length (L_max), defined as the length at which twitch force is maximal plus 0.5 mm]. On average, the peroneus tertius muscle in 2- to 2.5-kg cats is 27-30 mm long, with a maximal lengthening, that is, the difference DL between the minimal physiological length and L_max, of 3-3.5 mm. At L_max  2.5 mm, the muscle is still taut and would become slack around L_max  2.8 mm. Thus in this short muscle a 1-mm stretch is about one-third of DL which corresponds to a 3-mm stretch in the longer soleus muscle whose DL is ~10 mm.

Each sequence of sinusoidal stretches started with a 4-s period at 0.6 Hz. Then the frequency was linearly increased to 8-9 Hz in 12 s, allowing observations of primary ending responses to stretches of various velocities ranging from 1 to ~12.5 mm/s. The sequence ended with a 4-s period at a constant frequency of 8-9 Hz.

Stimulation of -axons

Periods of repetitive stimulation of -axons at 10, 20, or 30 Hz, lasting 12 s, started 2 s before and outlasted by 2 s the periods of sinusoidal stretch of linearly increasing frequency (marked by arrows in Figs. 2-5). These low frequencies of stimulation are within the range of discharge frequencies observed in tonically active static -motoneurons, <40 Hz (Murphy 1981, 1982; Murphy et al. 1984).

View larger version (43K): [in this window] [in a new window]   FIG. 2. Instantaneous frequency of the discharge of a primary ending and averaged traces (mean frequency and mean instantaneous frequency) observed during a sinusoidal stretch. The frequency of sinusoidal stretch linearly increased from 0.5 to 8 Hz during the period indicated by arrows. A and D: instantaneous frequency display of a single trace of the ending discharge, in passive conditions (A) and during repetitive stimulation of a single -axon supplying both chains and bag2 fibers at a frequency close to 30 Hz (D) with 2- to 3-Hz random variations (see METHODS). B and E: displays of mean frequency based on the calculation of the probability density function in passive condition (B, 2 records averaged) and during repetitive -stimulation (E, 4 records averaged). First stimulation at 30 Hz, the 3 others with random variations of a few Hz around 30 Hz. C and F: displays of mean instantaneous frequency calculated by averaging several records of instantaneous frequency in passive condition (C, 2 records averaged) and during repetitive stimulation (F, 4 records averaged). Broken line: period during which averaged records occasionally and temporarily fell to 0. Note that in that period the minimal value of instantaneous frequency for a single record (D) was >25 Hz.

View larger version (73K): [in this window] [in a new window]   FIG. 3. Effect of stimulating a single -axon coactivating bag2 and chain fibers in the same spindle on the discharge of a primary ending (mean instantaneous frequency) during sinusoidal stretch of linearly increasing frequency. P, passive response of a primary ending (2 records averaged). Stimulation at 10, 20, and 30 Hz of the axon: the period of stimulation (indicated by the stimulation line) precedes and outlasts the period (indicated by arrows) during which the frequency of the sinusoidal stretch linearly increased (4 records averaged). Note that the peak frequency during the 1st cycle was slightly higher than during the 2nd cycle because it was the 1st response to stretch to be recorded after a previous period of repetitive -stimulation and consequently was temporarily facilitated. This figure is the continuation of Fig. 2. Peak values of the mean frequency in the passive response (P) are slightly higher than in Fig. 2 because the average muscle length in that series was maximal physiological length (Lmax)  2.5 mm, whereas it was Lmax  1.5 mm for the series illustrated in Fig. 2.

View larger version (32K): [in this window] [in a new window]   FIG. 4. Responses of a primary ending to sinusoidal stretch 0.5 mm (A) and 1 mm (B) in amplitude, in passive conditions and during stimulation at 30 Hz of a -axon coactivating bag2 and chain fibers (same spindle as in Fig. 3). In A, the responses were recorded at 3 average muscle lengths (Lmax  0.5, 1.5, and 2.5 mm). In B, the average muscle length was Lmax  1.5 mm. Passive responses, 2 records averaged. Responses during stimulation of the axon, 4 records averaged.

Such low frequencies of stimulation elicit a moderate increase in the primary ending discharge frequency that can be modulated by periodic changes in muscle length of moderate amplitude.

Recording of primary ending discharges

At each of the three muscle lengths used, several records of the instantaneous frequency of the ending discharge observed during sinusoidal stretch were made at 30-s intervals. Subsequently two records in passive conditions and four records for each frequency of stimulation (10, 20, and 30 Hz and sometimes also 5 and 40 Hz) were averaged. The stimulation frequency was constant during the first record but randomly varied during the three others by a few hertz around the constant value to minimize, during the averaging process, the influence of the driving of the primary discharge by the stimulation (impulses occurring at a constant time after each stimulus).

Two averaging processes were used. The first, giving a mean frequency, was based on the calculation of the probability density function and was determined by counting the number of impulses that occurred within successive bins. The duration of the bins regularly decreased from one half-cycle of the sinusoidal stretch to the other, because each half-cycle was divided into five bins of equal duration. Then the number of impulses in each bin was divided by the binwidth and by the number of records used for averaging. The second averaging process, giving a mean instantaneous frequency, was calculated by averaging the instantaneous frequencies of impulses that occurred within successive bins. When no impulse occurred in a particular bin, the calculated value of both the mean frequency and the mean instantaneous frequency was 0 for that bin, irrespective of the actual time interval between impulses occurring in the previous and following bins.

Comparison of averaged traces of mean frequency and of mean instantaneous frequency (Fig. 2) observed in a passive spindle and during stimulation of a -axon coactivating bag₂ and chain fibers shows they are very similar. The mean instantaneous frequency was mainly used in the present study.

Apparent discrepancies between averaged traces and single records of instantaneous frequency that are due to the averaging processes are observed in two conditions.

1) In passive spindles discrepancies are observed when the ending ceases firing toward the end of each shortening phase (Fig. 2A). Both the mean frequency (Fig. 2B) and the mean instantaneous frequency (Fig. 2C) fall to 0 because no impulse had occurred in some bins during that part of the cycles, whereas the instantaneous frequency does not because an impulse is generated by the primary ending at the beginning of each lengthening phase. Consequently the instantaneous frequency of those impulses, approximately following the frequency of the sinusoidal stretch, rises from 0.6 to 8 Hz (see Fig. 2A, bottom).

2) In spindles activated by repetitive stimulation of a -axon supplying both chain and bag₂ fibers discrepancies are observed during the fastest cycles of the sinusoidal stretch. The discharge of the ending is now continuous (that is, without interruption during the shortening phases of the sinusoidal stretch as in passive spindles) but the averaged frequencies occasionally and temporarily fall to 0 (Fig. 2, E and F) although the lowest values of the instantaneous frequency of impulses are >25 Hz in the corresponding part of single records (Fig. 2D). These falls simply indicate that no impulse had occurred in at least one bin during some shortening phases. Most probably, bins without impulses would have been less frequent if more than four records had been averaged. However, because for each -axon a complete series of recordings already comprised 96 periods of stimulation lasting 12 s, it was feared that repeating each period of repetitive stimulation more than four times might affect the reproducibility of the responses. With only four repetitions, the activation of primary endings remained stable during the whole experiment.

	RESULTS

Abstract Introduction Methods Results Discussion References

Effects of coactivating bag₂ and chain fibers on the responses of primary endings to sinusoidal stretch

These effects were observed by stimulating, at various frequencies, single -axons previously shown to supply both types of fibers in a particular spindle (8 axons). In addition, in one instance, coactivation was obtained by stimulating together two -axons supplying, in the same spindle, chain fibers alone and bag₂ fibers alone, respectively.

When an axon with a mixed distribution was stimulated at 10 Hz, the ending continued to fire during the shortening phases of the slowest cycles, from 0.6 to 3-4 Hz, which it did not in passive condition (Fig. 3). Thus, in this limited range of stretch frequencies, the mean instantaneous frequency was continuously modulated by the sinusoidal stretch (that is, without fall to 0 of the averaged frequency). When the axon was stimulated at 20 and 30 Hz, the continuous modulation of the frequency persisted up to 5-6 and 8 Hz, respectively, with occasional and brief falls to 0 (see METHODS and Fig. 2). In all cases the modulation of the discharge was roughly sinusoidal and its amplitude progressively increased with the stretch frequency.

These responses were nearly identical at the three studied averaged muscle lengths (Fig. 4A), that is, at lengths 0.5, 1.5, and 2.5 mm shorter than L_max. However, at the shortest length (L_max  2.5 mm) the amplitude of the modulation during the slowest cycles tended to be larger than that observed at longer muscle lengths, and, in the upper range of stretch frequencies, falls to 0 of the mean instantaneous frequency during shortening phases began to occur for slightly lower stretch frequencies. These observations show that, irrespective of the average muscle length, bag₂ and chain fiber coactivation gives primary endings the capability of signaling changes of muscle length.

In nearly all experiments a 1-mm peak-to-peak amplitude sinusoidal stretch was also applied to the muscle at the initial length of L_max  1.5 mm. The responses of the primary endings were qualitatively similar to those obtained with an 0.5-mm stretch, but falls to 0 of the mean instantaneous frequency during shortening phases began to appear for lower stretch frequencies, which could be expected because for a given frequency the maximal and mean stretch velocities are doubled. For the same frequency of stretch and for the same frequency of -stimulation, the amplitude of the response to a 1-mm stretch was roughly twice as large as that of the response to an 0.5-mm stretch (Fig. 4B).

Contribution of bag₂ and chain fibers

The contribution of the two kinds of intrafusal muscle fiber was assessed by observing the effects elicited in individual spindles by the stimulation of -axons selectively activating either bag₂ fibers alone (7 axons) or chain fibers alone (3 axons). In addition, on one occasion, two axons supplying the same spindle, one only activating bag₂ fiber(s) and the other only activating chain fibers, were prepared. The effects separately exerted by these axons, rather than the identical effects observed in different spindles, were selected to illustrate the specific effects of bag₂ and chain fibers (Fig. 5) because the effects due to their joint stimulation could be compared with those elicited by single axons with mixed distribution, that is, supplying both kinds of fibers.

Activation at 30 Hz of bag₂ fibers alone (Fig. 5B) elicited a sinusoidal modulation whose pattern was comparable with that due to the coactivation of bag₂ and chain fibers (see Figs. 3 and 4) but only in the range of 0.6 to 4-5 Hz. On the other hand, activation of chain fibers alone at 30 Hz elicited for each cycle between 0.6 and 5-6 Hz a distorted modulation of large amplitude (Fig. 5C). At the end of each shortening phase the mean instantaneous frequency was maintained at a value close to that of the stimulation, which suggests that chain fiber contraction was fast enough to compensate for muscle shortening at velocities as high as 9.5 mm/s. Peak frequencies reached during lengthening phases were distinctly higher than those observed in the passive spindle.

Stimulating the two -axons together at 30 Hz elicited over nearly the whole range of sinusoidal stretch frequencies a modulation of primary ending discharge frequency identical (Fig. 5D) to that observed during the stimulation of single axons with a mixed distribution (Figs. 3 and 4).

The effects due to coactivation of bag₂ and chain fibers at 10 Hz (Fig. 5G) were very weak. Continuous modulation of the mean instantaneous frequency was observed only during the first cycles, the modulation amplitude being nearly the same for each cycle. However small, these modifications were more perceptible than those due either to bag₂ fibers only (Fig. 5E) or to chain fibers only (Fig. 5F).

	DISCUSSION

Abstract Introduction Methods Results Discussion References

The aim of this study was to find some functional consequences of the coactivation of bag₂ and chain fibers by the static -system, because it appears that the static control of primary ending is jointly exerted by these two very different intrafusal muscle fibers. In a recent study on the peroneus tertius muscle in which, on average, there are 14 spindles (Scott and Young 1987) supplied by 24 -axons (Horcholle-Bossavit et al. 1988), 35 of 42 single static -axons (83%) were found to activate both bag₂ and chain fibers in various proportions (Celichowski et al. 1994). Previous histophysiological observations had also shown the high incidence of static -axons distributed to chain and bag₂ fibers (Banks 1991; Barker et al. 1973).

Functional consequences were deduced from the comparison of primary ending responses to sinusoidal stretch of linearly increasing frequency in passive spindles, that is, without -stimulation, and during stimulation of -axons supplying bag₂ and chain fibers. Such comparisons are possible because the type(s) of intrafusal muscle fibers supplied in particular spindles by individual static -axons can be determined with a recently developed physiological method (Celichowski et al. 1994) with which the distribution of most static -axons was shown to vary among the several spindles each of the axons supplies.

The amplitude of sinusoidal stretches used in the present study (0.5 and 1.0 mm) greatly exceeded that of the linear range within which primary endings continuously fire throughout the cycle range (Goodwin et al. 1975; Matthews and Stein 1969; Poppele and Bowman 1970). Therefore, in passive spindles, the endings' responses were highly nonlinear and the endings nearly always stopped firing during shortening phases. It was in respect to that condition that changes of ending responses due to the activation of bag₂ and chain fibers were examined by averaging several responses recorded during repetitive stimulation of single -axons at frequencies randomly varying by a few hertz around a given value (see METHODS). These frequencies rarely exceeded 30 Hz, so as to remain within the presumed physiological range of static -motoneuron tonic activity.

During coactivation of bag₂ and chain fibers at 30 Hz, the modulation of the mean instantaneous discharge frequency became continuous (that is, without interruption during shortening phases) over nearly the whole range of sinusoidal stretch frequencies tested (0.6-8 Hz). Selective activation at the same frequency of either chain or bag₂ fibers alone also elicited continuous modulation, but within narrower ranges (up to 4-5 and 6-7 Hz for bag₂ and chain fibers, respectively) and of very different patterns (Fig. 5). These observations, which are in agreement with the function originally ascribed to the -system by Hunt and Kuffler (1951) of counteracting the effect of muscle shortening (see also Appenteng et al. 1982), show that the actions of bag₂ and chain fibers are complementary in the sense that the modifications elicited by their coactivation are observed over a broader range of stretch velocities. How those very different muscle fibers interact in determining primary ending responses cannot be deduced from the present experiments and remains to be investigated.

Stimulation at 10 Hz of single axons supplying both types of fibers elicited disappointingly very weak effects, probably because spindles were deefferented (L₆, L₇, and S₁ ventral roots had to be sectioned for preparing single -axons). Asynchronous stimulation at the same low frequency of several static -axons (of various distribution) supplying the same spindle would most probably elicit much larger effects because the number of activated chain fibers in both poles would be greater and focal contractions in bag₂ fiber(s) would summate.

The study of the responses at different muscle lengths shows that the cocontraction of these fibers stabilizes the responses of primary endings in that sense that the responses were nearly identical, irrespective of muscle length. The importance of the stabilizing action of the -system was already pointed out by Goodwin et al. (1975).

The large and progressive increase of the modulation amplitude of the discharge frequency that is consistently observed over the whole range of sinusoidal stretch when chain and bag₂ fibers were coactivated at a constant (or nearly constant) frequency appears to be primarily related to bag₂ fiber contraction. This is suggested by the fact that a comparable pattern of modulation is observed, although within a narrower range, when bag₂ fibers alone are activated (compare Fig. 5, B and D). Activation of chain fibers alone elicits a very different pattern of modulation (Fig. 5C), but their contribution appears to be essential, because it is only when they are coactivated that continuous modulation is observed over the whole range of the sinusoidal stretch.

Preliminary observations on bag₂ and chain fibers coactivation by ramp stimulation of their motor supply starting from very low values (1-2 Hz) up to 30-40 Hz showed that the stimulation elicited an entirely different pattern of modulation of the primary ending discharges: from the slowest to the fastest cycles of the sinusoidal stretch, large amplitude modulations of approximately constant size were observed. If this observation is confirmed, it would suggest that the primary ending sensitivity could be adjusted by the CNS to the expected velocity of a movement in such a way that length changes of the same amplitude but of different velocities would be signaled with an approximately constant gain. During fast movements with many static -motoneurons discharging at a comparatively high frequency, a large barrage of static impulses could attenuate that part of the primary ending response related to velocity, whereas during slow movements, weak (or no) static -activity would not modify it.

The present study gives no clues to the ways bag₂ and chain fiber contractions interact, but it shows that their coactivation at presumed physiological frequencies enables primary endings to continuously signal moderate-amplitude length changes over a large range of stretch velocities, independently of the average muscle length.

	ACKNOWLEDGEMENTS

  The authors are very grateful to P. B. C. Matthews for helpful comments during this investigation, to R. Banks, L. Jami, and A. Lundberg for critical reading of the manuscript, and to 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 Française.

	FOOTNOTES

  Address reprint requests to Y. Laporte.

  Received 27 June 1996; accepted in final form 1 November 1996.

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