J. Physiol. (1976), 255, pp. 635-649 With 5 text-ftgure8 Printed in Great Britain
635
THE EFFECT OF LOW AMPLITUDE MUSCLE VIBRATION ON THE DISCHARGE OF FUSIMOTOR NEURONES IN THE DECEREBRATE CAT
BY JUDY R. TROTT From the Department of Physiology, University College London, Cower Street, London WC1E 6BT (Received 4 July 1975) SUMMARY
1. Longitudinal vibration (50-100 /tm, 100-300 Hz) has been applied to the triceps surae tendon to examine its effect on the tonic discharges of gastrocnemius medialis fusimotor neurones in the decerebrated cat. 2. For nineteen out of twenty-seven fusimotor neurones vibration consistently caused a small rise in discharge frequency. The remaining eight neurones showed no response to the vibration which always evoked a considerable discharge in alpha motoneurones. 3. The reflex excitation of fusimotor neurones is attributed to activity in primary endings of muscle spindles since control experiments confirmed that these receptors were powerfully excited by the vibration used whereas secondary endings and Golgi tendon organs remained unaffected. 4. Tonic discharges of fusimotor neurones of unknown destination were also recorded from lumbar 7 and sacral 1 ventral root filaments in decerebrated cats. Of thirty cells, seven were inhibited, five were excited and the remaining eighteen units were unaffected by vibration of the triceps surae. 5. These findings are discussed in relation to the role of muscle stretch receptors in the autogenetic control of fusimotor neurones. INTRODUCTION
This study examines autogenetic control of fusimotor neurones by muscle stretch receptors. A controversy has existed ever since Hunt (1951) originally reported that some fusimotor neurones were inhibited by stretch of their own muscle. The effect was said to be present in both decerebrated and spinalized cats. In a later study however Hunt & Paintal (1958) failed to confirm the presence of autogenetic inhibition in spinalized cats but did not re-investigate the reflex in decerebrated preparations. A reduction in fusimotor activity during muscle stretch has also been described by Eldred, Granit & Merton (1953) who used the discharge frequency of muscle spindle afferents as an indirect index of fusimotor
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drive. However, since fusimotor neurones are extremely sensitive to a variety of afferent activity, this inhibition cannot be definitely attributed to an autogenetic effect since no denervation of surrounding tissues was carried out. In contrast, Kobayashi, Oshima & Tasaki (1952) had found that muscle stretch excited small, spontaneously active motor fibres (later equated with fusimotor neurones) which were destined for the stretched muscle. However, again no denervation of surrounding tissues had been performed. Recently muscle stretch has been found both to facilitate and inhibit some fusimotor neurones of the same muscle (Fromm, Haase & Noth, 1974). The authors ascribed the fusimotor inhibition to activity originating in muscle stretch receptors. The fusimotor facilitation was originally thought to be due to activity arising from nociceptors within the tendon, but Chr. Fromm and J. Noth now feel that part of this stretch-induced facilitation is also due to activity originating in muscle stretch receptors (personal communication). Muscle stretch excites both primary and secondary endings from muscle spindles and Golgi tendon organs (Matthews, 1933). In any attempt to attribute the reflex responses of motoneurones during muscle stretch to activity originating from a specific group of muscle receptors a more selective stimulus than muscle stretch is required. It is possible to excite selectively one group of muscle stretch receptors, the spindle primary endings, while leaving other stretch-sensitive muscle receptors unaffected, by using low amplitude muscle vibration (Brown, Engberg & Matthews, 1967). Furthermore, muscle vibration has been shown to inhibit activity in some fusimotor neurones recorded in ventral root filaments of decerebrated cats (Brown, Lawrence & Matthews, 1968). Although this result showed that activity in spindle primary afferents could inhibit discharges of fusimotor neurones, an autogenetic effect was not established since the peripheral destinations of the neurones could not be determined. In the present work low amplitude muscle vibration has been used to examine the responses of fusimotor neurones of a known muscle to activity in primary endings ofthat same muscle. It was found that a powerful input from primary endings sometimes resulted in a weak autogenetic facilitation of fusimotor neurones but autogenetic inhibition of such neurones was never observed. A preliminary communication of these results has been presented (Trott, 1975). METHODS Preparation. All experiments were performed on cats (1.6-4 3 kg) decerebrated intercollicularly under halothane (Fluothane, I.C.I.) in oxygen anaesthesia. Both carotid arteries were ligated and one wes cannulated in order to record blood pressure
VIBRATION AND FUSIMOTOR NEURONES
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throughout the experiment. The left hind limb was denervated below the hip leaving only the innervation of the triceps surae intact. When the experiment involved recording activity in spinal roots a lumbar laminectomy from lumbar (L) 4 to sacral (S) 1 was performed to expose dorsal and ventral roots L 7 and S 1. The Achilles tendon was severed from the calcaneus and separated from the plantaris and semimembranosus tendons. In some experiments, before severing the tendon from the heel, the positions of the Achilles tendon corresponding to various angles of the ankle were noted. The left leg was securely fixed to a myograph stand using clamps attached at the knee and ankle. Exposed spinal roots and hind limb muscles and nerves were submerged in warm liquid paraffin equilibrated with 0 9 % saline. After intercollicular decerebration the halothane anaesthesia was discontinued and the mean blood pressure was maintained in the range 80-120 mmHg using i.v. injections of dextran in 0.9 % saline (Dextraven) when necessary. The cat was then paralysed with a sufficient dose of gallamine triethiodide (Flaxedil, May and Baker) to block both intrafusal and extrafusal muscle contractions. The paralysis was maintained by additional injections of gallamine as necessary and the animal was artificially respired using a Starling Ideal pump. Nerve recordings were not made for at least 2 hr after discontinuing anaesthesia. The rectal temperature was monitored throughout the experiment and maintained in the range of 36-38° C by placing the animal on a heated myograph stand and supplementing this heating with a radiant I.R. lamp when necessary. I8Olation and identifization offusimotor neuronew. A branch of the gastrocnemius medialis nerve was cut distally, desheathed and split until the activity of a tonically discharging fusimotor neurone was recorded in isolation. Fusimotor neurones were identified by their characteristic discharge pattern, their small spike size and low threshold to cutaneous stimulation. The conduction velocities of the neurones were also determined whenever possible. This involved measuring the latency of an action potential evoked by electrical stimulation of the sciatic nerve at a point a few cm central to the recording electrodes. Most of the filaments used contained other conducting nerve axons in addition to the single tonically discharging fusimotor neurone under study. Electrical stimulation of the nerve thus produced a compound action potential. The contribution to this potential made by the neurone under study was indicated by its absence when the stimulus to the sciatic nerve fell in the refractory period of a naturally occurring fusimotor nerve impulse. No allowance was made for the initiation time of the action potential when measuring the conduction latency but all conduction velocities were corrected to 370 C using a temperature coefficient of 1-65, the Q10 value for nerve in the temperature range 30-400 C. Tonic discharges of fusimotor neurones were also recorded from L 7 and S 1 ventral root filaments. These units were identified by the preceding criteria except that it was not possible to measure their conduction latencies. Isolation and identification of afferent units. Activity in triceps sure afferent units was recorded in dorsal root filaments (L 7 and S 1). Spindle endings were distinguished from Golgi tendon organs by the pause of the former units during a muscle twitch. Primary and secondary spindle endings were distinguished by a number of criteria: (1) the conduction velocity of their afferent fibres was determined. Fibres conducting at over 80 m/sec were classified as innervating primary endings and those conducting at less than 60 m/sec were classified as innervating secondary endings; (2) the receptors were examined for their response to muscle stretch since primary endings show a greater dynamic response to this type of stimulus than do secondary endings; (3) secondary endings discharge more regularly than do primary endings at a given maintained muscle length. The interspike interval distribution of the discharges of some afferent axons was therefore studied and the regularity of discharge 21
PIIY
2~5.5
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was expressed in terms of the coefficient of variation (S.D. of a number of intervals/mean value of those intervals). The relative distributions of the coefficient of variation for primary and secondary spindle endings (Matthews & Stein, 1969) were used as a guide to the identity of afferent units. Despite the use of these criteria afferent units were occasionally found which could not be definitely classified as innervating primary or secondary endings. The response of these units to muscle vibration was nevertheless examined. Recording of nerve activity. Nervous activity was studied using a loudspeaker and an oscilloscope from which photographic records could be obtained. Additionally, action potentials obtained from fusimotor neurones were converted into standard electrical pulses which were fed into a rate-meter consisting of a 'leaky integrator' having a time constant of decay of 180 msec. The output of this integrator provided a display of discharge frequency for which calibration could be carried out. The standard electrical pulses were also suitable for analysis using a Linc 8 computer to examine the distribution of interspike intervals. An additional index of nerve activity was obtained by displaying the mass activity of alpha motoneurones using another 'leaky integrator' having a time constant of decay of 65 msec. In this latter situation it was not necessary to calibrate the final discharge frequency and the input to the integrating capacitor consisted simply of the original amplified action potentials. Application and measurement of vibration. Longitudinal sinusoidal vibration was applied to the Achilles tendon by an electromagnetic puller (Goodmans 390A) to which the tendon was firmly attached using a strong thread. The vibrator, which was not servocontrolled, had a compliance of 1-5 mm/kg wt. over the range used. The frequency and amplitude of vibration were controlled by a Goodmans power oscillator. When examining the response of afferent units, vibration at frequencies of 70500 Hz and amplitudes of up to 550 Kim was applied to the tendon. However, as described later, the range of vibration was restricted when its effect on fusimotor activity was studied. The amplitude of vibration through which the tendon moved was dependent not only on the size of the oscillator signal but also the frequency of vibration and the degree of loading to which the puller was subjected. It was therefore necessary to measure the tendon vibration directly using a microscope graticule previously calibrated against a stage micrometer. Before applying vibration to the triceps sure the muscles were stretched about 9 mm beyond the point at which passive tension had started to develop. It would have been possible to stretch the muscles a further 5-7 mm beyond this point before reaching the maximum physiological length of the muscles. RESULTS
Only fusimotor neurones having a tonic discharge in the absence of any intentional stimulus were selected for study. They characteristically exhibited. regular sustained activity ranging from 30 to 80 impulses/sec. All units had a low threshold to cutaneous stimulation, their discharge frequency being readily raised by stroking the fur or twisting the ear of the animal. Activity was also recorded from filaments containing alpha motoneurones. These units rarely showed tonic activity but were readily excited by stretch or vibration oftheir muscle of origin (gastrocnemius medialis) and its two synergists (gastrocnemius lateralis and soleus).
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Effect of vibration on gastrocnemius mediatis fuimotor neurones Twenty-seven gastrocnemius medialis fusimotor neurones in fifteen cats were examined for their response to vibration (frequency range 150-250 Hz) of the triceps surae. Nineteen units showed an increase in discharge
A -
Vibration (200 Hz, 50 pm)
Impulses/sec Gamma (22 m/sec) u sk&i A discharge frequency
so
10 sec
B Before vibration
During vibration (200 Hz, 50 pm)
After vibration
7=54/sec
671sec f=55/sec
1 sec Fig. 1. Facilitation of a gastrocnemius medialis fusimotor neurone during vibration of the triceps sure. A, discharge frequency of a single gamma motoneurone. The irregularities in the rate-meter output are genuine reflexions of irregularity in the fusimotor discharge. Application of vibration is indicated by an upward deflexion of marker bar, this was manually executed. B, activity of fusimotor neurone from three different sections of record A; J is the mean frequency of discharge for each of the 2-2 see periods of firing illustrated.
frequency which usually appeared at amplitudes of 40-60 ,um but sometimes required an amplitude of up to 100 gim. The facilitation was weak, the increase in discharge frequency observed ranging from 5 to 24 impulses/ sec with an average of 10 impulses/sec. A typical result is shown in Fig. 1. At the onset of vibration most fusimotor neurones excited by vibration exhibited a sharp rise in frequency but the frequency also continued to rise 2T-2
640 JUDY R. TROTT gradually during maintained vibration. On some occasions the discharge frequency was still rising when vibration was removed (about 15 sec after its application). However other neurones exhibited a plateau of discharge Vibration
(170 Hz, 50 pm)
Integrated Alpha electrical _
activity
Gam ma
discharge
Gain ma
Impulses/sec 25
frequency
Fig. 2. Different patterns of response of an alpha and a gamma motoneurone during vibration of the triceps surae (vibration signalled by upward deflexion of marker bar). The upper trace shows the integrated electrical activity recorded from a gastrocnemius medialis filament containing an alpha motoneurone which was silent in the absence of muscle vibration but was excited to discharge at a frequency around 14 impulses/sec during vibration (frequency noted at time of recording). The lower trace shows the increase in discharge frequency of a single gastrocnemius medialis fusimotor neurone in response to muscle vibration. The discharge frequency continues to rise gradually throughout maintained vibration and takes several seconds to decline to its previous level on the removal of vibration. In contrast the alpha motoneurone, once excited by vibration, discharges at a fairly steady level thoughout vibration and rapidly ceases firing on the removal of vibration.
frequency after about 10 sec of vibration and on the two occasions when vibration was maintained for 1 min, a plateau level of firing had been reached after about 20 sec. When vibration was removed the frequency of fusimotor discharge usually showed an initial sharp fall and then a more prolonged decline, several seconds elapsing before the neurone resumed its previous discharge level. Fig. 2 shows that a somewhat different pattern of response was seen in alpha motoneurones which showed a sharp rise in frequency at the onset of vibration but no gradual increase in discharge rate during maintained vibration. When vibration was removed alpha motoneurones rapidly
VIBRATION AND FUSIMOTOR NEURONES
641 ceased discharging. The measured conduction velocities of fusimotor neurones excited by muscle vibration spanned the range 19-31 m/sec, all being well within the range for gamma motoneurones. A, triceps nerves largely intact
Vibration (50 pm, 200 Hz) 60 s
I Impulses/sec
40
B, after cutting medial gastrocnemius nerve Vibration (50 am, 200 Hz) Gamma discharge
frequency C. after cutting all triceps nerves Vibration (50 pm, 200 Hz)
10 sec
Fig. 3. Effect of cutting the nerves from the triceps surae on fusimotor facilitation induced by vibration. A, triceps nerves intact except for the branch of gastrocnemius medialis nerve containing the neurone under study. B, only the nerve to gastrocnemius lateralis and soleus intact. C, no triceps nerves intact.
It was established that activity in both the gastrocnemius medialis and gastrocnemius lateralis/soleus nerves could contribute to the excitation of gastrocnemius medialis fusimotor neurones. This point is illustrated in Fig. 3 which shows the response of a fusimotor neurone to muscle vibration when the triceps nerves were intact, except for the small branch of the gastrocnemius medialis nerve containing the neurone under study (Fig. 3A) -and the response to vibration when only the gastrocnemius lateralis/soleus nerve was intact (Fig. 3B). The response to vibration was invariably abolished when both the triceps nerves had been sectioned (Fig. 3C). It was often possible to excite fusimotor neurones by squeezing the Achilles tendon. This effect could always be abolished by applying a cotton wool swab soaked in 5 % procaine solution to the tendon. This procedure never altered the response of the fusimotor neurone to vibration (Fig. 4).
JUDY R. TROTT 642 It therefore seems unlikely that stimulation of tendon nociceptors by mechanical irritation was responsible for the fusimotor excitation produced by muscle vibration. B A -
-
Vibration (50 pm, 200 Hz)
Tendon squeeze
jImpulseslsec 4
t
Gischamma discharge
D
frequency C
%5 procaine applied to tendonVibration (50 pm, 200 Hz)
-
0 sec
________Tendon squeezes
Gamma 22 m/sec
Fig. 4. Resistance of vibration-induced fusimotor facilitation to anaesthetization of the Achilles tendon. A and C, excitation of gastrocnemius medialis fusimotor neurone during vibration of the triceps sure both before and after 5 % procaine was applied to the Achilles tendon. B and D, response of the neurone to squeezing the tendon before and after the application of procaine. Note disappearance of the facilitation in response to squeezing once the tendon has been anaesthetized. It was thought desirable to examine the response of fusimotor neurones to muscle stretch in addition to muscle vibration. However, only a small range of stretches could be tested because it was impossible to move the vibrator once the Achilles tendon had been secured in place. Thus, the only stimulus tested was that of a sudden stretch of 1-2 mm applied to the probe of the vibrator to which the Achilles tendon was attached. In this position, the muscles were already stretched 9-10 mm beyond the point at which passive tension had started to develop. Sudden stretch of 1-2 mm, excited thirteen of the nineteen fusimotor neurones which were excited by vibration. Stretch-induced facilitation was usually smaller than that induced by vibration. Of the other six units excited by vibration four were unaffected by muscle stretch and the response of the other two was not examined. One of the eight neurones uuiaffected by vibration was weakly inhibited by the stretch stimulus used. It should be emphasized that the response of fusimotor neurones to stretch was not studied in depth and control experiments such as the effect of cutting the triceps nerves and anaesthetizing the Achilles tendon were not performed.
Identity of the receptor excited by vibration In the work described above muscle vibration was specifically chosen as being a stimulus which selectively excites primary endings from muscle spindles. It was therefore thought desirable to carry out control experiments
643 VIBRATION AND FUSIMOTOR NEURONES recording from muscle stretch receptors during vibration to examine whether, under the experimental conditions employed here, this stimulus was selectively exciting primary endings. Afferent activity was recorded from spindle afferents and Golgi tendon organs of triceps surae in L 7 and S 1 dorsal rootlets. Primary endings from muscle spindles were found to be appreciably more sensitive to vibration than were spindle secondary endings or Golgi tendon organs (Fig. 5). 200
@ 150" E
4'100 0r
0
I/ 0
100
300 400 200 Amplitude of vibration (pam)
500
600
Fig. 5. Responses of triceps surae afferents to vibration of 200 Hz applied to the Achilles tendon. Two primary endings (El), two secondary endings (*), and two Golgi tendon organs (A) are illustrated. The discharge frequency of each ending is plotted against the amplitude of vibration. The pairs of endings have been chosen because they represent the extremes of sensitivity to vibration found within each of the three different receptor types.
Primary endings Of the eleven primary endings studied all could be driven to discharge at frequencies of vibration 100-300 Hz applied to the triceps surae when these muscles were stretched by about 9 mm as described before. The more sensitive endings showed such a response at amplitudes of vibration of 16-50 ,um and, with one exception, all the primary endings were driven to discharge at 200 Hz when the vibration measured at the tendon had been increased to 200 Mum. The least sensitive primary ending required vibration of 350 #sm to be applied to the tendon before driving at this frequency occurred. The reason for this insensitivity is not known but it is perhaps
JUDY R. TROTT worth noting that this unit was one of only two primary endings which did not show a background discharge in the absence of muscle vibration although it was very sensitive to muscle stretch. 644
Secondary endings Out of nine secondary endings four were unaffected by vibration up to 500 ,um at a frequency of 200 Hz. None of these four units showed a background discharge in the absence of vibration. The threshold amplitude required to excite the remaining units ranged from 100 to 400 ,um at a frequency of 200 Hz. The most sensitive secondary endings were those having the highest discharge frequency in the absence of muscle vibration. In general the secondary endings did not attain high discharge rates under the influence of vibration, although one unit was driven by vibration at 200 Hz and 600 ,um.
Golgi tendon organs Of the four tendon organs studied three were unaffected by vibration up to 500 ,um at a frequency of 200 Hz. The threshold amplitude for excitation of the remaining tendon organ was 275 ,lm at 200 Hz and the unit achieved a maximum discharge frequency of 40 impulses/sec under the influence of vibration with an amplitude of 550 /sm. None of the tendon organs studied showed a background discharge in the absence of muscle vibration. It can therefore be stated that the amplitudes and frequencies of vibration which were found to excite fusimotor neurones would have 'driven' about half of the primary endings examined here, strongly excited the rest of the primary endings (with the one exception mentioned) but would not have influenced either secondary endings or Golgi tendon organs.
Spindle endings with axons of intermediate conduction velocity Attempts were made to classify spindle afferents conducting between 60 and 80 m/sec as either primary or secondary endings on the basis of their dynamic sensitivity and the regularity of their tonic discharge pattern. Difficulty was found however in quantifying the dynamic sensitivity under the conditions employed so more emphasis was placed on examining the regularity of discharge of the units under study. This was expressed as the coefficient of variation of the interspike interval distribution. Matthews & Stein (1969) found that for de-efferented spindle endings discharging at frequencies around 30/sec the range for the coefficients of variation shown by primary endings was 0-027-0-16 whereas that for secondary endings was 0-016-0024. Of the afferent units whose interspike interval distributions were analysed in the present study all units definitely classified as primary or secondary on the basis of their conduction velocity fell within
645 VIBRATION AND FUSIMOTOR NEURONES the respective ranges quoted above with the exception of one primary ending which had a coefficient of variation of 0-025. However the coefficients of variation of units conducting between 60 and 80 m/sec occupied a range 0-023-0-031 spanning the trough between the two distributions established by Matthews & Stein (1969) as being typical of primary and secondary units. Since the regularity of discharge of such endings having axons of intermediate conduction velocities has not previously been documented and since the coefficients of variation for such units in the present study were so near the borderline between primary and secondary endings, it was thought unwise to assign these units to either population on the basis of their regularity of discharge. The response of these units to vibration was very varied. Several of them were as sensitive as primary endings, for example one unit was driven to discharge at 200 Hz by vibration of 60 jum amplitude. Other intermediate endings behaved similarly to secondary endings in that they were not driven in response to vibration even at amplitudes up to 500 ,um but such units did show a slight response to vibration of 50-100 jsm amplitude, unlike typical secondary endings which were always insensitive to vibration of less than 100 ,um. The sensitivity of primary endings to muscle vibration is critically dependent on the degree of muscle stretch. At short muscle lengths it was impossible to drive some primary endings at the frequency of applied vibration but the threshold for driving dropped with the application of muscle stretch. Nevertheless, in the present study the sensitivity of all muscle receptors to vibration was apparently less than that described by Brown et al. (1967). There are two possible explanations for this: firstly, in the present experiments disturbance in the region of the triceps sure was kept to a minimum and the muscles were not dissected away from surrounding skin and connective tissue. Vibration transmitted to the belly of the muscles will therefore undoubtedly have been attenuated compared with vibration applied (and measured) at the tendon; secondly, although moderate stretch corresponding to an angle of the ankle of 90-110O was applied to the muscles before vibration at no time did the triceps surae reach their maximum physiological length (cf. Brown et al. 1967) and the difference in resting tension within the muscles may well have contributed to the relative receptor sensitivity in the two investigations.
Afferent units from non-denervated muscles other than the triceps sure, for example, those of the hip and rump, were examined for their sensitivity to vibration applied to the Achilles tendon. Most of these were appreciably less sensitive (threshold above 450 /m) than triceps surae primary endings. However, two units were excited by vibration of about 100 jsm so activity in some such units may have contributed to the reflex effects observed in the present experiments. However any such contribution must have been negligible since vibration produced no reflex effects on fusimotor neurones after the triceps surae nerves had been cut.
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Activity in ventral root filaments Muscle vibration has been shown to inhibit unidentified fusimotor neurones isolated from ventral root filaments (Brown et al. 1968). In the light of the present findings it seemed worth while to examine the response of such neurones to vibration which I knew was capable of exciting gastrocnemius medialis fusimotor neurones. Accordingly, background activity in thirty fusimotor neurones was recorded in L 7 and S 1 ventral root filaments. The peripheral destinations of these units were of course unknown. Five fusimotor neurones were excited during vibration of the triceps surae. The extent and time course of the facilitation was similar to that shown by medial gastrocnemius neurones and was produced by the same parameters of vibration. Of these five neurones, two were also facilitated by stretch of the triceps surae but the effect of muscle stretch on the three remaining units was not tested. Seven fusimotor neurones were inhibited by vibration and by stretch of the triceps surae. The remaining eighteen fusimotor neurones could not be influenced by muscle vibration despite the fact that alpha motoneurones responded to the stimulus. These fusimotor neurones were however responsive to other reflex inputs such as cutaneous stimulation. DISCUSSION
The present results have shown that longitudinal muscle vibration of a form which excited only spindle primary endings produced weak autogenetic facilitation of some fusimotor neurones. The facilitation, although weak, was a repeatable response and was maintained throughout the period of vibration. Excitation of gastrocnemius medialis fusimotor neurones was produced not only by afferent activity from that muscle, that is a strictly autogenetic effect, but was also elicited by activity in primary endings of the synergistic gastrocnemius lateralis and soleus muscles. None of the gastrocnemius medialis fusimotor neurones in the present study was inhibited during low amplitude (50-100 ,sum) vibration of the triceps surae. However, fusimotor neurones in ventral roots can be inhibited by vibration of the triceps surae in decerebrated cats (Brown et al. 1968). These authors report that 'in some preparations perhaps a fifth of the spontaneously active units were inhibited by vibration'. This was confirmed in the present experiments where activity in seven out of thirty fusimotor neurones recorded in ventral root filaments was reduced by vibration. The peripheral destinations of these neurones were of course unknown but in the light of the results gained from peripheral nerve recordings they are unlikely to have been destined for the triceps surae.
647 VIBRATION AND FUSIMOTOR NEURONES In the same way it is possible that the five fusimotor neurones in ventral root filaments which were facilitated by vibration of the triceps surae were in fact destined for those muscles, although it is also possible that activity in primary endings could facilitate fusimotor activity destined for other muscles. In the present experiments a deliberate attempt was made to use vibration of a sufficiently low amplitude such that it produced considerable excitation of primary endings without exciting less sensitive muscle receptors. This was essential since it is known that both spindle secondary endings and Golgi tendon organs are excited by muscle vibration but they are appreciably less sensitive than primary endings (Brown et al. 1967). Moreover, the threshold amplitude of vibration required to excite muscle receptors is dependent on factors such as the degree of tension in the muscle undergoing vibration (both active and passive) and the extent of fusimotor tone. To ensure that only primary endings were excited by the vibratory stimuli used in the present experiments, afferent activity in dorsal root filaments was studied in a separate series of experiments. This study showed that amplitudes of vibration up to 100 jum caused considerable excitation of primary endings from muscle spindles without having any effect on secondary endings or Golgi tendon organs. Since fusimotor facilitation was achieved by vibration at amplitudes 50-100 jtm this facilitation can definitely be attributed to activity in Ta afferents. Approximately half the primary endings studied were driven to discharge at the frequency of applied vibration (usually 200 Hz) at amplitudes of up to 100 jam. The thresholds for 'driving' of the remaining endings (with the one exception previously described in the Results section) were in the range 100-200 jsm. However, the threshold amplitudes required to excite the more sensitive of the secondary endings also lay in the range 100-200 um, making it impossible in the present experiments to excite maximally the primary ending population without exciting some secondary endings. For this reason the effect on fusimotor neurones of a maximal Ia input could not be tested. Golgi tendon organs proved to be less sensitive to muscle vibration than secondary endings from muscle spindles, the only one sensitive to vibration having a threshold of 275 ,um. In the present study primary and secondary endings were thus found to be less clearly separable on the basis of their sensitivity to vibration than was previously found (Brown et al. 1967). It is interesting to note in this context that Stuart, Mosher, Gerlach & Reinking (1970) found a higher dynamic sensitivity of group II afferents to brief muscle pulls (60 sum or less) than might have been expected, such transient stretch activating 95 % of Ia afferent units and 41 % of group II units. There is no reason to
648 JUDY R. TROTT suppose that some secondary endings, perhaps those situated near the elastic, non-viscous equatorial zone of the intrafusal muscle fibres, or alternatively the few secondary endings innervating nuclear bag fibres as opposed to nuclear chain fibres would not be more sensitive to vibration than the secondary ending population as a whole. Alternatively, there may be slight differences of sensitivity between muscle receptors of the triceps surae (studied in the present experiments) compared with those of the soleus muscle alone (Brown et al. 1967). The situation was further complicated by the behaviour of spindle afferents which could not be definitely classified as primary or secondary endings. As described before some of these showed sensitivity to vibration typical of primary endings, others gave a response intermediate between that of typical primary and secondary endings. Those which were responsive to vibration of 50-100 gm may well have contributed to the reflex effects observed on fusimotor neurones and such afferent activity, although not widespread, should not be overlooked in studying the reflex responses of motoneurones to muscle vibration. A short communication which has appeared since the present work was completed describes autogenetic inhibition of fusimotor neurones being produced by muscle vibration of 50-100 Mum (Fromm & Noth, 1975). The authors found twelve out of forty-six fusimotor neurones tested to be inhibited by muscle vibration. The authors also mention that a further twenty of the forty-six cells were, in fact, excited by vibration. This excitation appeared at a lower amplitude of vibration (15 #sm) than that described as inhibiting fusimotor neurones. Vibration at such amplitudes (15 Mum) would almost certainly have been exciting spindle primary endings rather than any other receptors. The facilitation of fusimotor neurones found by Fromm & Noth is therefore not incompatible with the results of the present study. The present experiments do not shed any light on the fusimotor inhibition which Fromm & Noth describe as appearing at an amplitude of vibration considerably higher than that required to facilitate fusimotor activity. As mentioned before the amplitude of vibration was deliberately kept low in the present study to ensure selective excitation of
primary endings. In conclusion, the demonstration of a facilitatory action of Ia afferents to some fusimotor neurones of the same muscle raises the possibility that, being an example of positive feed-back, such an action could lead to instability in the stretch reflex. This would obviously constitute a hazard in normal muscle activity. However, it should be emphasized that the facilitation is a weak response to an almost maximal input from primary endings and as such is unlikely to occur under normal conditions of muscle
activity.
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I am grateful to Dr P. H. Ellaway and Mr J. E. Pascoe for valuable help and discussion throughout the work and constructive criticism of the text. I also thank Mrs Maria Winder for skilful technical assistance and the M.R.C. for the provision of a scholarship for training in research methods.
REFERENCES BROWN, M. C., ENGBERG, I. & MATTHEWs, P. B. C. (1967). The relative sensitivity to vibration of muscle receptors of the cat. J. Phy8iol. 192, 773-800. BROWN, M. C., LAWRENCE, D. G. & MATrREws, P. B. C. (1968). Reflex inhibition by Ia afferent input of spontaneously discharging motoneurones in the decerebrate cat. J. Phy8iol. 198, 5-7P. ELDRED, E., GIIANrr, R. & MERTON, P. A. (1953). Supraspinal control of the muscle spindles and its significance. J. Physiol. 122, 498-523. FROMM, CHR., HAASE, J. & NoTH, J. (1974). Length-dependent autogenetic inhibition of extensor y-motoneurones in the decerebrate cat. Pfluger8 Arch. ge8. Phy8iol. 346, 251-262. FROMM, CHR. & NoTH, J. (1975). Vibration-induced autogenetic inhibition of gamma motoneurons. Brain Res. 83, 495-497. HUNT, C. C. (1951). The reflex activity of mammalian small-nerve fibres. J. Phy8iol. 115, 456-469. HUNT, C. C. & PANTAL, A. S. (1958). Spinal reflex regulation of fusimotor neurones. J. Physiol. 143, 195-212. KOBAYASHI, Y., OsimA, K. & TASAIx, I. (1952). Analysis of afferent and efferent systems in the muscle nerve of the toad and cat. J. Physiol. 117, 152-171. MATTHEWS, B. H. C. (1933). Nerve endings in mammalian muscle. J. Phy8iol. 78, 1-53. MArT ws, P. B. C. & STEIN, R. B. (1969). The regularity of primary and secondary muscle spindle afferent discharges. J. Physiol. 202, 59-82. STUART, D. G., MOSHER, C. G., GERLACH, R. L. & REINKING, R. M. (1970). Selective activation of Ia afferents by transient muscle stretch. Expl Brain Res. 10, 477487. TROTr, JUnDY R. (1975). Reflex responses of fusimotor neurones during muscle vibration. J. Physiol. 247, 20-22P.
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THE EFFECT OF LOW AMPLITUDE MUSCLE VIBRATION ON THE DISCHARGE OF FUSIMOTOR NEURONES IN THE DECEREBRATE CAT
BY JUDY R. TROTT From the Department of Physiology, University College London, Cower Street, London WC1E 6BT (Received 4 July 1975) SUMMARY
1. Longitudinal vibration (50-100 /tm, 100-300 Hz) has been applied to the triceps surae tendon to examine its effect on the tonic discharges of gastrocnemius medialis fusimotor neurones in the decerebrated cat. 2. For nineteen out of twenty-seven fusimotor neurones vibration consistently caused a small rise in discharge frequency. The remaining eight neurones showed no response to the vibration which always evoked a considerable discharge in alpha motoneurones. 3. The reflex excitation of fusimotor neurones is attributed to activity in primary endings of muscle spindles since control experiments confirmed that these receptors were powerfully excited by the vibration used whereas secondary endings and Golgi tendon organs remained unaffected. 4. Tonic discharges of fusimotor neurones of unknown destination were also recorded from lumbar 7 and sacral 1 ventral root filaments in decerebrated cats. Of thirty cells, seven were inhibited, five were excited and the remaining eighteen units were unaffected by vibration of the triceps surae. 5. These findings are discussed in relation to the role of muscle stretch receptors in the autogenetic control of fusimotor neurones. INTRODUCTION
This study examines autogenetic control of fusimotor neurones by muscle stretch receptors. A controversy has existed ever since Hunt (1951) originally reported that some fusimotor neurones were inhibited by stretch of their own muscle. The effect was said to be present in both decerebrated and spinalized cats. In a later study however Hunt & Paintal (1958) failed to confirm the presence of autogenetic inhibition in spinalized cats but did not re-investigate the reflex in decerebrated preparations. A reduction in fusimotor activity during muscle stretch has also been described by Eldred, Granit & Merton (1953) who used the discharge frequency of muscle spindle afferents as an indirect index of fusimotor
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drive. However, since fusimotor neurones are extremely sensitive to a variety of afferent activity, this inhibition cannot be definitely attributed to an autogenetic effect since no denervation of surrounding tissues was carried out. In contrast, Kobayashi, Oshima & Tasaki (1952) had found that muscle stretch excited small, spontaneously active motor fibres (later equated with fusimotor neurones) which were destined for the stretched muscle. However, again no denervation of surrounding tissues had been performed. Recently muscle stretch has been found both to facilitate and inhibit some fusimotor neurones of the same muscle (Fromm, Haase & Noth, 1974). The authors ascribed the fusimotor inhibition to activity originating in muscle stretch receptors. The fusimotor facilitation was originally thought to be due to activity arising from nociceptors within the tendon, but Chr. Fromm and J. Noth now feel that part of this stretch-induced facilitation is also due to activity originating in muscle stretch receptors (personal communication). Muscle stretch excites both primary and secondary endings from muscle spindles and Golgi tendon organs (Matthews, 1933). In any attempt to attribute the reflex responses of motoneurones during muscle stretch to activity originating from a specific group of muscle receptors a more selective stimulus than muscle stretch is required. It is possible to excite selectively one group of muscle stretch receptors, the spindle primary endings, while leaving other stretch-sensitive muscle receptors unaffected, by using low amplitude muscle vibration (Brown, Engberg & Matthews, 1967). Furthermore, muscle vibration has been shown to inhibit activity in some fusimotor neurones recorded in ventral root filaments of decerebrated cats (Brown, Lawrence & Matthews, 1968). Although this result showed that activity in spindle primary afferents could inhibit discharges of fusimotor neurones, an autogenetic effect was not established since the peripheral destinations of the neurones could not be determined. In the present work low amplitude muscle vibration has been used to examine the responses of fusimotor neurones of a known muscle to activity in primary endings ofthat same muscle. It was found that a powerful input from primary endings sometimes resulted in a weak autogenetic facilitation of fusimotor neurones but autogenetic inhibition of such neurones was never observed. A preliminary communication of these results has been presented (Trott, 1975). METHODS Preparation. All experiments were performed on cats (1.6-4 3 kg) decerebrated intercollicularly under halothane (Fluothane, I.C.I.) in oxygen anaesthesia. Both carotid arteries were ligated and one wes cannulated in order to record blood pressure
VIBRATION AND FUSIMOTOR NEURONES
637
throughout the experiment. The left hind limb was denervated below the hip leaving only the innervation of the triceps surae intact. When the experiment involved recording activity in spinal roots a lumbar laminectomy from lumbar (L) 4 to sacral (S) 1 was performed to expose dorsal and ventral roots L 7 and S 1. The Achilles tendon was severed from the calcaneus and separated from the plantaris and semimembranosus tendons. In some experiments, before severing the tendon from the heel, the positions of the Achilles tendon corresponding to various angles of the ankle were noted. The left leg was securely fixed to a myograph stand using clamps attached at the knee and ankle. Exposed spinal roots and hind limb muscles and nerves were submerged in warm liquid paraffin equilibrated with 0 9 % saline. After intercollicular decerebration the halothane anaesthesia was discontinued and the mean blood pressure was maintained in the range 80-120 mmHg using i.v. injections of dextran in 0.9 % saline (Dextraven) when necessary. The cat was then paralysed with a sufficient dose of gallamine triethiodide (Flaxedil, May and Baker) to block both intrafusal and extrafusal muscle contractions. The paralysis was maintained by additional injections of gallamine as necessary and the animal was artificially respired using a Starling Ideal pump. Nerve recordings were not made for at least 2 hr after discontinuing anaesthesia. The rectal temperature was monitored throughout the experiment and maintained in the range of 36-38° C by placing the animal on a heated myograph stand and supplementing this heating with a radiant I.R. lamp when necessary. I8Olation and identifization offusimotor neuronew. A branch of the gastrocnemius medialis nerve was cut distally, desheathed and split until the activity of a tonically discharging fusimotor neurone was recorded in isolation. Fusimotor neurones were identified by their characteristic discharge pattern, their small spike size and low threshold to cutaneous stimulation. The conduction velocities of the neurones were also determined whenever possible. This involved measuring the latency of an action potential evoked by electrical stimulation of the sciatic nerve at a point a few cm central to the recording electrodes. Most of the filaments used contained other conducting nerve axons in addition to the single tonically discharging fusimotor neurone under study. Electrical stimulation of the nerve thus produced a compound action potential. The contribution to this potential made by the neurone under study was indicated by its absence when the stimulus to the sciatic nerve fell in the refractory period of a naturally occurring fusimotor nerve impulse. No allowance was made for the initiation time of the action potential when measuring the conduction latency but all conduction velocities were corrected to 370 C using a temperature coefficient of 1-65, the Q10 value for nerve in the temperature range 30-400 C. Tonic discharges of fusimotor neurones were also recorded from L 7 and S 1 ventral root filaments. These units were identified by the preceding criteria except that it was not possible to measure their conduction latencies. Isolation and identification of afferent units. Activity in triceps sure afferent units was recorded in dorsal root filaments (L 7 and S 1). Spindle endings were distinguished from Golgi tendon organs by the pause of the former units during a muscle twitch. Primary and secondary spindle endings were distinguished by a number of criteria: (1) the conduction velocity of their afferent fibres was determined. Fibres conducting at over 80 m/sec were classified as innervating primary endings and those conducting at less than 60 m/sec were classified as innervating secondary endings; (2) the receptors were examined for their response to muscle stretch since primary endings show a greater dynamic response to this type of stimulus than do secondary endings; (3) secondary endings discharge more regularly than do primary endings at a given maintained muscle length. The interspike interval distribution of the discharges of some afferent axons was therefore studied and the regularity of discharge 21
PIIY
2~5.5
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JUDY B. TROTT
was expressed in terms of the coefficient of variation (S.D. of a number of intervals/mean value of those intervals). The relative distributions of the coefficient of variation for primary and secondary spindle endings (Matthews & Stein, 1969) were used as a guide to the identity of afferent units. Despite the use of these criteria afferent units were occasionally found which could not be definitely classified as innervating primary or secondary endings. The response of these units to muscle vibration was nevertheless examined. Recording of nerve activity. Nervous activity was studied using a loudspeaker and an oscilloscope from which photographic records could be obtained. Additionally, action potentials obtained from fusimotor neurones were converted into standard electrical pulses which were fed into a rate-meter consisting of a 'leaky integrator' having a time constant of decay of 180 msec. The output of this integrator provided a display of discharge frequency for which calibration could be carried out. The standard electrical pulses were also suitable for analysis using a Linc 8 computer to examine the distribution of interspike intervals. An additional index of nerve activity was obtained by displaying the mass activity of alpha motoneurones using another 'leaky integrator' having a time constant of decay of 65 msec. In this latter situation it was not necessary to calibrate the final discharge frequency and the input to the integrating capacitor consisted simply of the original amplified action potentials. Application and measurement of vibration. Longitudinal sinusoidal vibration was applied to the Achilles tendon by an electromagnetic puller (Goodmans 390A) to which the tendon was firmly attached using a strong thread. The vibrator, which was not servocontrolled, had a compliance of 1-5 mm/kg wt. over the range used. The frequency and amplitude of vibration were controlled by a Goodmans power oscillator. When examining the response of afferent units, vibration at frequencies of 70500 Hz and amplitudes of up to 550 Kim was applied to the tendon. However, as described later, the range of vibration was restricted when its effect on fusimotor activity was studied. The amplitude of vibration through which the tendon moved was dependent not only on the size of the oscillator signal but also the frequency of vibration and the degree of loading to which the puller was subjected. It was therefore necessary to measure the tendon vibration directly using a microscope graticule previously calibrated against a stage micrometer. Before applying vibration to the triceps sure the muscles were stretched about 9 mm beyond the point at which passive tension had started to develop. It would have been possible to stretch the muscles a further 5-7 mm beyond this point before reaching the maximum physiological length of the muscles. RESULTS
Only fusimotor neurones having a tonic discharge in the absence of any intentional stimulus were selected for study. They characteristically exhibited. regular sustained activity ranging from 30 to 80 impulses/sec. All units had a low threshold to cutaneous stimulation, their discharge frequency being readily raised by stroking the fur or twisting the ear of the animal. Activity was also recorded from filaments containing alpha motoneurones. These units rarely showed tonic activity but were readily excited by stretch or vibration oftheir muscle of origin (gastrocnemius medialis) and its two synergists (gastrocnemius lateralis and soleus).
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639
Effect of vibration on gastrocnemius mediatis fuimotor neurones Twenty-seven gastrocnemius medialis fusimotor neurones in fifteen cats were examined for their response to vibration (frequency range 150-250 Hz) of the triceps surae. Nineteen units showed an increase in discharge
A -
Vibration (200 Hz, 50 pm)
Impulses/sec Gamma (22 m/sec) u sk&i A discharge frequency
so
10 sec
B Before vibration
During vibration (200 Hz, 50 pm)
After vibration
7=54/sec
671sec f=55/sec
1 sec Fig. 1. Facilitation of a gastrocnemius medialis fusimotor neurone during vibration of the triceps sure. A, discharge frequency of a single gamma motoneurone. The irregularities in the rate-meter output are genuine reflexions of irregularity in the fusimotor discharge. Application of vibration is indicated by an upward deflexion of marker bar, this was manually executed. B, activity of fusimotor neurone from three different sections of record A; J is the mean frequency of discharge for each of the 2-2 see periods of firing illustrated.
frequency which usually appeared at amplitudes of 40-60 ,um but sometimes required an amplitude of up to 100 gim. The facilitation was weak, the increase in discharge frequency observed ranging from 5 to 24 impulses/ sec with an average of 10 impulses/sec. A typical result is shown in Fig. 1. At the onset of vibration most fusimotor neurones excited by vibration exhibited a sharp rise in frequency but the frequency also continued to rise 2T-2
640 JUDY R. TROTT gradually during maintained vibration. On some occasions the discharge frequency was still rising when vibration was removed (about 15 sec after its application). However other neurones exhibited a plateau of discharge Vibration
(170 Hz, 50 pm)
Integrated Alpha electrical _
activity
Gam ma
discharge
Gain ma
Impulses/sec 25
frequency
Fig. 2. Different patterns of response of an alpha and a gamma motoneurone during vibration of the triceps surae (vibration signalled by upward deflexion of marker bar). The upper trace shows the integrated electrical activity recorded from a gastrocnemius medialis filament containing an alpha motoneurone which was silent in the absence of muscle vibration but was excited to discharge at a frequency around 14 impulses/sec during vibration (frequency noted at time of recording). The lower trace shows the increase in discharge frequency of a single gastrocnemius medialis fusimotor neurone in response to muscle vibration. The discharge frequency continues to rise gradually throughout maintained vibration and takes several seconds to decline to its previous level on the removal of vibration. In contrast the alpha motoneurone, once excited by vibration, discharges at a fairly steady level thoughout vibration and rapidly ceases firing on the removal of vibration.
frequency after about 10 sec of vibration and on the two occasions when vibration was maintained for 1 min, a plateau level of firing had been reached after about 20 sec. When vibration was removed the frequency of fusimotor discharge usually showed an initial sharp fall and then a more prolonged decline, several seconds elapsing before the neurone resumed its previous discharge level. Fig. 2 shows that a somewhat different pattern of response was seen in alpha motoneurones which showed a sharp rise in frequency at the onset of vibration but no gradual increase in discharge rate during maintained vibration. When vibration was removed alpha motoneurones rapidly
VIBRATION AND FUSIMOTOR NEURONES
641 ceased discharging. The measured conduction velocities of fusimotor neurones excited by muscle vibration spanned the range 19-31 m/sec, all being well within the range for gamma motoneurones. A, triceps nerves largely intact
Vibration (50 pm, 200 Hz) 60 s
I Impulses/sec
40
B, after cutting medial gastrocnemius nerve Vibration (50 am, 200 Hz) Gamma discharge
frequency C. after cutting all triceps nerves Vibration (50 pm, 200 Hz)
10 sec
Fig. 3. Effect of cutting the nerves from the triceps surae on fusimotor facilitation induced by vibration. A, triceps nerves intact except for the branch of gastrocnemius medialis nerve containing the neurone under study. B, only the nerve to gastrocnemius lateralis and soleus intact. C, no triceps nerves intact.
It was established that activity in both the gastrocnemius medialis and gastrocnemius lateralis/soleus nerves could contribute to the excitation of gastrocnemius medialis fusimotor neurones. This point is illustrated in Fig. 3 which shows the response of a fusimotor neurone to muscle vibration when the triceps nerves were intact, except for the small branch of the gastrocnemius medialis nerve containing the neurone under study (Fig. 3A) -and the response to vibration when only the gastrocnemius lateralis/soleus nerve was intact (Fig. 3B). The response to vibration was invariably abolished when both the triceps nerves had been sectioned (Fig. 3C). It was often possible to excite fusimotor neurones by squeezing the Achilles tendon. This effect could always be abolished by applying a cotton wool swab soaked in 5 % procaine solution to the tendon. This procedure never altered the response of the fusimotor neurone to vibration (Fig. 4).
JUDY R. TROTT 642 It therefore seems unlikely that stimulation of tendon nociceptors by mechanical irritation was responsible for the fusimotor excitation produced by muscle vibration. B A -
-
Vibration (50 pm, 200 Hz)
Tendon squeeze
jImpulseslsec 4
t
Gischamma discharge
D
frequency C
%5 procaine applied to tendonVibration (50 pm, 200 Hz)
-
0 sec
________Tendon squeezes
Gamma 22 m/sec
Fig. 4. Resistance of vibration-induced fusimotor facilitation to anaesthetization of the Achilles tendon. A and C, excitation of gastrocnemius medialis fusimotor neurone during vibration of the triceps sure both before and after 5 % procaine was applied to the Achilles tendon. B and D, response of the neurone to squeezing the tendon before and after the application of procaine. Note disappearance of the facilitation in response to squeezing once the tendon has been anaesthetized. It was thought desirable to examine the response of fusimotor neurones to muscle stretch in addition to muscle vibration. However, only a small range of stretches could be tested because it was impossible to move the vibrator once the Achilles tendon had been secured in place. Thus, the only stimulus tested was that of a sudden stretch of 1-2 mm applied to the probe of the vibrator to which the Achilles tendon was attached. In this position, the muscles were already stretched 9-10 mm beyond the point at which passive tension had started to develop. Sudden stretch of 1-2 mm, excited thirteen of the nineteen fusimotor neurones which were excited by vibration. Stretch-induced facilitation was usually smaller than that induced by vibration. Of the other six units excited by vibration four were unaffected by muscle stretch and the response of the other two was not examined. One of the eight neurones uuiaffected by vibration was weakly inhibited by the stretch stimulus used. It should be emphasized that the response of fusimotor neurones to stretch was not studied in depth and control experiments such as the effect of cutting the triceps nerves and anaesthetizing the Achilles tendon were not performed.
Identity of the receptor excited by vibration In the work described above muscle vibration was specifically chosen as being a stimulus which selectively excites primary endings from muscle spindles. It was therefore thought desirable to carry out control experiments
643 VIBRATION AND FUSIMOTOR NEURONES recording from muscle stretch receptors during vibration to examine whether, under the experimental conditions employed here, this stimulus was selectively exciting primary endings. Afferent activity was recorded from spindle afferents and Golgi tendon organs of triceps surae in L 7 and S 1 dorsal rootlets. Primary endings from muscle spindles were found to be appreciably more sensitive to vibration than were spindle secondary endings or Golgi tendon organs (Fig. 5). 200
@ 150" E
4'100 0r
0
I/ 0
100
300 400 200 Amplitude of vibration (pam)
500
600
Fig. 5. Responses of triceps surae afferents to vibration of 200 Hz applied to the Achilles tendon. Two primary endings (El), two secondary endings (*), and two Golgi tendon organs (A) are illustrated. The discharge frequency of each ending is plotted against the amplitude of vibration. The pairs of endings have been chosen because they represent the extremes of sensitivity to vibration found within each of the three different receptor types.
Primary endings Of the eleven primary endings studied all could be driven to discharge at frequencies of vibration 100-300 Hz applied to the triceps surae when these muscles were stretched by about 9 mm as described before. The more sensitive endings showed such a response at amplitudes of vibration of 16-50 ,um and, with one exception, all the primary endings were driven to discharge at 200 Hz when the vibration measured at the tendon had been increased to 200 Mum. The least sensitive primary ending required vibration of 350 #sm to be applied to the tendon before driving at this frequency occurred. The reason for this insensitivity is not known but it is perhaps
JUDY R. TROTT worth noting that this unit was one of only two primary endings which did not show a background discharge in the absence of muscle vibration although it was very sensitive to muscle stretch. 644
Secondary endings Out of nine secondary endings four were unaffected by vibration up to 500 ,um at a frequency of 200 Hz. None of these four units showed a background discharge in the absence of vibration. The threshold amplitude required to excite the remaining units ranged from 100 to 400 ,um at a frequency of 200 Hz. The most sensitive secondary endings were those having the highest discharge frequency in the absence of muscle vibration. In general the secondary endings did not attain high discharge rates under the influence of vibration, although one unit was driven by vibration at 200 Hz and 600 ,um.
Golgi tendon organs Of the four tendon organs studied three were unaffected by vibration up to 500 ,um at a frequency of 200 Hz. The threshold amplitude for excitation of the remaining tendon organ was 275 ,lm at 200 Hz and the unit achieved a maximum discharge frequency of 40 impulses/sec under the influence of vibration with an amplitude of 550 /sm. None of the tendon organs studied showed a background discharge in the absence of muscle vibration. It can therefore be stated that the amplitudes and frequencies of vibration which were found to excite fusimotor neurones would have 'driven' about half of the primary endings examined here, strongly excited the rest of the primary endings (with the one exception mentioned) but would not have influenced either secondary endings or Golgi tendon organs.
Spindle endings with axons of intermediate conduction velocity Attempts were made to classify spindle afferents conducting between 60 and 80 m/sec as either primary or secondary endings on the basis of their dynamic sensitivity and the regularity of their tonic discharge pattern. Difficulty was found however in quantifying the dynamic sensitivity under the conditions employed so more emphasis was placed on examining the regularity of discharge of the units under study. This was expressed as the coefficient of variation of the interspike interval distribution. Matthews & Stein (1969) found that for de-efferented spindle endings discharging at frequencies around 30/sec the range for the coefficients of variation shown by primary endings was 0-027-0-16 whereas that for secondary endings was 0-016-0024. Of the afferent units whose interspike interval distributions were analysed in the present study all units definitely classified as primary or secondary on the basis of their conduction velocity fell within
645 VIBRATION AND FUSIMOTOR NEURONES the respective ranges quoted above with the exception of one primary ending which had a coefficient of variation of 0-025. However the coefficients of variation of units conducting between 60 and 80 m/sec occupied a range 0-023-0-031 spanning the trough between the two distributions established by Matthews & Stein (1969) as being typical of primary and secondary units. Since the regularity of discharge of such endings having axons of intermediate conduction velocities has not previously been documented and since the coefficients of variation for such units in the present study were so near the borderline between primary and secondary endings, it was thought unwise to assign these units to either population on the basis of their regularity of discharge. The response of these units to vibration was very varied. Several of them were as sensitive as primary endings, for example one unit was driven to discharge at 200 Hz by vibration of 60 jum amplitude. Other intermediate endings behaved similarly to secondary endings in that they were not driven in response to vibration even at amplitudes up to 500 ,um but such units did show a slight response to vibration of 50-100 jsm amplitude, unlike typical secondary endings which were always insensitive to vibration of less than 100 ,um. The sensitivity of primary endings to muscle vibration is critically dependent on the degree of muscle stretch. At short muscle lengths it was impossible to drive some primary endings at the frequency of applied vibration but the threshold for driving dropped with the application of muscle stretch. Nevertheless, in the present study the sensitivity of all muscle receptors to vibration was apparently less than that described by Brown et al. (1967). There are two possible explanations for this: firstly, in the present experiments disturbance in the region of the triceps sure was kept to a minimum and the muscles were not dissected away from surrounding skin and connective tissue. Vibration transmitted to the belly of the muscles will therefore undoubtedly have been attenuated compared with vibration applied (and measured) at the tendon; secondly, although moderate stretch corresponding to an angle of the ankle of 90-110O was applied to the muscles before vibration at no time did the triceps surae reach their maximum physiological length (cf. Brown et al. 1967) and the difference in resting tension within the muscles may well have contributed to the relative receptor sensitivity in the two investigations.
Afferent units from non-denervated muscles other than the triceps sure, for example, those of the hip and rump, were examined for their sensitivity to vibration applied to the Achilles tendon. Most of these were appreciably less sensitive (threshold above 450 /m) than triceps surae primary endings. However, two units were excited by vibration of about 100 jsm so activity in some such units may have contributed to the reflex effects observed in the present experiments. However any such contribution must have been negligible since vibration produced no reflex effects on fusimotor neurones after the triceps surae nerves had been cut.
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Activity in ventral root filaments Muscle vibration has been shown to inhibit unidentified fusimotor neurones isolated from ventral root filaments (Brown et al. 1968). In the light of the present findings it seemed worth while to examine the response of such neurones to vibration which I knew was capable of exciting gastrocnemius medialis fusimotor neurones. Accordingly, background activity in thirty fusimotor neurones was recorded in L 7 and S 1 ventral root filaments. The peripheral destinations of these units were of course unknown. Five fusimotor neurones were excited during vibration of the triceps surae. The extent and time course of the facilitation was similar to that shown by medial gastrocnemius neurones and was produced by the same parameters of vibration. Of these five neurones, two were also facilitated by stretch of the triceps surae but the effect of muscle stretch on the three remaining units was not tested. Seven fusimotor neurones were inhibited by vibration and by stretch of the triceps surae. The remaining eighteen fusimotor neurones could not be influenced by muscle vibration despite the fact that alpha motoneurones responded to the stimulus. These fusimotor neurones were however responsive to other reflex inputs such as cutaneous stimulation. DISCUSSION
The present results have shown that longitudinal muscle vibration of a form which excited only spindle primary endings produced weak autogenetic facilitation of some fusimotor neurones. The facilitation, although weak, was a repeatable response and was maintained throughout the period of vibration. Excitation of gastrocnemius medialis fusimotor neurones was produced not only by afferent activity from that muscle, that is a strictly autogenetic effect, but was also elicited by activity in primary endings of the synergistic gastrocnemius lateralis and soleus muscles. None of the gastrocnemius medialis fusimotor neurones in the present study was inhibited during low amplitude (50-100 ,sum) vibration of the triceps surae. However, fusimotor neurones in ventral roots can be inhibited by vibration of the triceps surae in decerebrated cats (Brown et al. 1968). These authors report that 'in some preparations perhaps a fifth of the spontaneously active units were inhibited by vibration'. This was confirmed in the present experiments where activity in seven out of thirty fusimotor neurones recorded in ventral root filaments was reduced by vibration. The peripheral destinations of these neurones were of course unknown but in the light of the results gained from peripheral nerve recordings they are unlikely to have been destined for the triceps surae.
647 VIBRATION AND FUSIMOTOR NEURONES In the same way it is possible that the five fusimotor neurones in ventral root filaments which were facilitated by vibration of the triceps surae were in fact destined for those muscles, although it is also possible that activity in primary endings could facilitate fusimotor activity destined for other muscles. In the present experiments a deliberate attempt was made to use vibration of a sufficiently low amplitude such that it produced considerable excitation of primary endings without exciting less sensitive muscle receptors. This was essential since it is known that both spindle secondary endings and Golgi tendon organs are excited by muscle vibration but they are appreciably less sensitive than primary endings (Brown et al. 1967). Moreover, the threshold amplitude of vibration required to excite muscle receptors is dependent on factors such as the degree of tension in the muscle undergoing vibration (both active and passive) and the extent of fusimotor tone. To ensure that only primary endings were excited by the vibratory stimuli used in the present experiments, afferent activity in dorsal root filaments was studied in a separate series of experiments. This study showed that amplitudes of vibration up to 100 jum caused considerable excitation of primary endings from muscle spindles without having any effect on secondary endings or Golgi tendon organs. Since fusimotor facilitation was achieved by vibration at amplitudes 50-100 jtm this facilitation can definitely be attributed to activity in Ta afferents. Approximately half the primary endings studied were driven to discharge at the frequency of applied vibration (usually 200 Hz) at amplitudes of up to 100 jam. The thresholds for 'driving' of the remaining endings (with the one exception previously described in the Results section) were in the range 100-200 jsm. However, the threshold amplitudes required to excite the more sensitive of the secondary endings also lay in the range 100-200 um, making it impossible in the present experiments to excite maximally the primary ending population without exciting some secondary endings. For this reason the effect on fusimotor neurones of a maximal Ia input could not be tested. Golgi tendon organs proved to be less sensitive to muscle vibration than secondary endings from muscle spindles, the only one sensitive to vibration having a threshold of 275 ,um. In the present study primary and secondary endings were thus found to be less clearly separable on the basis of their sensitivity to vibration than was previously found (Brown et al. 1967). It is interesting to note in this context that Stuart, Mosher, Gerlach & Reinking (1970) found a higher dynamic sensitivity of group II afferents to brief muscle pulls (60 sum or less) than might have been expected, such transient stretch activating 95 % of Ia afferent units and 41 % of group II units. There is no reason to
648 JUDY R. TROTT suppose that some secondary endings, perhaps those situated near the elastic, non-viscous equatorial zone of the intrafusal muscle fibres, or alternatively the few secondary endings innervating nuclear bag fibres as opposed to nuclear chain fibres would not be more sensitive to vibration than the secondary ending population as a whole. Alternatively, there may be slight differences of sensitivity between muscle receptors of the triceps surae (studied in the present experiments) compared with those of the soleus muscle alone (Brown et al. 1967). The situation was further complicated by the behaviour of spindle afferents which could not be definitely classified as primary or secondary endings. As described before some of these showed sensitivity to vibration typical of primary endings, others gave a response intermediate between that of typical primary and secondary endings. Those which were responsive to vibration of 50-100 gm may well have contributed to the reflex effects observed on fusimotor neurones and such afferent activity, although not widespread, should not be overlooked in studying the reflex responses of motoneurones to muscle vibration. A short communication which has appeared since the present work was completed describes autogenetic inhibition of fusimotor neurones being produced by muscle vibration of 50-100 Mum (Fromm & Noth, 1975). The authors found twelve out of forty-six fusimotor neurones tested to be inhibited by muscle vibration. The authors also mention that a further twenty of the forty-six cells were, in fact, excited by vibration. This excitation appeared at a lower amplitude of vibration (15 #sm) than that described as inhibiting fusimotor neurones. Vibration at such amplitudes (15 Mum) would almost certainly have been exciting spindle primary endings rather than any other receptors. The facilitation of fusimotor neurones found by Fromm & Noth is therefore not incompatible with the results of the present study. The present experiments do not shed any light on the fusimotor inhibition which Fromm & Noth describe as appearing at an amplitude of vibration considerably higher than that required to facilitate fusimotor activity. As mentioned before the amplitude of vibration was deliberately kept low in the present study to ensure selective excitation of
primary endings. In conclusion, the demonstration of a facilitatory action of Ia afferents to some fusimotor neurones of the same muscle raises the possibility that, being an example of positive feed-back, such an action could lead to instability in the stretch reflex. This would obviously constitute a hazard in normal muscle activity. However, it should be emphasized that the facilitation is a weak response to an almost maximal input from primary endings and as such is unlikely to occur under normal conditions of muscle
activity.
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I am grateful to Dr P. H. Ellaway and Mr J. E. Pascoe for valuable help and discussion throughout the work and constructive criticism of the text. I also thank Mrs Maria Winder for skilful technical assistance and the M.R.C. for the provision of a scholarship for training in research methods.
REFERENCES BROWN, M. C., ENGBERG, I. & MATTHEWs, P. B. C. (1967). The relative sensitivity to vibration of muscle receptors of the cat. J. Phy8iol. 192, 773-800. BROWN, M. C., LAWRENCE, D. G. & MATrREws, P. B. C. (1968). Reflex inhibition by Ia afferent input of spontaneously discharging motoneurones in the decerebrate cat. J. Phy8iol. 198, 5-7P. ELDRED, E., GIIANrr, R. & MERTON, P. A. (1953). Supraspinal control of the muscle spindles and its significance. J. Physiol. 122, 498-523. FROMM, CHR., HAASE, J. & NoTH, J. (1974). Length-dependent autogenetic inhibition of extensor y-motoneurones in the decerebrate cat. Pfluger8 Arch. ge8. Phy8iol. 346, 251-262. FROMM, CHR. & NoTH, J. (1975). Vibration-induced autogenetic inhibition of gamma motoneurons. Brain Res. 83, 495-497. HUNT, C. C. (1951). The reflex activity of mammalian small-nerve fibres. J. Phy8iol. 115, 456-469. HUNT, C. C. & PANTAL, A. S. (1958). Spinal reflex regulation of fusimotor neurones. J. Physiol. 143, 195-212. KOBAYASHI, Y., OsimA, K. & TASAIx, I. (1952). Analysis of afferent and efferent systems in the muscle nerve of the toad and cat. J. Physiol. 117, 152-171. MATTHEWS, B. H. C. (1933). Nerve endings in mammalian muscle. J. Phy8iol. 78, 1-53. MArT ws, P. B. C. & STEIN, R. B. (1969). The regularity of primary and secondary muscle spindle afferent discharges. J. Physiol. 202, 59-82. STUART, D. G., MOSHER, C. G., GERLACH, R. L. & REINKING, R. M. (1970). Selective activation of Ia afferents by transient muscle stretch. Expl Brain Res. 10, 477487. TROTr, JUnDY R. (1975). Reflex responses of fusimotor neurones during muscle vibration. J. Physiol. 247, 20-22P.
