primary endings. endings of muscle spindles without exciting the secondary endings of

Transcription

1 J. Physiol. (1967), 192, pp With 15 text-figures Printed in Great Britain THE RELATIVE SENSITIVITY TO VIBRATION OF MUSCLE RECEPTORS OF THE CAT By M. C. BROWN, I. ENGBERG* AND P. B. C. MATTHEWS From the University Laboratory of Physiology, Oxford (Received 12 April 1967) SUMMARY 1. Longitudinal vibration was applied to the de-efferented soleus muscle of anaesthetized cats while recording the discharge of single afferent fibres from the proprioceptors within the muscle. Conditions were defined under which vibration can be used to excite selectively the primary endings of muscle spindles without exciting the secondary endings of muscle spindles or Golgi tendon organs. 2. Frequencies of vibration of c/s were used. The maximum amplitude of vibration which the vibrator could produce fell with increasing frequency; it was 250,t (peak to peak) for 100 c/s and 20,u for 500 c/s. 3. Primary endings of muscle spindles were very sensitive to vibration. Most could be 'driven' to discharge one impulse for each cycle of vibration over the whole of the above range of frequencies, provided the initial tension was moderate ( g wt.). The amplitude of vibration required to produce driving usually varied by less than a factor of two over the whole range of frequencies. The most sensitive endings could be driven by vibrations of below 10 t amplitude. 4. Stimulation of single fusimotor fibres, whether static or dynamic fusimotor fibres, increased the sensitivity of primary endings to vibration. Contraction of the main muscle, produced by stimulating ac motor fibres, reduced the sensitivity of primary endings even when fusimotor fibres were also being stimulated. 5. The secondary endings were very insensitive to longitudinal vibration and with the amplitudes available not one of twenty-five endings could be driven at 150 c/s or above; one ending could be driven at 100 c/s by vibration of 250 t amplitude. Stimulation of single fusimotor fibres, probably all of which were static fusimotor fibres, made them slightly more sensitive to vibration but none of them approached the sensitivity of the primary endings. 6. The Golgi tendon organs were as insensitive as the secondary endings * Wellcome-Swedish travelling research fellow. Present address: Department of Physiology, University of Gbteborg, Sweden.

2 774 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS when the muscle was not contracting and none could be driven at any frequency in spite of quite high tensions in the muscle. However, when the muscle was made to contract by stimulating z fibres in ventral root filaments the tendon organs became appreciably more sensitive, the degree of sensitization increasing approximately with the strength of the contraction. They never became as sensitive as the primary endings, and with the amplitudes of vibration available none was driven at frequencies of over 250 c/s. 7. When the amplitude of vibration was somewhat below that required to produce driving of an ending it still produced some increase in its mean frequency of discharge. However, amplitudes of vibration of 25-50,t applied to a non-contracting muscle, whether with or without fusimotor stimulation, produced driving of nearly all primary endings without any significant increase in the mean frequency of firing of secondary endings or Golgi tendon organs. Such vibration can therefore be used as a specific stimulus for the primary endings in order to investigate the central effects of repetitive discharge of the Ia afferent fibres from them. 8. Experiments on endings in the peroneus longus muscle showed that these behaved similarly to those in soleus. INTRODUCTION High frequency vibration of small amplitude has recently been found to be a potent stimulus for eliciting various reflexes when applied to the belly of a muscle or to its tendon. In conscious man, it produces a slow involuntary contraction of the vibrated muscle (Hagbarth & Eklund, 1966; De Gail, Lance & Neilson, 1966; Rushworth & Young, 1966). In the decerebrate cat, vibration excites a tonic stretch reflex which persists as long as the vibration continues (Matthews, 1966a, b). These effects have been attributed to the vibration exciting the primary endings of muscle spindles, for several workers have previously shown the great ease with which these receptors may be so excited (Kuffler, Hunt & Quilliam, 1951; Granit & Henatsch, 1956; Bianconi & Van der Meulen, 1963; Crowe & Matthews, 1964). Detailed interpretation of the reflex effects of vibration has been hindered by our relative ignorance of the sensitivity to vibration of the two other main proprioceptors of muscle, namely the secondary endings of the muscle spindle and the Golgi tendon organs. The Golgi tendon organs of limb muscles have never previously been studied in this respect. The secondary endings were studied by Bianconi & Van der Meulen (1963), who applied localized vibration to points on the belly of a muscle by means of a stylus of 1-5 mm diameter attached to a vibrator. By careful adjustment of the position of the vibrating stylus they were able to 'drive' nearly half the secondary endings studied to

3 VIBRATION AND MUSCLE RECEPTORS 775 discharge one impulse for each cycle of vibration up to maximal frequencies of c/s, the precise value depending upon the individual ending. All primary endings could be driven on appropriate localization of the vibrating stylus, and on average the maximum frequency to which they could be driven was slightly higher than that to which the sensitive secondary endings could be driven. Bianconi & Van der Meulen (1963) also found that none of the secondary endings could be driven when the stylus was applied to the tendon of the muscle, though apparently most or all of the primary endings could be so excited. Thus the secondary endings were shown to be somewhat less sensitive to vibration than were the primary endings. The only measurement they gave, however, was the maximum frequency to which each receptor could be driven by their particular vibrator with its glass stylus. Very recently, Euler & Peretti (1966) have shown that different spindle endings in the external intercostal muscle may differ markedly in their sensitivity to vibration applied to the ribs and suggested, without additional evidence for their identification, that those which were relatively insensitive to vibration were secondary endings. They also noted that Golgi tendon organs showed only a moderate sensitivity to vibration under their particular conditions. The present experiments were performed to make a more systematic comparison of the effects of longitudinal vibration on all three types of receptor. The experiments have permitted us to define certain conditions under which vibration can be used to selectively excite the primary endings, as already briefly described (Brown, Engberg & Matthews, 1967). METHODS The experiments were performed on twenty-five cats which were anaesthetized with pentobarbitone sodium (Nembutal, Abbott laboratories) given intraperitoneally. The main study was on receptors in the soleus muscle, and nineteen of the experiments were on this muscle. The peroneus longus muscle and the medial head of gastrocnemius were each studied in three experiments to see how far the receptors in another muscle behaved similarly to those in soleus. The methods were generally similar to those used before (Matthews, 1962, 1963b; Crowe & Matthews, 1964). The hind-limb was widely denervated except for the muscle being studied, and the appropriate dorsal and ventral spinal nerve roots were severed so as to isolate the muscle from the spinal cord. The discharge of functionally single afferent fibres supplying individual receptors in the muscle studied was recorded from thin dorsal-root filaments. Functionally-single fusimotor fibres to the soleus muscle were isolated in thin ventral root filaments. They were sought by means of their excitatory action on a previously selected spindle ending (Matthews, 1962); all those isolated proved to be y motor fibres producing no detectable contraction of the muscle and with conduction velocities below 45 m/sec. The afferents were classified as belonging to spindle endings or Golgi tendon organs on their behaviour during contraction of the muscle elicited by atimulation of its nerve (B. H. C. Matthews, 1933). To permit unequivocal classification of every ending studied it was found helpful on occasion to stimulate thick ventral root filaments as well as the muscle nerve, and to use repetitive as well as single shock stimulation. The spindle endings were classified as primary or secondary endings on the conduction velocities of their

4 776 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS afferent fibres (Hunt, 1954; Matthews, 1964). To avoid any possible confusion, endings with afferent fibres with velocities near the arbitrary dividing line (72 m/sec) are not included in the main results to be presented; all the endings which were classified as primary endings had afferents which conducted at velocities over 80 m/sec, and all endings classified as secondary endings had afferents with conduction velocities below 60 m/sec. The rectal temperature of the cat varied from 35 to 390 0, and the temperature of the paraffin pool covering the muscle studied was usually similar. Generation and measurement of vibration. The muscle was vibrated longitudinally by an electromagnetic puller which was attached to its tendon and which could also be used to stretch the muscle by varying amounts. The tendon was attached directly to a rigid extension of the puller, without any intervening threads or chains which might have attenuated the vibration. The puller, which has been described before (Matthews, 1962), consists of a vibrator (Goodmans, model 390 A) controlled by means of length feed-back from a linear transducer to form a positional servomechanism. To produce the vibration a sine wave from an audio oscillator was resistively fed into a late stage of the control amplifier. The relation between the amplitude of movement and the frequency of the input signal was determined by direct observation of the moving element of the vibrator using a micrometer eyepiece (Matthews, 1966b). This showed that for a constant input voltage the movement fell off approximately as the square of the frequency. It was also observed that the movement for any particular input was not significantly reduced by connecting the vibrator to the muscle, nor was it when the muscle was made to contract by stimulating thick ventral root filaments. To simplify the conduct of the experiment the output from the oscillator was led to the vibrator circuit through a filter network which had the inverse frequency characteristic of the vibrator, so that a given voltage from the oscillator produced a constant movement irrespective of its frequency in the range c/s. The adjustment of this system and the routine checking of the amplitude of vibration under various conditions was greatly facilitated by using a piezo-electric accelerometer (Ether, XA 3) whose output was doubly integrated by operational amplifier circuits to measure the movement. All measurements given in this paper are of peak-to-peak amplitude. The stated amplitudes of vibration are only accurate to about + 10 %, but this appeared adequate for the present investigation. The maximum amplitudes of vibration conveniently available at various frequencies were as follows: loo c/s, 250 je; 150 c/s, 250,; 200 c/s, 190 /i; 250 c/s, 100 /t; 300 c/s, 70,; 400 c/s, 35,u; 500 c/s, 20,4. These were the magnitudes of vibration which could still be produced when the stretcher was producing a steady force of up to 1 kg wt., to overcome any steady tension in the muscle; slightly larger amplitudes of vibration could be produced at lower tensions. The tension developed in the muscle at any time was recorded by an isometric myograph on the end of the vibrator and through which the vibration was transmitted. The tendon was connected directly to the myograph. The myograph utilized a semi-conductor strain gauge (Ether 3A-lA-350P) and was rigidly isometric (compliance less than 041 mm/kg wt.). The vibrator under servo-control was, however, appreciably less stiff (compliance 0-6 mm/ kg wt.). Recording. The discharge of the afferent fibres was recorded in three different ways. For each of them the spikes were used to trigger a short pulse, which then served as a representation of the existence and the timing of a spike. The first recording method was to photograph a reciprocal pulse interval display which has been described previously (Matthews, 1963 b). This permitted ready detection of whether or not an ending was being 'driven' by the vibration, but as it was recorded on slowly moving photographic paper it did not easily permit determination of the average frequency of the ending when the pulse interval was variable. The second method was to use the pulses to brighten momentarily the spot of a cathode ray tube beam which was sweeping vertically, with a constant velocity, at 10 repetitions/sec. This was photographed on horizontally moving paper. The number of spots in each vertical sweep could then be counted to determine the average frequency over any

5 VIBRATION AND MUSCLE RECEPTORS 777 chosen number of tenths of a second. The third method was to feed the pulses into a gated counter with a digital display (Elesta type CPT 41 AO1). The gate was set to open for a period of half a second beginning 100 msec after the start of a 1 see period of vibration, and the resulting count then read off visually. Cathode ray tube displays of the length of the muscle (from the linear transducer) and of the tension in it (from the strain gauge myograph mounted on the vibrator) were usually also photographed alongside the records giving the frequency of the action potentials. During a typical experiment the muscle would be vibrated for a period of about 1 sec every 10 or 15 sec, and in between each period of vibration the amplitude or frequency of vibration would be altered. When appropriate, the vibration would be superimposed on a 2 see period of stimulation of a single fusimotor fibre (to excite a spindle ending), or on a 2 sec period of muscular contraction evoked by ventral root stimulation (to excite a Golgi tendon organ). In order to provide a definite slight tension in the muscle during the vibration without running the risk of interfering with its circulation the muscle was automatically stretched a few millimetres a second or two before applying the vibration (see Fig. 1), and released again shortly afterwards. All these various eventts were triggered automatically at constant time intervals within any one experiment. The final length of the muscle when stretched was within a few mm of the maximum length it could take up in the body and the tension in it was g wt. 600 _ O sec E 200 Length Tension ~- - E. Vibration 300/sec 500/sec Fig. 1. Response of a primary ending to vibration of an amplitude sufficient to 'drive' it. Top, reciprocal pulse interval display showing the instantaneous frequency of discharge of the ending. Bottom, diagrammatic marker showing the period of vibration. The peak-to-peak amplitude of the vibration was 25 /%; its frequency was either 300 or 500 e/s. Middle, records of the stretch applied to the muscle to make it taut (6 mm applied at 10 mm/sec, note typical response of primary ending), and the tension in the muscle (tension increase 60 g wt.). The thickening of the myograph trace during the vibration is largely the direct response of the myograph to being vibrated, and does not represent a true vibratory tension change. (Conduction velocity of afferent fibre of ending, 114 m/sec.) RESULTS Primary endings. As expected from previous work the primary endings proved to be very sensitive to longitudinal vibratory stretching of the muscle. A typical example of the behaviour of an ending in a de-efferented spindle is shown in Fig. 1, where as always the vibration was applied after stretching the muscle to make it taut. The ending was 'driven' by vibration of 25 It peak to peak amplitude to discharge one impulse for each

6 778 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS cycle of the vibration for frequencies of vibration of both 300 and 500 c/s. The reciprocal pulse interval display would have shown, by the occurrence of a spot at half the vibratory frequency, if even a single cycle of the vibration had failed to excite an impulse. Direct recording of the action potentials on a sweep locked to the vibration signal showed that during 'driving' the action potentials always occurred at a constant position with regard to the vibration (cf. Granit & Henatsch, 1956; Bianconi & Van der Meulen, 1963). Because of the unknown conduction delay from the spindle to the site of recording no significance could be attached to the precise phase of the vibration at which the potential occurred, nor was the variation of this phase angle with frequency studied. As might be expected (Granit & Henatsch, 1956), the threshold amplitude of vibration required to produce driving at any particular frequency varied with the initial tension. An example is shown in Fig. 2 where, as was typical, the amplitude required for driving fell to an approximately constant value as the length of the muscle and thus the tension in it were increased. Measurements of the vibration sensitivity of endings were therefore taken with the muscle stretched to near the maximum length it could take up in the body and at a length at which the vibration sensitivity of each ending appeared to be little altered by small changes in length. Figure 3 shows for three different primary endings the minimum amplitudes of vibration required at a series of frequencies to produce driving. (An ending was accepted as being driven provided that within a period of vibration lasting 1 sec it responded to all but 3 or 4 cycles.) Unit B, the ending of Fig. 1, behaved typically and was driven at all frequencies by vibration of 25 u amplitude. Unit C was one of the most sensitive endings studied and was driven at most frequencies by amplitudes of below 5,u. Unit A was relatively insensitive. The range of the sensitivity of all the primary endings studied is dealt with later (Figs. 11, 12). For Units B and C the relation between amplitude for driving and frequency is largely flat and this was the usual finding, but a number of endings showed a definite 'hump' on the curve around 200 c/s, as did Unit A. Very much larger amplitudes of vibration at low frequencies sometimes caused the more sensitive endings to fire more than one spike for each cycle of the vibration (cf. Granit & Henatsch, 1956; Bianconi & Van der Meulen, 1963). Secondaryfactors influencing sen8sitivity to vibration. The threshold amplitude of vibration required to produce driving was repeatable to about + 10 % on successive determinations under the same conditions. This was also the accuracy to which the amplitude of any vibration was known, though relative values at any particular frequency were appreciably more accurate and so also was the accuracy with which a vibration of any particular amplitude could be repeated. For a minute or two after stretching the muscle to physiological full extension, the vibration responsiveness of an ending at any shorter length of the muscle was less than it otherwise would have been. The vibration sensitivity then rose

7 VIBRATION AND MUSCLE RECEPTORS t 0.> k b4 N -, d2 80 *_r Extension (mm) Full extension Fig. 2. Threshold amplitudes of vibration required to 'drive' a primary ending at 200 c/s with the muscle at a series of different lengths. Abscissa, increase in the length of the muscle above its shortest physiological length. Ordinate, either the threshold amplitude of vibration in,u ( 0), or the tension in the muscle in g wt. (0); the figures and scale apply to both measurements. The initial length of the muscle (see Fig. 1) was always 7 mm on the above scale; at this length driving could not be produced by the maximum amplitude of vibration available. 50 r- 40F F Unit A Unit B 0. '. 0 20j- 1- Z 10 M Unit C l 1- ^].-~. * v I Vibration frequency (c/s) Fig. 3. Threshold amplitudes ofvibration required to 'drive' three different primary endings at a series of frequencies. Unit A was of relatively high threshold and showed a 'hump' on the graph at low frequencies. Unit B was typical, and is the ending of Fig. 1. Unit C was one of the most sensitive endings studied. The symbols on the base line give the frequencies at which the endings were firing in the absence of vibration.

8 780 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS progressively to its 'normal' value. In contrast, after a period of fusimotor stimulation the threshold to vibration sometimes appeared to be lowered for a time (cf. post-excitatory facilitation, Kuffler et al. 1951; Harvey & Matthews, 1961). The responsiveness of the endings to vibration was tested about 1 sec after the muscle had been stretched up to the desired length (cf. Fig. 1). On altering this interval between the stretching and the vibration, the threshold amplitude required for driving sometimeschanged slightly. There was, however, no consistent effect; lengthening the interval sometimes increased the sensitivity of the ending and sometimes decreased it. These various second-order effects were not explored systematically. They may well have depended upon a variation in the mechanical properties of the muscle affecting the transmission of the vibration as well as upon spindle properties. 'Humps.' For about a third of the endings in soleus there was a range of frequencies for which considerably higher amplitudes of vibration were required to produce driving, thus producing a 'hump' on the curve relating the amplitude for driving to the frequency. An example is Unit A in Fig. 3; this ending had rather a high threshold, but 'humps' were also seen for endings with lower thresholds, and Unit B shows a small 'hump'. The width of the base of any hump was usually around 100 c/s and its maximum fell in the range c/s; at the peak of the hump the threshold for driving could be as much as double its value at other frequencies. Granit & Henatsch (1956) described similar humps on applying vibration to the surface of the muscle with a stylus attached to a vibrator and these were taken to indicate a 'specific resonance in the end organs'. In three of the present experiments receptors in the medial head of gastrocnemius were studied and even more marked humps were found for eight out of the ten primary endings investigated. We believe that such humps are due largely if not wholly to the mechanical properties of the muscle determining the way in which vibration is transmitted, and that they cannot be taken to demonstrate a specific 'resonant' behaviour of the spindle or its ending. The reasons for this belief are: first, on increasing the tension in the muscle the frequency at which the maximum of the hump occurred sometimes increased, and sometimes the hump decreased in size; second, fusimotor stimulation, whether of static or dynamic fibres, did not appreciably alter the frequency at which the hump occurred nor the relative sensitivity of the ending at the hump frequency relative to its sensitivity at other frequencies third, and most cogently, microscopic observation of small glass balls or of hairs on the surface of the gastrocnemius showed a decrease in amplitude of the vibratory movement at a given point in the muscle for certain frequencies in the range in which humps were observed. Sometimes the movement was as much side to side as it was to and fro (i.e. a moving point traced out a circle or an ellipse rather than a line), and the angle along the muscle at which maximum movement occurred varied with the frequency. The occurrence of local resonances within a vibrated muscle need cause little suprise, and would be adequate to explain the observed humps. Velocity of transmission of vibration. It is of interest to know the velocity of propagation of the vibration along the muscle because the stimulating efficacy of a given amplitude of vibration applied to the tendon may depend on its wave-length in the muscle. When the velocity is high the wavelength will be long and the amplitude of the vibratory deformation of any spindle will only be a fraction of that of the applied vibration, because it occupies only a fraction of the length of the muscle. If, however, the velocity of propagation of the vibration were low enough for its wavelength to become comparable to the length of a muscle spindle, its stimulating efficacy would be greatly increased. In the extreme, if the spindle lay close to the tendon and were exactly half a wave-length long, the

9 VIBRATION AND MUSCLE RECEPTORS 781 amplitude of the vibratory deformation of the spindle would be twice that applied to the tendon because the two ends of the spindle would always be moving in opposite directions. For example, if 50 It vibration were applied to a 5 cm long muscle then a 5 mm long spindle could be stretched by up to 100 /t if the wave-length were appropriately short, while it would be stretched by only 5,t if the wave-length were long. In fact, the experiments described below show that under all the present conditions the wave-length of the vibration is long in relation to the length of the spindle. Probe A Probe B < Separation (cm) Fig. 4. Records showing the finite velocity of propagation of the vibration along the muscle. Two piezo-electric probes were placed at different points on the muscle while it was being vibrated at 400 c/s with an amplitude of 35,P. Probe A remained at the same point on the muscle. Probe B was separated from it by the distance shown below the records. The amplification of the record from probe B is not constant for the different records. Muscle tension 25 g wt., muscle not contracting. The propagation of the vibration along the muscle was studied by means of a piezo-electric crystal to which was attached a fine rigid metal stylus about 1 cm long with a sharp point on its end. This was very lightly pushed on to the surface of the belly of the muscle at a series of points along its length, and the muscle then vibrated in the usual way. Altering the pressure with which the stylus was inserted into the muscle did not change the phase of the record obtained, but did change its amplitude. The progressive attenuation of the vibration as it travelled along the muscle could not therefore be studied by this means. Usually, two piezo-electric probes were used simultaneously and the distance between them altered. Figure 4 shows records obtained in this way on vibrating at 400 c/s. The velocity of propagation of the vibration was about 40 m/sec. Similar values for the propagation velocity were obtained on using vibration at different frequencies, and when the exciting mechanical stimulus was a brief jerk of the muscle (a sudden displacement of the stretcher lasting just under 10 msec). Increasing the tension in the muscle by stretching it increased the velocity and in another experiment in which the muscle was finally stretched several millimetres beyond its maximum physiological length (producing a tension of 400 g wt.) the velocity increased to over 200 m/sec. In addition, when the muscle was made to contract by repetitively stimulating thick ventral root filaments the velocity also increased greatly; for example, in the experiment illustrated in Fig. 4 it was considerably

10 782 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS above 200 m/sec during a contraction developing 250 g wt. In three experiments on non-contracting muscle the velocity varied between 40 and 80 m/sec when the tension in the muscle was at the lowest value at which the effect of vibration on the endings had been studied. A velocity of 40 m/sec corresponds to a wave-length of 40 cm for 100 c/s vibration, and to 8 cm for 500 c/s vibration. The reduction of wave-length with increase in frequency may tend to increase the stimulating efficacy of the higher frequencies, but the effect does not seem likely to be great, as the muscle spindles of soleus are only a few millimetres long (Boyd, 1962). When the muscle is under appreciable tension, whether from contraction or stretch, the wave-length of the vibration will be so long that the progressive reduction of wave-length with frequency must be unimportant (i.e. for 250 m/sec propagation the wave-length at 500 c/s would be 50 cm.) A.Vibration alone, Vibration 5 7*5, 125,a 5 c) _- Stimulation B. Vibration during fusimotor stimulation.0,.. sec Vibration 5,c 7.5,j Fig. 5. Sensitization of a primary ending to vibration by fusimotor stimulation. The frequency of vibration was 400 c/s; its amplitude is shown below each record. In B, a single static fusimotor fibre was stimulated at 100/sec for the periods shown by the marker. (Conduction velocity of fusimotor fibre, 35 m/sec, and of afferent fibre 100 m/sec.) Fusimotor stimulation. Crowe & Matthews (1964) found that stimulation of either static or dynamic fusimotor fibres enabled individual primary endings to be driven by an amplitude of vibration which otherwise would not have done so. They suggested that fusimotor stimulation produced a sensitization of the ending to vibration by causing an alteration in the mechanical properties of the intrafusal fibres consequent on their contraction. In support of this was their finding that the increase in the frequency at which an ending could be driven by vibration of a particular amplitude was greater than the increase in the discharge of the ending produced by fusimotor stimulation acting alone. We have now fortified their conclusion by studying the matter in more detail. The present experiments were performed by vibrating the muscle at a

11 VIBRATION AND MUSCLE RECEPTORS 783 fixed frequency with a series of different amplitudes of vibration both in the presence and in the absence of fusimotor stimulation. Figure 5A shows typical responses to vibration at 400 c/s obtained in the absence of fusimotor stimulation. It will be seen that the vibration still had a considerable excitatory action on the ending when its amplitude was somewhat below that required to cause driving; the ending then tended to fire u 400 With stimulation Passive X 200 I ~ ~ ~ I ~~II I Amplitude of vibration (u) Fig. 6. Sensitization of a primary ending to vibration by fusimotor stimulation. The mean frequency of discharge of the ending during vibration at 400 c/s is plotted against the amplitude of the vibration. 0, no fusimotor stimulation; *, during stimulation of a single static fusimotor fibre at 100/sec. The frequency of discharge was measured over a period of 0 5 sec. Same experiment as Fig. 5. at various subharmonics of the vibration frequency. The bottom curve of Fig. 6 shows the mean frequency of firing of the ending produced by a range of amplitudes of vibration. An amplitude of 5,t had rather little excitatory action, and vibrations of 15 #tt and above produced driving. Figures 5 and 6 also show the effects produced by the same amplitudes of vibration applied during stimulation of a static fusimotor fibre at 100/sec. In terms of impulses/sec the effect of fusimotor stimulation was much greater than a simple additive one. Fusimotor stimulation acting alone added only 20 impulses/sec to the discharge of the ending. The top curve of Fig. 6, which shows the effects of combining vibration with fusimotor stimulation, lies much more than 20 impulses/sec above the bottom curve. Thus it may be concluded that stimulation of this particular static fusimotor fibre sensitized the ending to vibration, over and above any effect to be expected from the simple increase in the frequency of firing of the ending. Similar sensitization to vibration was demonstrated over a range of frequencies of vibration on stimulating each of 10 static fusimotor fibres and each of 4 dynamic fusimotor fibres, though the whole range of frequencies and amplitudes was not studied in every case. The static fusimotor fibres were studied the more intensively, as the result is perhaps less to be expected 50 Physiol. I92

12 784 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS and because only four of them were studied in the earlier experiments (Crowe & Matthews, 1964). On stimulation of any particular fusimotor fibre its sensitizing action increases with the frequency of stimulation up to a maximum of about 150/sec. The sensitizing action as assessed by the extra impulses produced by the combined effects of vibration and fusimotor stimulation (compared to the sum of individual actions) was most easily demonstrable using the higher frequencies of vibration; this might be expected. As Fig. 6 shows, the threshold amplitude of vibration required to produce driving was reduced by fusimotor stimulation and in this we have regularly confirmed Crowe & Matthews (1964). The action of fusimotor stimulation in this respect was somewhat more marked when the muscle was only slightly stretched so that the threshold for driving in the absence of fusimotor stimulation was relatively high, but the effect was seen at all lengths. It may be noted also that by chance none of the fusimotor fibres which were stimulated caused the ending to fire with the rhythm of the stimulus, as sometimes occurs on stimulation of static fusimotor fibres (Crowe & Matthews, 1964; Appelberg, Bessou & Laporte, 1966). It is of interest to enquire whether static and dynamic fusimotor fibres differ in the strength of their action in sensitizing the primary ending to vibration. Crowe & Matthews (1964) found no gross qualitative difference and the present experiments have done no more than confirm this general impression. In making a precise comparison of the action of the two kinds of fusimotor fibre it is desirable to study the action of either kind of fibre upon a single spindle, and moreover it is desirable that both fibres should produce approximately the same excitatory action on the ending when the muscle is at a constant length, for it was found that the strength of action of different fusimotor fibres in sensitizing an ending to vibration tended to be correlated with their direct excitatory action on the endings. In the present limited series such similarity of effects was not obtained for the action of different fusimotor fibres in the same spindle, and arguments based on results obtained with different fusimotor fibres acting on different spindles did not lead to a convincing conclusion. Secondary endings. The secondary endings proved to be very much less sensitive than the primary endings to longitudinal vibration of the muscle applied under the same conditions. Figure 7A shows as an example the response of a secondary ending to vibration of various amplitudes at 200 c/s. An amplitude of vibration of 25,u produced virtually no increase in the frequency of discharge of the ending and when the amplitude of vibration was increased some seven times to 190 It (the maximum available) the ending was still not 'driven' although its mean frequency of discharge was increased by the vibration. (Its frequency during the vibration was 54 impulses/sec compared with 30 impulses/sec in the absence of vibration.) This was a typical finding. None of the 25 secondary endings studied could be driven at frequencies of 150 c/s or above using the maximum amplitudes available with our apparatus (see Methods); and only one ending could be driven at 100 c/s, when it required an amplitude of 250 /t. Nor was any appreciable increase in the frequency of discharge of the endings produced by vibration of 50,u amplitude or below. Vibrations of It amplitude increased the discharge of most endings only slightly (cf. Fig. 8). The insensitivity of the secondary endings was not changed by stretching the muscle up to and beyond the maximum in situ physiological length.

13 VIBRATION AND MUSCLE RECEPTORS 785 The effect of fusimotor stimulation on the vibration sensitivity of eight secondary endings from four cats was studied using sixteen single fusimotor fibres. Six of these were shown to be static fusimotor fibres because it was possible to find a primary ending which was influenced in the appropriate manner by them, and the remaining ten were also probably static fusimotor fibres, for these are the only ones with an appreciable action on the secondary ending (Appelberg et al. 1966). Figure 7B shows the results obtained A. Vibration alone o sec Vibration 25# 50# 190,u o 200 B. Vibration during fusimotor stimulation Stimulation Vibration , 50,c 190G Fig. 7. Response of a secondary ending to vibration. The frequency of vibration was 200 c/s; its amplitude is shown below each record. In B a single fusimotor fibre was stimulated at loo/sec for the period shown by the marker. (Conduction velocity of fusimotor fibre 27 m/sec, and of afferent fibre 45 m/sec. As in Fig. 1 the vibration was applied shortly after stretching the muscle to make it taut.) on stimulating a fusimotor fibre with a particularly powerful excitatory action on a secondary ending. It did not, however, enable the ending to be driven securely by the vibration even when the amplitude of vibration was 190 set. Vibration of 25 #t applied during fusimotor stimulation again led to very little increase in the firing rate of the ending, but 190 /u gave a greater increase in the discharge during fusimotor stimulation than it did in its absence. Figure 8 brings together the results obtained with vibration for eight secondary endings which were studied both in the presence and absence of fusimotor stimulation. The frequency of each ending in impulses/sec during the period of vibration is plotted on the ordinate against its frequency with the vibration switched off. The axes of the graphs are symmetrical and therefore any increase in frequency brought about by the vibration shows up as a point displaced upwards from the dashed 450 line. 50-2

14 786 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS The open symbols give the results from the eight receptors without fusimotor stimulation, and show that 25,u vibration had no appreciable excitatory action, but that 190 /t had a definite effect and three of the endings then fired at 100 impulses/sec or half the vibration frequency. 200 A. 300 c/s, 25#u B. 200 c/s, 190Qs ** ~~~~~~~~~~~~~~ o / I~~~~ 0 I~~~~~~~ IS so *o, IS Frequency without vibration (impulses/see) Fig. 8. Responsiveness of secondary endings to vibration, both in the presence (-) and in the absence of fusimnotor stimulation (O). Each point relates the frequency of firing of an endling in the absence of vibration (abscisam) to its frequency of firing during vibration (ordlinate), applied under the same conditions. If the vibration had no excitatory effect the frequency is the same in both cases and the point lies on the 450 line (dashed). A, vibration of 25,u amplitude at 300 c/s, B, 190 /cf amplitude at 200 cls. Frequency of fusimotor stimulation, 100/sec. The frequency of firing was measured over 0-5 sec. Results from eight endings, most of which were studied during stimulation of more than one fusimotor fibre. The filled symbols show the results found during fusimotor stimulation. The endings remain insensitive to vibration of 25 jc amplitude, but did become somewhat more responsive to the 190#j vibration and three of them were driven by this amplitude. It may be concluded that though fusimotor stimulation produces some increase in the responsiveness of secondary endings to vibration, they none the less all remain relatively insensitive in terms of the standard set by the primary endings. Gol,gi tendon organ8. In the absence of muscle contraction Golgi tendon organs proved to be largely insensitive to vibration of the amplitudes presently available. This was regularly so when the muscle was stretched up to approximately the maximum length of which it was capable in the body, and was producing a tension of up to 200 g wt. At these lengths and tensions the majority of endings were not firing (cf. Jansen & Rudjord,

15 VIBRATION AND MUSCLE RECEPTORS ). Larger stretches producing tensions up to 500 g wt. were tested in one experiment without effect on the vibration sensitivity, but were not used regularly for fear of damaging the muscle or of altering the behaviour of the spindle endings by interference with the blood supply (Matthews, 1933). None of the Golgi tendon organs studied could be driven at frequencies of 100 c/s or over (lower frequencies were not tested), and their mean frequency of discharge was usually only very slightly or not at all increased by the vibration. An example of this is shown in Fig. 9 for an ending which was particularly sensitive to simple stretch of the muscle. 200_ E A. Vibration alone I sec Vibration 50u 150,u B. Vibration during stimulation Tension O.'''' - - Stimulation g Vibration 50# 150,u Fig. 9. Response of a Golgi tendon organ to vibration. The frequency of vibration was 200 c/s; its amplitude is shown below each record. In B, a thick ventral root filament was stimulated at 30/sec for the periods shown by the marker. (Tension in absence of contraction about 15 g wt.) In contrast, when the soleus was made to contract by stimulating ventral root filaments the Golgi tendon organs became appreciably sensitive to vibration, though still not nearly as sensitive as the primary endings in the non-contracting muscle. This increase in responsiveness to vibration is shown in Fig. 9B, where the ending was driven by vibration at 200 c/s of 150 It amplitude when the muscle was producing a tension of 250 g wt. and the ending was firing at 85/sec in the absence of vibration. The amplitudes of vibration required to produce driving of one particular Golgi tendon organ on stimulation of three different ventral root filaments is shown graphically in Fig. 10. This was the most sensitive of the soleus endings. The greater the initial frequency of discharge of the ending the more readily could it be driven, and this finding was typical. In contrast to the behaviour of the primary endings, the Golgi tendon organs could only be driven when the frequency of vibration was within about 100 c/s of the

16 788 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS frequency of discharge of the ending without vibration. When the frequency of vibration was set just below the basic frequency of discharge this would usually still produce driving with a consequent decrease in the discharge of the ending; this was also seen for some primary endings during stimulation of fusimotor fibres. The precise mechanism of the action of contraction in sensitizing the Golgi tendon organs to vibration has not been established. One factor is that any increase in the initial rate of firing of an ending before applying the vibration might be expected to make it more sensitive, particularly when the vibration frequency is close 250. ^t 200 tmaximum output from vibrator v-61 I A/. I' Vibration frequency (c/s) Fig. 10. Threshold amplitudes of vibration required to drive a Golgi tendon organ at a series of frequencies when the muscle was contracting. This ending was the most sensitive to vibration of all the Golgi tendon organs studied in soleus. The different symbols were obtained on stimulating different ventral root filaments. The symbols on the base line give the frequencies at which the ending was firing in the absence of vibration. V, was obtained in the absence of muscle contraction and the ending could not then be driven by any amplitude of vibration available. The tensions during stimulation were 100 g wt. for 0, 190 g wt. for 0, and 540 g wt. for A. The maximum output conveniently available from the vibrator is shown to indicate the range of amplitudes of vibration which were tested. to the initial frequency of firing. Stretch alone never produced high frequencies of firing. In addition, on contraction the muscle might be expected to become stiffer so that the vibratory change in tension produced by a given amplitude of movement would increase. Whether or not this occurred could not be readily tested with the present myograph because, owing to its mass, it produced a considerable oscillatory output when it was vibrated without being attached to anything.

17 VIBRATION AND MUSCLE RECEPTORS 789 The increase in vibration sensitivity produced by contraction was not due to some nonspecific effect of the contraction on the whole muscle since the production of approximately equal tensions in the muscle on stimulating different ventral root filaments sometimes had quite different actions in sensitizing a particular ending to vibration. The action of contraction in sensitizing any one ending was, however, approximately related to the effect it had in directly exciting the ending. Different filaments producing approximately the same tension differed greatly in this respect, and some filaments produced no excitation of some individual Golgi tendon organs even though they produced large over-all tensions in the tendon. These relations might be expected from the work of Houk & Henneman (1967), who found that any particular Golgi tendon organ of soleus is only influenced by four to fifteen motor units out of the possible c/s 300 c/s 400 c/s 15 0 z 05 0! > >35 Amplitude for driving (A) Fig. 11. Histograms showing the amplitudes of vibration required to produce driving of each of the forty-nine primary endings studied at frequencies of 200, 300 and 400 c/s. The maximum amplitudes ofvibration available at these frequencies were respectively 190, 70 and 35 A. The class 30-40, in the last histogram only includes observations up to 35,t. Comparison of sensitivities of different types of receptors. In the noncontracting muscle and in the absence of fusimotor activity the primary endings were always readily driven by vibration; none of the secondary endings or the Golgi tendon organs could be driven under the same conditions with the amplitudes available. Figure 11 shows the amplitudes of vibration required to drive all the primary endings studied at three different frequencies. Another way of assessing the vibration sensitivity of an ending is to measure the amount by which its frequency of discharge is increased by vibration of certain fixed amplitudes and frequencies. Figures 5, 7 and 9 have already shown that vibration may still have an

18 790 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS appreciable excitatory effect on all three types of receptor even when the amplitude of the vibration is below that required to produce driving. From the point of view of understanding the reflex effect of vibration, it is more important to know whether vibration has an excitatory effect at all, and if so how great the effect is, rather than merely knowing the amplitude Primaries 25,u O 300 Primaries 10,u =,200 o 100,*' * * * * * * Secondaries 50# A A A A Golgis 50,u I Vibration frequency (c/s) Fig. 12. The average excitatory effect of vibration of certain particular amplitudes at a range of frequencies on a number of primary endings, secondary endings and Golgi tendon organs. The mean frequency of discharge of the endings (ordinate) is plotted against the frequency of vibration (abscissa) for the stated amplitudes. The frequency of discharge of each ending was measured over a period of 0 5 sec. There was no fusimotor stimulation, nor was the muscle contracting. The results were obtained from twenty-four primary endings, twenty-five secondary endings and twenty-seven Golgi tendon organs. The dashed line at 450 shows the frequencies at which the endings would be discharging if they were all being driven by the vibration. required for driving. Figure 12 shows the averaged results from fourteen experiments in which this matter was investigated. The top line shows the average frequency of discharge of twenty-four primary endings during vibration of 25 It amplitude at a series of frequencies. For each point the average frequency of discharge of the endings is practically the same as the frequency of the vibration, since at all frequencies the majority of endings were driven by the vibration. The few endings which were not securely driven by the vibration were still powerfully excited by it. For vibrations of 10 /t amplitude the average frequency of discharge of the

19 VIBRATION AND MUSCLE RECEPTORS 791 primary endings was considerably below the vibration frequency, but in general vibration was still having a powerful excitatory effect. Figure 12 also shows the excitatory effect of 50,u vibration on twenty-five secondary endings and twenty-seven Golgi tendon organs. The effect is negligible. When smaller amplitudes of vibration at 400 and 500 c/s were tested on these two kinds of ending, no excitatory effect was observed either. The graphs of Fig. 12 serve to define the conditions under which vibration can be used as a selective stimulus to the primary endings of the spindles and therefore for the Ia fibres of soleus. Fusimotor activity, as judged by the effects of stimulating single fusimotor fibres, further lowers the threshold of the primary endings. Fortunately the sensitivity of the secondary endings to vibration of amplitudes of 50 #u or below remains inappreciable during fusimotor activity. This is shown in Fig. 13, which gives the mean frequency of discharge for a range 400, rimaries-fusimotor stimulation 10 t 300 Primaries-passive 10g e Secondaries- fusimotor stimulation> 50, Secondaries-passive> Vibration frequency (c/s) Fig. 13. Effects of fusimotor stimulation on the average excitatory effect of vibration at a range of frequencies on primary endings and on secondary endings. The results were averaged from those obtained from eight primary endings on vibrating with an amplitude of 10 /t and seven secondary endings on vibrating with an amplitude of 50 /z or in a few cases greater than 50 It (100, 150, 190 or 250,u). (The larger amplitude of vibration has been used when the response to 50,u vibration had not been studied; this occurred when the response to a large amplitude was judged insignificant. The fusimotor fibres were stimulated at 100/sec; they were proven to be static ones for the primary endings, and may be presumed also to have been so for the secondary endings. Several of the endings were studied during stimulation of more than one fusimotor fibre. Altogether there were ten combinations of a primary ending with a fusimotor fibre and sixteen combinations of a secondary ending with a fusimotor fibre.)

20 792 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS of vibration frequencies for eight primary endings vibrated at 10, amplitude with and without static fusimotor stimulation, and for seven secondary endings at 50,u amplitude (or above) with and without fusimotor stimulation. Thus in a non-contracting soleus it may be concluded that 300 A. Golgis 50,, B. Golgis 1004u Vbainfrequency (c/s) Fig. 14. GEraphs showing the average sensitivity of Golgi tendon organs to vibration when the muscle was contracting. The mean frequency of discharge of the endings during vibration (ordinate, frequency measured over 0.5 sec) is plotted against the frequency of the vibration (abscissa) for an amplitude of vibration of 50 jc4 (A) and of 100 ju (B). Each line gives the average behaviour ofa number of endings, The endings have been grouped on the basis of their frequency of discharge during contraction, but in the absence of vibration. *, results from endings whose discharge in the absence of vibraton was between 50 and 75 impulses/sec (mean strength of contraction 310 g wt.); 0, results from endings with a discharge between 75 and 100 impulses/sec (mean contractile tension, 360 g wt.); *, results from endings with a discharge between 125 and 150 impulses/sec (mean tension 1805 kg wt.). The points on the ordinate show the frequency of discharge in the absence of vibration. The dashed line at 450 shows the frequencies at which the endings would discharge ifthey were driven by the vibration. The figure is based on results obtained from twenty-five Golgi tendon organs. (Only some of the endings contributed to each of the above lines as ventral root filaments were not always studied which produced the appropriate excitatory effect for the above groupings. Some endings contributed more than one set of observations to a given line because they were studied during stimulation of different ventral root filaments, all of which pro. duced the appropriate effects. The numbers studied were as follows: A, nineteen sets of observation on twelve endings; 0, eighteen observations on thirteen endings; *, fifteen observations on fourteen endings.) vibrations of 25-SO,ct amplitude at frequencies from 100 to 500 c/s will have a very powerful excitatory action on all primary endings while having no appreciable excitatory action on secondary endings or Golgi tendon organs, and this is so whether or not there is any fusimotor activity. Unfortunately, when the muscle is contracting the position is not so simple, since the Golgi tendon organs then become appreciably sensitive to vibration. Figure 14 shows the average excitatory effects of 50 and 100 jut

21 VIBRATION AND MUSCLE RECEPTORS 793 vibration over a range of frequencies applied during contraction; from below upwards the different lines show the effect of increasing strength of contraction. Neither amplitude of vibration had any appreciable effect during the weakest contractions, even though these were producing a considerable excitatory effect on the endings. Both amplitudes of vibration had an appreciable excitatory effect during the strongest contractions, the 100,t vibration considerably more so than the 50,u vibration. As discussed earlier, the significant factor in sensitizing the Golgi tendon organ is not the over-all strength of the contraction of the whole muscle but rather the strength of contraction of the particular muscle fibres with which it is associated, as judged by the increase in the firing frequency of the ending. It is for this reason that the endings have been grouped on the basis of their frequency of discharge during the contraction, rather than on the actual over-all strength of the contractions. Effect of muscle contraction on responsiveness of primary endings. If the responsiveness of primary endings to vibration were to remain unchanged during muscular contraction, then it would still be possible to excite them without exciting the Golgi tendon organs by using amplitudes of vibration of about 25 4tt. Unfortunately, contraction of itself was found to reduce the responsiveness of the primary endings. The contraction was produced by repetitively stimulating the peripheral end of a moderately thick ventral root filament, as was done for the Golgi tendon organs. The shocks used were considerably supramaximal for producing contraction and probably excited most or all of the y fibres as well as the a fibres. On stimulating most filaments, the discharge of any individual ending studied would slow or cease during the contraction, owing to the unloading of the muscle spindle. The action of any fusimotor fibres in the filament which supplied the ending was then presumably not strong enough to compensate for this. Invariably, when the discharge of a primary ending slowed during contraction its sensitivity to vibration was greatly reduced for all frequencies of vibration, and it might prove impossible to drive it with any of the amplitudes of vibration available. Unloading the muscle by releasing it by 1 mm shortly after stretching it also decreased or silenced the discharge of any primary ending (Matthews, 1933), and such release also greatly reduced the responsiveness of the ending to vibration (in comparison with its responsiveness when the muscle was stretched up to the same length as that to which it had been released). It is hardly surprising that the sensitivity to vibration of an ending should be reduced when its firing frequency is lowered during muscle contraction or following a release, but this finding needs to be kept in mind on using vibration as a tool for studying reflexes (cf. the silent period occurring at the beginning of the stretch reflex produced by vibration; Matthews, 1966b).

22 794 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS Stimulation of some ventral root filaments caused an increase in the discharge of the primary ending studied, in spite of the fact that stimulation also caused a considerable contraction of the muscle. Such excitation of an ending could be confidently attributed to the excitation of a fusimotor fibre or fibres, because the excitatory effect of the stimulation a L b c d,400 - sec P4 Tension Stimulation 1 kg Vibration Fig. 15. Reduction of the vibration responsiveness of a primary ending on stimulating a thick ventral root filament which caused both contraction of the muscle and an increase in the discharge of the ending. a, unmodified response of ending; b, excitation of ending by ventral root stimulation at 100/sec; c, driving of ending by 10 1 vibration at 400 c/s; d, combined vibration and ventral root stimulationno driving. increased on increasing the frequency of stimulation above that required to produce a maximal contraction of the extrafusal fibres. The frequency of stimulation producing a maximal fusimotor effect is well known to be much higher than that producing a maximal contraction of the main muscle (Bessou, Emonet-D6nand & Laporte, 1965; Harvey & Matthews, 1961; Matthews, 1962). Unexpectedly, it was regularly found that the vibration responsiveness of primary endings was still reduced by ventral root stimulation, even though the frequency of firing of the ending had increased. This effect is shown in Fig. 15, where during contraction the ending was no longer driven by vibration (d) which was otherwise adequate (c), even though the frequency of discharge of the ending had increased by about 100 impulses/sec. As shown earlier, stimulation of fusimotor fibres in isolation from muscle contraction increases the vibratory responsiveness of primary endings. The present reduction in vibration sensitivity cannot be attributed to unloading of the spindle by the contraction, for judging by the discharge of the ending the spindle became 'tauter' during the stimulation. Possibly the contraction changed the mechanical properties of the main muscle so that rather less vibration was actually applied to the muscle spindle. This would happen if contraction caused the muscle to resist the high frequency deformations of the vibration more strongly than before; i.e. if the contractile component were to become stiffer more of the applied deformation would then be taken up in the tendon (or series

23 VIBRATION AND MUSCLE RECEPTORS 795 elastic element) and less in the muscle. One finding favoured such a possibility; when the soleus muscle was observed microscopically about half way along its length it was seen that on contraction of the muscle the amplitude of movement at this point decreased to about a third of its original value, even though the vibration applied to the tendon remained unchanged. Whatever the mechanism, the effect of contraction in decreasing the vibration sensitivity of primary endings while increasing that of Golgi tendon organs, means that during contraction vibration is no longer a specific stimulus for the primary endings. Possibility of intermediate ending8. We have not systematically studied afferent fibres with conduction velocities close to the presumed dividing line between those from primary and those from secondary endings (75 m/sec; Hunt, 1954). Indeed, it is not yet clear just how far all endings with such border-line afferent fibres can be cleanly classified on their functional behavour into primary and secondary endings (Matthews, 1963a, 1964; Rack & Westbury, 1966; Renkin & Vallbo, 1964). In the present experiments in order to avoid any possible confusion we arbitrarily defined primary endings as those with afferents conducting at over 80 m/sec and secondary endings as those with afferents conducting at below 60 m/sec (see Methods). However, recordings were also taken from seven afferent fibres with conduction velocities falling between these limits, though the results have not been included in the preceding analysis. Five of the endings supplied by these afferents behaved as typical pr mary endings with regard to vibration, and were suitably sensitive. One of the endings (afferent fibre conduction velocity 75 m/sec) was less sensitive than the majority of the primary endings but was far more sensitive than any of the secondary endings (for 300 c/s vibration the threshold amplitude for driving was 50 it). The last ending (afferent fibre conduction velocity 61 m/sec) was rather more sensitive than any of the secondary endings, but was far less sensitive than the primary endings (50 As at 300 c/s increased its firing by 30 impulses/sec from a pre-existing level of 45 impulses/sec; cf. Fig. 12). Though our methods of selection may have been biased against such endings with intermediate properties (when attempting to isolate single units we persevered to obtain those with a definitely high or low conduction velocity), it seems unlikely that they are common. In any case, there are rather few endings in soleus with afferent fibres conducting in the range m/sec (Hunt, 1954; Lloyd & Chang, 1948). Peroneus longus muscle. The behaviour of endings in the peroneus longus was studied in three experiments to ensure that the endings in the soleus were reasonably typical of those in other muscles; the general behaviour of receptors in this muscle was described by Matthews (1933). The findings with vibration proved to be very similar to those for soleus and vibration of 25, amplitude applied to a non-contracting muscle again provided a specific stimulus for the primary endings. The seven primary endings studied were all driven over a wide range of frequencies by 25,u vibration, whereas six secondary endings and nine Golgi tendon organs were uninfluenced by 50 It vibration. Both in the presence and absence of muscle contraction the tendon organs in peroneus longus were in general slightly more sensitive to vibration than the endings in soleus and two of them could be driven at 300 c/s by vibration of 70, amplitude

24 796 M. C. BROWN, 1. ENGBERG AND P. B. C. MATTHEWS when the muscle was contracting. The two muscles are about the same over-all length and produce about the same maximal tetanic tensions, but they differ in that soleus is the heavier, redder and has a wider range of movement. Medial gastrocnemiu8 muscle. Ten primary endings and seven secondary endings were studied in three early experiments on this muscle before the techniques and methods of taking observations had been fully developed. Using the maximum amplitude of vibration available all the primary endings could be securely driven at frequencies of c/s. None of the secondary endings could be driven in this range of frequencies, and maximal vibration at these frequencies produced on average only a very slight increase in their mean frequency of discharge. Further experiments were not performed because mechanical resonances in the muscle appeared to be influencing the transmission of the vibration from the tendon to the receptors. It may be provisionally concluded, however, that the properties of the spindle endings in the medial head of gastrocnemius are similar to those of soleus. Golgi tendon organs were not studied. DISCUSSION The chief finding of the present work is that the sensitivity of the primary endings to longitudinal vibration is very much greater than that of the secondary endings or the Golgi tendon organs. Thus there is little doubt that the production of a tonic reflex contraction by vibration in the decerebrate cat (Matthews, 1966a, b) is due to the excitation of primary endings, for these are the only endings with a low enough threshold to vibration to have produced the reflex, which was usually present for vibration of 10,t amplitude (Matthews, 1966a, b). When the muscle is not contracting, the threshold of the primary endings is so much lower than that of the other endings that they may be 'driven' to discharge one impulse per cycle of vibration at amplitudes of vibration which do not significantly increase the mean frequency of discharge of either secondary endings or Golgi tendon organs. This is so whether or not the spindle endings are also being activated by stimulation of single fusimotor fibres. Thus, vibration of 25-50,u amplitude applied to a non-contracting muscle provides a way of selectively activating nearly all the Ia fibres from the primary endings to discharge repetitively over a wide range of frequencies. This method of activation is probably rather more selective than that achieved by the graded electrical stimulation of muscle nerve trunks, particularly for activating Ia fibres without Ib fibres (Bradley & Eccles, 1953; Eccles, Eccles & Lundberg, 1957; Laporte & Bessou, 1957; McIntyre, 1965). Vibration is further to be preferred over electrical stimulation for the repetitive activation of I a fibres because of the progressive rise which occurs in the electrical threshold of those fibres which have been excited, relative to those which have not been. Lundberg & Winsbury (1960) have already shown that a brief tap to a non-contracting soleus excites a

25 VIBRATION AND MUSCLE RECEPTORS 797 virtually pure I a volley, and our method may be regarded as an extension of theirs. One qualification to the present conclusions is that the vibratory sensitivity of the afferent endings supplied by group III and non-medullated fibres has not been studied. It is improbable, however, that they are significantly excited by small-amplitude vibration, for the majority of them are completely insensitive to muscle stretch or contraction, and the few that can be so excited are very insensitive to these stimuli in comparison with spindle endings and tendon organs (Bessou & Laporte, 1961; Iggo, 1961; Paintal, 1960). Unfortunately, vibration was no longer so selective a stimulus when the muscle was contracting, both because the Golgi tendon organs then became appreciably more sensitive to vibration, and also because the sensitivity of primary endings to vibration was reduced by muscle contraction, even in the presence of fusimotor activation. This latter effect of contraction is likely to be the reason why in the previous work on the stretch reflex the amplitude of vibration required to give a plateau of the reflex contraction was as much as #t, suggesting that such amplitudes were required to drive all primary endings. These amplitudes of vibration would certainly have given some excitation of Golgi tendon organs during the resultant reflex contraction, so that interpretation of the relationship between the frequency of vibration and the plateau size of the reflex is complicated (cf. Matthews, 1966b, 1967). It should also be noted that in the present experiments the fixation of the preparation was not sufficiently good to stop all transmission of vibration to the rest of the animal, and the larger amplitudes of vibration produced a palpable vibration of the pelvis. Thus, unlike electrical stimulation, vibration carries the risk of exciting some afferent fibres at a distance from the region of stimulation. In man, vibration applied to a tendon in the absence of muscle contraction can be presumed to be a specific stimulus for the primary endings, though just what amplitude is required to activate them maximally in any particular muscle remains unknown. However, on the basis of the findings of Bianconi & Van der Meulen (1963) vibration applied to the surface of the muscle is unlikely to be a specific stimulus and probably excites secondary as well as primary endings. If the muscle is contracting, then in man as well as in the cat Golgi tendon organs must be assumed to be somewhat activated by the vibration. Euler & Peretti (1966), studying receptors in the intercostal muscles and applying much larger amplitudes of vibration than we have used (i.e. apparently up to 500,c at 800 c/s applied to the ribs), produced rather more excitation of presumed secondary endings and Golgi tendon organs than we found, again emphasizing that selective stimulation ofprimary endings can only be obtained with careful control ofthe amplitude, and the mode of application, ofthe vibration.

26 798 M. C. BROWN, I. ENGBERG AND P. B. C. MATTHEWS The sensitivity of primary endings to vibration is really remarkably high by any standard except that of the ear. The most sensitive primary ending studied was driven at c/s by vibration of the muscle of only 4,u peak to peak amplitude representing an extension of the muscle of about 1 part in 10,000. As the spindle would have occupied at most a quarter of the length of the muscle (i.e. a 7 mm long spindle among 3 cm long muscle fibres) and as the wave-length of the vibration may be assumed to have been much longer, the vibration actually applied between the ends of the spindle could have been only about 1 Ac peak-to-peak amplitude. This figure is very similar to that for vibratory excitation of the Pacinian corpuscle, as deduced from the records published by Sato (1961). Hair receptors in the rabbit (Miller, 1967) and touch receptors in the monkey or toad (Lindblom, 1958, 1965; Mountcastle, Talbot & Kornhuber, 1966) can only rarely be excited by a displacement of skin of 10 Itt. If the deformation of the muscle spindle occurred uniformly along its length, then a 1 At stretching of a 7 mm spindle corresponds to a stretching of only 0 04 It of the central 300,u, which is the region innervated by the primary ending. It seems more likely that differences in the visco-elastic properties of the intrafusal fibres along their length cause most of the deformation of the spindle to occur in its innervated equatorial region. This would happen if the central region behaved in a relatively elastic manner in relation to a more viscous behaviour of the striated polar regions (cf. Matthews, 1964). In favour of this idea is our finding of a low sensitivity for the secondary ending to longitudinal vibration, while Bianconi & Van der Meulen (1963) found that about half of the secondary endings could be driven by vibration applied locally to the surface of the muscle just over the spindle. This difference in response to the two modes of applying vibration would be expected if the insensitivity of the secondary endings to longitudinal vibration depended not upon the properties of the receptor terminals themselves, but upon their lying on a region of intrafusal fibre which was relatively stiff for deformations applied at these high frequencies (i.e. was relatively viscous). The secondary endings lie on a striated region of the intrafusal fibres, whether nuclear-bag or nuclearchain, whereas the primary endings lie on poorly striated regions of both kinds of intrafusal fibre (Boyd, 1962). In this respect it is interesting that even during stimulation of static fusimotor fibres the secondary endings remained largely insensitive to vibration. Crowe & Matthews (1964), on finding that the primary ending is sensitized to vibration by stimulation of either static or dynamic fusimotor fibres, suggested that the central regions of both the nuclear-bag and the nuclear-chain intrafusal muscle fibres behaved in a simple elastic manner and were not particularly viscous. This suggestion also involves the assumption that when they are contract-

27 VIBRATION AND MUSCLE RECEPTORS 799 ing the polar regions of both kinds of intrafusal fibre are sufficiently viscous to cause these regions to be rather 'stiff' when tested by the high frequency deformations of vibration, even though their 'stiffnesses' probably differ for low-velocity stretches (Crowe & Matthews, 1964). Such suggestions remain tenable in spite of the changes and uncertainties that surround hypotheses on the internal functioning of the muscle spindle (cf. Granit, 1966; Brown & Matthews, 1966). The chief interest of vibration at present, however, lies in its use as a tool for studying the central action of Ia fibres, rather than for any light it can throw on the working of the muscle spindle itself. We should like to thank Mr P. Scearce for help with both the experiments and the apparatus. REFERENCES APPELBERG, B., BESSOU, P. & LAPORTE, Y. (1966). Action of static and dynamic fusimotor fibres on secondary endings of cat's spindles. J. Physiol. 185, BESssou, P., EMONET-DENAND, F. & LAPORTE, Y. (1965). Motor fibres innervating extrafusal and intrafusal muscle fibres in the cat. J. Physiol. 180, BEssou, P. & LAPORTE, Y, (1961). Etude des recepteurs musculaires innerv6s par les fibres afferentes du groupe III (fibres myelinisees fines) chez le chat. Arch8 ital. Biol. 99, BiANcom, R. & VAN DER MEULEN, J. F. (1963). The response to vibration of the endorgans of mammalian muscle spindles. J. Neurophy8iol. 26, BOYD, I. A. (1962). The structure and innervation of the nuclear-bag muscle fibre system and the nuclear chain muscle fibre system in mammalian muscle spindles. Phil. Trans. R. Soc. B 245, BRADLEY, K. & EcciEs, J. C. (1953). Analysis of the fast afferent impulses from thigh muscles. J. Physsol. 122, BROWN, M. C., ENGBERG, I. & MATTHEWS, P. B. C. (1967). The use of vibration as a selective repetitive stimulus for Ia afferent fibres. J. Physiol. (In the Press.) BROWN, M. C. & MATTHEWS, P. B. C. (1966). On the subdivision of the efferent fibres to muscle spindles into static and dynamic fusimotor fibres. In Control and Innervation of Skeletal Mu8cle, ed. ANDREW, B. L. Dundee: Thomson and Co. CROWE, A. & MATTHEWS, P. B. C. (1964). Further studies of static and dynamic fusimotor fibres. J. Physiol. 174, ECCLES, J. C., ECCLES, R. M. & LUNDBERG, A. (1957). Synaptic actions on motoneurones in relation to the two components of the group I muscle afferent volley. J. Physiol. 136, EuLER, C. v. & PERETTI, G. (1966). Dynamic and static contributions to the rhythmic y activation of primary and secondary spindle endings in external intercostal muscle. J. Physiol. 187, DE GAIL, P., LANCE, J. W. & NEILSON, P. O. (1966). Differential effects on tonic and phasic reflex mechanisms produced by vibration of muscles in man. J. Neurol. Neurosurg. Psychiat. 29, GRANIT, R. (1966). Nobel Symposium I, Muscular Afferenta and Motor Control. Stockholm: Almqvist and Wiksell. GRANIT, R. & HENATSCH, H. D. (1956). Gamma control of dynamic properties of muscle spindles. J. Neurophysiol. 19, HAGBARTH, K. E. & EKLUND, G. (1966). Motor effects of vibratory muscle stimuli in man. In Nobel Symposium I Muscular Afferents and Motor Control, ed. GRANIT, R. Stockholm: Almqvist and Wiksell. HARVEY, R. J. & MATTHEWS, P. B. C. (1961). The response of de-efferented muscle spindle endings in the cat's soleus to slow extension of the muscle. J. Physiol. 157, S I Physiol. I 92

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