Diete-Spiff, Dodsworth & Pascoe (1961) reported that many fusimotor. spinalized rabbit. Their results show, however, that the brain stem exerts

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1 200 J. Physiol. (1965),181, pp (13 With 5 text-figures Printed in Great Britain DISCHARGES OF SEMITENDINOSUS FUSIMOTOR NEURONES IN THE DECEREBRATED AND SPINALIZED RABBIT BY P. H. ELLAWAY AND J. E. PASCOE From the Department of Physiology, University College London (Received 7 April 1965) Diete-Spiff, Dodsworth & Pascoe (1961) reported that many fusimotor fibres to the gastrocnemius muscle were tonically active in the decerebrated or anaesthetized rabbit, but this activity ceased after complete section of the spinal cord. In the spinal cat, Hunt & Paintal (1958) saw tonic discharges of gastrocnemius medialis fusimotor neurones in fourteen out of thirty-one neurones isolated, but Voorhoeve & van Kanten (1962) reported that they saw no spontaneous activity of gastrocnemius fusimotor neurones in either the acutely or the chronically spinalized cat. Diete-Spiff et al. concluded that by taking elaborate precautions to preserve the general condition of the animal it might be possible to show spontaneous discharging of fusimotor neurones to the gastrocnemius muscle of the spinalized rabbit. Their results show, however, that the brain stem exerts a facilitatory effect on the gastrocnemius fusimotor neurones, and this effect is conducted by a diffuse pathway, since it is not completely interrupted by multiple staggered bilateral hemisections of the cord. Also electrical or mechanical stimulation of the peripheral end of the cut spinal cord caused the gastrocnemius fusimotor neurones to discharge repetitively after a long latent period; with electrical stimulation there was no temporal relation between each stimulus and the discharges of the neurones, i.e. there was no driving. This pathway from the brain stem to the gastrocnemius fusimotor neurones resembles the diffuse pathway described by Granit & Holmgren (1955). The semitendinosus muscle is a knee flexor in the rabbit, and our interest in it was aroused by the suggestion of Professor C. C. Hunt (personal communication, 1961) that flexor fusimotor neurones are probably more excitable than extensor fusimotor neurones in the spinal animal. This particular muscle was chosen because of the ease with which its nerves can be separated from those to neighbouring muscles. The first experiments clearly showed that there was abundant activity in its fusimotor neurones in the spinal animal, and the accumulated results show that the effect of spinalization is to cause some neurones to discharge at a higher mean frequency (see also Voorhoeve & van Kanten, 1962). The

2 SEMITENDINOSUS FUSIMOTOR NEURONES 201 most striking effect of spinalization has been however the reduction in the regularity of the fusimotor discharge. It is noteworthy that Hunt & Paintal (1958) in their report on fusimotor fibres of the spinal cat state that 'interval between impulses is typically irregular in background discharge of fusimotor fibres'. The work reported in this paper show-s that the brain stem somewhere between the level of the obex and the superior colliculi exerts a regularizing action on the discharges of the fusimotor neurones of the semitendinosus muscle. A preliminary account of this work has appeared (Pascoe, 1963). METHODS Dissection. The rabbits were anaesthetized with a 1 g/15) ml. solution of hexobarbitone sodium by marginal ear vein. The trachea was cannulated, and both carotid arteries tied. Holes were made in the auricular cartilage on each side to take the ear plugs of the headholder. The right femoral vein was cannulated and anaesthesia maintained with hexobarbitone sodium by this route. In later experiments anaesthesia was maintained with a Inixture of halothane in oxygen, and the hexobarbitone discontinued after tracheal cannulation. In the left leg the following nerves were cut: femoral, obturators, semimembranosus, anterior and posterior biceps, sural, medial and lateral popliteal. The left leg was held with clamps at ankle, knee and head of the femur. Laminectomy, where necessary, was performed at this stage, in most instances at the mid-thoracic level. The calcaneus was cut off, and the tendons of gastrocnemius, soleus and plantaris muscles cut, leaving only the tendon of semitendinosus. The animal was mounted either on its left side or prone on a warmed stand with its head held rigidly in a holder. At this stage a large mid-line craniotomy was made, the dura cut and the whole of the fore-brain sucked away. Suction was applied from a filter pump and the internal diameter of the nozzle was 2-3 mm. A fairly large nozzle sucks away the nervous tissue leaving long lengths of blood vessels which were occluded with clips made of soft aluminium wire. Anaesthetic was discontinued and allowed to wear off for not less than an hour. A pool of liquid paraffin was made in the thigh, and one of the nerves to the semitendinosus muscle, usually the longer, was separated from the surrounding tissues. At this stage the preparation was usually paralysed with a solution of gallamine 4 mg,'ml. given in sufficient quantity through the femoral vein. The doss was repeated at half-hourly intervals or as necessary to maintain paralysis. The rabbit was respired from a Starling Ideal pump, frequency 53/min and stroke approximately 35 ml. The semitendinosus nerve sheath was slit with scissors and small strands of nerve fibres dissected free and placed on twin platinum electrodes so as to record activity coming from the cord to the muscle. The electrodes were connected to a capacity-coupled differential amplifier through cathode followers. The inputs to the grids of the cathode followers were through 0-01 juf capacitors with 3-3 mf. leaks to ground. This avoids the risk of destroying the nerve filaments with grid current. The amplified potentials were displayed in the usual fashion on a cathode-ray oscilloscope, and in addition a direct display of reciprocal intervals or 'instantaneous frequency' was devised (Huxley & Pascoe, 1963). This device causes a dot to appear on the screen of the cathode ray oscilloscope coinciding in time with the nerve action potential. The height of the dot above the base line is equal to the reciprocal of the interval between this potential and the preceding one (MacNichol & Jacobs, 1955). Care was taken when using this device to record only from single units. In those instances where there were two active fibres in the same filament, it was possible with an amplitude discriminator to record only from the fibre with the larger action potential. In those instances where a treatment caused an additional fibre to diseharge, and where its action potential

3 202 P. H. ELLA WA Y AND J. E. PASCOE was large enough to trigger the display device, the fact was noted and that part of the record discarded. Some of the illustrations were made from records obtained with the frequency display device before it was perfected. This accounts for the non-linearity of some of the scales. Identification of fusitnotor fibres. Ideally we should have liked to measure the conduction velocity of each of the fibres studied. Attempts were made but were not successful since invariably the filaments from which unitary discharges were recorded proved to be composed of several functional nerve fibres when the sciatic nerve was stimulated electrically above the nerve to the hamstrings. Activity in filaments isolated from the central end of the semitendinosus nerve could come from extrafusal motor nerves, fusimotor nerves, or be due to injury discharges in any of the nerve fibres which make up the nerve, both motor and sensory. Pascoe (1963) showed that the activity categorized as Type II fusimotor by Diete-Spiff & Pascoe (1959) arose locally from the nerve and must be considered as a form of injury discharge. For this reason any unit whose activity could not be reflexly influenced by stroking or pinching the skin was rejected. The easiest method of differentiating between fusimotor and extrafusimotor fibres is by the smaller amplitude of the action potentials of the former. This is a reliable method when using a differential amplifier, but it has to be used with caution and additional criteria are desirable. A frequently encountered activity consisted of short staccato bursts of action potentials at a high frequency. The amplitude of these potentials was invariably large, and it was concluded that they originated in extrafusimotor fibres. This form of activity most often occurred after strong stimulation of the skin or whilst making lesions in the spinal cord. It was assumed to be serving the flexor reflex, and was observed to coincide with phasic contractions of the semitendinosus muscle at times when the gallamine paralysis had worn off. Diete-Spiff & Pascoe (1959) used the absence of recurrent inhibition on fusimotor neurones (Granit, Pascoe & Steg, 1957; Hunt & Paintal, 1958), as a means of differentiating between fusimotor and extrafusimotor fibres. In several of the early experiments the sciatic nerve was stimulated below the hamstrings at a strength sufficient to excite the large motor fibres whilst recording from a tonically discharging fibre of the semitendinosus nerve. It was hoped that if the fibre was an extrafusimotor fibre there would be a pause of at least 50 msec after the stimulus. However, one fibre was isolated showing a discharge of 20 impulses/sec, and there was no pause in the discharge after sciatic nerve stimulation; but this fibre gave double discharges clearly audible from the loudspeaker, and recorded by the frequency display. Diete-Spiff & Pascoe (1959) did not observe double discharge in any of the fusimotor neurones of the gastrocnemius muscle but such discharges are common with tonic extrafusimotor neurones. When the semitendinosus muscle was stretched the discharge frequency of this particular fibre increased, confirming that it was a tonic extrafusimotor fibre, since there is now abundant evidence that fusimotor fibres are not influenced by stretching the muscle they innervate (Hunt & Paintal, 1958; Diete-Spiff & Pascoe, 1959). Only one other tonically discharging extrafusimotor fibre was isolated. Its discharge frequency was 12 impulses/sec, and this could be increased by stretching the muscle. The recurrent inhibition test was not made with this fibre so that there is insufficient evidence to decide whether the semitendinosus extrafusimotor neurones are lacking in recurrent inhibition. The effect of diethyl ether and ethyl chloride on the discharges of thirteen fusimotor fibres from four rabbits was noted. In all cases there was a marked rise in the discharge frequency, and in some this was terminated by an abrupt cut-off similar to that described for gastrocnemius fusimotor neurones by Diete-Spiff et al. (1961). This is supporting evidence that the fibres selected were indeed fusimotor.

4 SEMITENDINOSUS FUSIMOTOR NEURONES 203 RESULTS Reflex behaviour of semitendinosus fusimotor neurones Closely concerned with the identification of fusimotor neurones is their reflex behaviour, since they are capable of discharging over a much wider range of frequencies than the tonic extrafusimotor fibres. The responses of fifteen fusimotor fibres from decerebrated preparations to pinching the contralateral heel were as follows. In nine there was a transitory rise in frequency followed by a longer maintained fall. In five there was a pure rise in frequency, and in one an abrupt cessation of the discharge followed by recovery to a lower frequency than before the stimulus. The semitendinosus muscle in the rabbit is a knee flexor, and the fall in frequency of firing of its fusimotor neurones is what one would expect to accompany a crossed extensor reflex. The rise in frequency would tend to oppose such a reflex. It is possible that skin receptors of the contralateral heel having a low threshold to pinches are facilitatory in their reflex effects on semitendinosus fusimotor neurones, but that higher threshold and/or more deeply situated receptors are inhibitory. Diete-Spiff & Pascoe (1959) could only obtain facilitation of gastrocnemius fusimotor neurones by stroking the fur of the rabbit but strong pinches of the forepaws did produce inhibition. The responses of semitendinosus fusimotor neurones to pinches of the forepaws and the skin of the ipsilateral thigh were once again variable with different fibres. The responses to all three stimuli were mainly excitatory but in some instances the response was followed by inhibition, and in a few pure inhibition alone was seen. In one experiment it was noted repeatedly that strong pinches to the ipsilateral forepaw caused a reduction in the discharge frequency but that immediately the pinch stopped there was a sharp rise in the frequency to well above the control level. This again suggests two kinds of receptors having opposing actions. However, it is clear that more standardized stimuli will be required to elucidate the reasons for the variability in the responses of fusimotor neurones. In three cases the effects of repetitive stimulation of the ipsilateral sural nerve were recorded. In two, the first effect was a rise in the frequency of firing of the semitendinosus fusimotor fibre followed by a return to the prestimulation frequency. Then, at from 15 to 20 sec after the end of the stimulation period, the fibres stopped discharging for periods up to 32 see, after which the discharge started again reaching a steady plateau at the prestimulation level in 1-2 sec. In the third fibre where repetitive stimulation of the ipsilateral sural nerve was tried, the result was a prolonged reduction in the discharge frequency. In addition to these long-lasting inhibitory effects of sural nerve stimulation, single shocks to

5 204 P. H. ELLA WA Y AND J. E. PASCOE the ipsilateral sural nerve drive semitendinosus fusimotor neurones with a latency of from 10 to 12 msec. In four decerebrated preparations, very long-lasting reflex responses were seen; after the initial effects of the stimulus the discharges became steady at a new level higher or lower than the prestimulation value. In two of the preparations from which these long-lasting effects were obtained the spinal cord was later cut, and it is worth recording, though perhaps not significant, that they could now no longer be produced. Pattern of discharge of fusimotor neurone In the decerebrated preparation it seemed that tonically discharging fusimotor neurones were difficult to isolate. This is a subjective observation and was not made quantitative in any way. The pattern of the discharge was very regular, and in many rabbits, if they were left undisturbed, the frequency of the discharge remained constant for long periods. In others apparently spontaneous changes in the frequency of discharge occurred. These could take the form of short-lasting rises or falls in the frequency or more prolonged changes. The expression 'apparently spontaneous' is used to mean that the changes in frequency did not follow any deliberate stimulation of the animal. However, it must be borne in mind that fusimotor fibres are very reactive to stimulation of the skin and that of necessity in these experiments there were many extensive skin lesions. In this connexion also it is worth reporting that in several preparations loud noises affected the frequency of discharge. The degree of scatter in the frequency appeared to be approximately related to the frequency in any particular fibre. This appeared to be true for spontaneous and reflex changes in frequency. However, to the eye and ear the discharges seemed to be less regular at the lower frequencies than at the higher. This is because the eye and ear detect changes in the intervals between impulses and not the reciprocals of these intervals. The histograms in Fig. 1 were made from records taken from a fusimotor fibre which gave a 'spontaneous' abrupt and sustained change of frequency. The mean frequency was originally 57-6/sec, and this dropped to 34.5/see, corresponding to mean impulse intervals of 17-4 and 29-0 msec, respectively. The standard deviation at the short interval was 0'85 msec compared to 1*54 msec at the longer. Thus the fibre was more regular when discharging at the higher than at the lower frequency. If, however, one calculates the frequencies corresponding to the mean intervals plus and minus twice the standard deviation, there is a scatter of 11-4/sec at the higher frequency compared to 8-6/sec at the lower. The frequency display accentuates changes of interval between impulses, particularly below 40 msec. For this reason also it can give an appearance of skewness which is not borne

6 SEMITENDINOSUS FUSIMOTOR NEURONES 205 out when one plots intervals. For example, if a fibre is discharging at a mean interval of msec, this converts to a frequency of 50/sec + 17 and -10/sec Fig. 1. Histograms of intervals between action potentials recorded from a single semitendinosus fusimotor neurone. Each histogram was constructed from approximately 12 sec of record. The change in mean interval between the two histograms occurred 'spontaneously'. Note that the degree of dispersion was less when the fibre discharged with the shorter impulse interval. Abscisae: grouping intervals of 1 msec. Ordinates: percentage of total number of intervals falling in any one group. In the spinal preparation it has seemed easier to find and isolate tonically discharging fusimotor neurones. The discharge patterns were noticeably less regular, in fact one can judge correctly from the frequency display records whether they were obtained from spinal or decerebrated animals. Indeed the records looked so different that it seemed possible that they were obtained from two different populations of neurones. To test this point, tonically discharging fusimotor neurones were isolated from decerebrated rabbits, and after recording the pattern of their discharges for several minutes the neuraxis was cut across. This was done in nineteen experiments, fifteen at the thoracic level, three at the level of the obex and

7 206 P. H. ELLA WA Y AND J. E. PASCOE one at the level of the acoustic tubercles. In twelve of these experiments the pattern of the discharge changed to that associated with the spinal condition. In the remaining seven the discharge ceased, the animal died or the recording filament gave multifibre discharges. The change in the pattern of discharge produced by spinalization is shown in Fig. 2. The effect comes on slowly, and is fully developed about 4 min after making the section. In seven of the twelve experiments where spinalization changed the pattern of the discharge, the discharges were not maintained, but ceased at from 2-5 to 20 min after the time of section. In the case of the remaining five fibres the discharges were maintained for more than 1 hr. a b s- ~~~~~~~~~~~~~~~~~s 100 d 0 5 min Fig. 2. Reciprocal interval records from a single fusimotor fibre. a, Control. b, c and d 1, 3 ancd 4 min, respectively, after cutting across the spinal cord. Note the very great decrease in regularity of the discharge after spinalization. Abscissae: time, ordinates: reciprocal time or frequency in impulses per sec. The increase in the scatter of frequencies after spinalization was always accompanied by an increase in the mean frequency of discharge. Thus as pointed out previously the increase in scatter may not indicate a reduction in the regularity of the discharge. However, the eye tends to pick out the modal frequency from such records, i.e. the most commonly occurring frequency. In many instances there was little if any change in the modal frequency as judged by eye following spinalization. The increase in the mean frequency and in the scatter was brought about by an increase in the number of higher frequencies. This is substantiated by the histograms

8 SEMITENDINOSUS FUSIMOTOR NEURONES 207 in Fig. 3. These were constructed from records taken from the same fibre before, and 4 min after, spinalization. It is seen that the mean interval before spinalization was 22-9 msec, and this fell to 19-5 mseo after; these correspond with frequencies of 43 8/sec and 51-4/sec, respectively. The standard deviation rose from 1-5 to 3-4 msec indicating a genuine reduction in the regularity of the discharge. In addition the histogram from Fig. 3. Histograms constructed from 12 sec of record of the discharge of a semi tendinosus fusimotor neurone. Unshaded before, shaded 4 min after cutting across the spinal cord. Note the decrease in the mean interval, the increase in scatter of intervals and the skew distribution of intervals after spinalization. Abscissae: grouping intervals of 1 msec. Ordinates: percentage of total number of intervals falling in any one group. the post-spinalization data shows a significant deviation from normal; there are many more short intervals than would fit a normal distribution. These effects could be due to a removal of some influence from the headward side of the point of section or to an irritation at the point of section. The latter possibility seems unlikely from a study of Fig. 2 since irritation

9 208 P. H. ELLA WAY AND J. E. PASCOE would most likely be greatest at the time of, and immediately after making the cut. The irregular discharge, however, developed progressively for 4 min. Furthermore, no return to a regular discharge pattern has been observed even though the discharges of single fibres have been studied for several hours after cutting across the spinal cord. The experiment illustrated in Fig. 4 showed that the effect of cord irritation is to cause a return to the prespinal pattern of discharging. At 42 min after cutting across the cord, the caudal stump of the cord was pinched with forceps. After a short period during which the discharge frequency was raised, the mean frequency fell progressively and the pattern became regular. Indeed the pattern of discharge illustrated in Fig. 4b is to the eye no different from that expected from an animal with an intact spinal cord. The change of pattern and the rise of mean frequency brought about by cord section is therefore due to the removal of some descending influence which can be mimicked by stimulation of the peripheral stump of the cord. The effect of electrical stimulation of the cord as illustrated in Fig. 5 emphasizes this. Single shocks to the cord were without effect but short bursts of tetanic stimulation, in this instance 50/sec for 1-8 sec led to a progressive decline in the irregularity of the discharge and in the mean frequency. As expected the effects of whole spinal-cord stimulation are complex. In many experiments at least two effects of cord stimulation have been seen, first an excitatory action followed by the longer-lasting reduction in the scatter and lower mean frequency already noted. The short-lived excitatory action is best seen after repeated tetanic stimulation of the cord has completely suppressed the tonic activity of the fusimotor neurone. Then each period of cord stimulation is followed by a few discharges of the neurone. The excitatory effect of cord stimulation has been little studied because it is usually masked by the more powerful and longer lasting inhibition. More discrete forms of excitation than whole cord stimulation will be necessary to study the facilitatory action. Fffect of chlorpromazine It was shown by Henatsch & Ingvar in 1956 that chlorpromazine in a dose of 0-5 mg/kg abolished discharges of fusimotor neurones in the decerebrated but not in the spinal animal. This we have confirmed on several occasions; in the decerebrated rabbits, single semitendinosus fusimotor discharges ceased discharging 1-2 min after 0 5 mg/kg of chlorpromazine hydrochloride given intravenously. In spinal animals, however, this dose was without effect and in some animals a tenfold increase in the dose caused only a slight reduction in mean frequency.

10 SEMITENDINOSUS FUSIMOTOR NEURONES 209 a b _1150 _1100 d 0 I 0 5 min Fig. 4. Reciprocal interval records from a single fusimotor neurone taken 42 min after cutting across spinal cord. a, Control, and at arrow compression of spinal cord with forceps. b, c and d starting 1-5, 3 and 6 min, respectively, after the compression. Note that the stimulus caused the fibre to discharge at a lower and much more regular frequency. Abscissae: time in min; ordinates: l/t, or frequency in impulses/sec. I a 150 b I _j min I Fig. 5. Reciprocal time-interval records from a fusimotor neurone, beginning 55 min after cutting across the spinal cord. a, Control. b, At signal stimulation ofthe cut end of the spinal cord at 50/sec, 5 V and 05 msec for 1-8 sec. Note that stimulation caused a gradual reduction in the frequency of the discharge and an increase in its regularity. Abscissae: time in min. Ordinates: llt, or frequency in impulse/sec. 14 Physiol. 181

11 210 P. H. ELLA WA Y AND J. E. PASCOE DISCUSSION It is clear from the results that in the decerebrated rabbit the fusimotor fibres to the semitendinosus muscle have many properties in common with those to the gastrocnemius muscle. The fibres discharge regularly for long periods, and this discharge may be influenced by stimulating the skin of the animal. In addition the excitant effects of ethyl chloride closely resemble those described for gastrocnemius fusimotor neurones. There seems, however, to be more variability between fibres in their reflex responses in these experiments than in those on the gastrocnemius fusimotor neurones. It must be emphasized that in the experiments on decerebrated rabbits we deliberately chose those fusimotor neurones which showed tonic discharges since we were interested in what happened to the discharges after spinalization. As stated in the results there often did not appear to be many fibres which showed a tonic discharge, so that the fibres studied were a highly selected sample of the whole population of semitendinosus fusimotor neurones. When one looks at the spinal animal there are many differences between the behaviour of gastrocnemius and semitendinosus fusimotor neurones. Diete-Spiff et al. (1961) never saw tonic activity of gastrocnemius fusimotor neurones in the spinal animal. Experiments in progress at the moment suggest that occasionally it is possible to see tonic activity of rabbit gastrocnemius fusimotor neurones after spinalization, but the fact remains that after spinalization these neurones become much less excitable. With the semitendinosus muscle in the spinal animal there is an abundance of tonically discharging fusimotor neurones. The effect of spinalization upon particular fusimotor neurones gives a less clear-cut difference between the two muscles. Out of nineteen experiments in which the cord was cut whilst observing the effect on the discharge of a fusimotor neurone, in five instances there was a sustained rise in the mean frequency of the discharge (see also Voorhoeve & van Kanten, 1962). In seven there was an initial rise in frequency followed by a fall to complete silence. In the remaining trials the fibre was silenced without any preceding rise in frequency. In all instances the cord was cut with the preparation unanaesthetized since anaesthetics have potent actions upon fusimotor neurones; the volatile anaesthetics excite and the barbiturates depress. There is no doubt that cutting the cord in this way has a deleterious effect upon the general condition of the animal, and in one of the experiments the heart was stopped and in another the heart was noted to be very weak after the section. Poor nutrition of the cord can undoubtedly account for some of the instances where cord section silenced a fusimotor fibre, but one must consider also whether in some instances the fibre

12 SEMITENDINOSUS FUSIMOTOR NEURONES 211 stopped because of the removal of facilitatory influences from above the cut. If this is the case then in this respect there is little difference between fusimotor neurones of the gastrocnemius and semitendinosus muscles. However, if one stimulates the cut end of the spinal cord gastrocnemius fusimotor neurones are excited but the most prominent effect on semitendinosus neurones is inhibition. There is, nevertheless, clear evidence of a shorter lasting excitatory effect also. The fact that the inhibitory effect does not reach a maximum until several seconds after the end of the stimulus seems most likely to be due to the time taken for the decay of an excitatory process. The differences in the distributions in intervals of firing in the decerebrated and the spinalized preparations suggest that two different mechanisms are involved. It is likely that a regular discharge frequency of a fusimotor neurone is due to a steady depolarization of the cell. This frequency would be determined by the level of depolarization of the cell and the properties of the cell membrane. The rhythm in such a system would be an intrinsic property of the cell, and the frequency could be controlled extrinsically by any influence which could vary the resting potential. If to such a system be added another which imposes large phasic potential swings, i.e. synaptic potentials on the cell membrane, then these would disturb the intrinsic rhythm and make it less regular. In the gastrocnemius fusimotor neurones there is evidence of two such mechanisms. Diete-Spiff et al. (1959) showed that with spinal-cord stimulation in the spinalized rabbit the gastrocnemius fusimotor neurones were caused to discharge regularly. The discharge frequency was related to the frequency and the strength of spinal-cord stimulation but there was no fixed temporal relation between any one stimulus and any of the fusimotor discharges. Granit & Holmgren (1955) described previously a similar pathway in the cat's spinal cord, but in addition they described a pathway which on electrical stimulation could drive the fusimotor neurone at a fixed latency. They described similar driving by stimulation of skin nerves,and this was also seen by Diete-Spiff et al. Thus pathways of two sorts converge on to gastrocnemius fusimotor neurones, one influencing frequency of discharge, and the other capable of driving. Stimulation of the sural nerve also activates a driving pathway for the semitendinosus fusimotor neurones. It is suggested that in the decerebrated preparation such pathways are inhibited tonically. Experiments are in hand to test this hypothesis. The finding, in agreement with Henatsch & Ingvar (1956), that chlorpromazine abolished the discharges of the semitendinosus fusimotor neurones in the decerebrated but not in the spinalized preparation again 14-2

13 212 P. H. ELLA WA Y AND J. E. PASCOE suggests that in the decerebrated animal there is a descending facilitatory influence from the mid brain. The suggestion that there is a single diffuse descending pathway which is facilitatory to gastrocnemius and inhibitory to semitendinosus fusimotor neurones (Pascoe, 1963) is thus unlikely. In summary, to account for the differences in the pattern of the discharges in the decerebrated and spinal preparation, we postulate two supraspinal influences. The first is capable of influencing only the frequency of discharge, the second is inhibitory to those pathways capable of interpolating discharges in the steady rhythm produced by the first. SUMMARY 1. Functionally single fusimotor fibres were isolated from the nerves to the semitendinosus muscle of the rabbit. In the decerebrated preparation they discharged regularly whereas in the spinalized preparation the discharge was irregular. There was an abundance of tonically discharging fusimotor neurones in the spinalized animals. 2. Stimulation of the peripheral end of the cut spinal cord caused a lowering in the mean frequency of discharge, and an increase in its regularity. 3. Chlorpromazine abolished fusimotor discharges in the decerebrated but not in the spinalized preparations. 4. To account for the results two supraspinal influences were postulated, one which causes the cells to discharge at a steady frequency, and a second which inhibits pathways capable of driving the cells. We wish to express our thanks to Professor Huxley for helpful suggestions and for reading the manuscript. The work has been supported throughout by the financial assistance of the M.R.C. REFERENCES DIETE-SPIFF, K., DODSWORTH, H. & PASCOE, J. E. (1961). An analysis of the effect of ether and ethyl chloride on the discharge frequency of gastrocnemius fusimotor neurones in the rabbit. In Symposium on Mwscle Receptors, ed. BARKxR, D. Hong-Kong: University Press. DIETE-SPIFF, K. & PASCOE, J. E. (1959). The spindle motor nerve to the gastrocnemius muscle of the rabbit. J. Physiol. 149, GRANIT, R. & HOLMGREN, B. (1955). Two pathways from brainstem to gamma ventral horn cells. Acta physiol. 8cand. 35, GRANIT, R., PASCOE, J. E. & STEG, G. (1957). The behaviour of tonic a and y motor neurones during stimulation of recurrent collaterals. J. Physiol. 138, HENATSCH, H. D. & INGvAR, D. H. (1956). Chlorpromazin und Spastizitat. Arch. Psychiat. Nervenkr. 195, HUNT, C. C. & PAINTAL, A. S. (1958). Spinal reflex regulation of fusimotor neurones. J. Physiol. 143, HuxLEy, A. F. & PASCOE, J. E. (1963). Reciprocal time-interval display unit. J. Physiol. 167, 40-42P.

14 SEMITENDINOSUS FUSIMOTOR NEURONES 213 AIAC(NICHOL, E. F. & JACOBS, J. A. H. (1955). Electronic device for measuring reciprocal time intervals. Rev. sci. Instrum. 26, PASCOE, J. E. (1963). Fusimotor neurones of the semitendinosus muscle of the rabbit. J. Physiol. 167, 34-35P. VOORHOEVE, P. E. & VAN KANTEN, R. W. (1962). Reflex behaviour of fusimotor neurones of the cat upon electrical stimulation of various afferent fibres. Acta physiol. pharm. neerl. 10,