J. Physiol. (1975), 247, pp. 131-143 131With 1 plate and 4 text-ftgureMPrinted in Great Britain

AN INVESTIGATION INTO THE SITE OFTERMINATION OF STATIC GAMMA FIBRES WITHIN MUSCLE

SPINDLES OF THE CAT PERONEUS LONGUS MUSCLE

By M. C. BROWN AND R. G. BUTLER*

From the University Laboratory of Physiology,Parks Road, Oxford OX1 3PT

(Received 6 August 1974)

SUMMARY

1. The distribution of static fusimotor fibres to intrafusal musclefibres of cat peroneus longus muscle spindles was investigated using theglycogen-depletion technique of Edstr6m & Kugelberg (1968). Singlestatic y fibres were stimulated intermittently at high rates for 3 hr withthe blood supply occluded for some of this time. Subsequently the portionof muscle containing the activated spindles was fixed, sectioned and stainedfor glycogen with the periodic acid-Schiff (PAS) method.

2. Ten static axons caused depletion in eleven spindles. In five ofthese the only glycogen-depleted fibres were nuclear chain fibres. In theother six spindles one nuclear bag fibre was depleted in addition to chainfibres and this was always the larger of the two within the spindle.

3. These results on a medium-sized hind limb muscle are comparedwith findings concerning the distribution of static y fibre axons previouslyinvestigated only in very small muscles. The results agree in showing thatnearly all static y fibres innervate nuclear chain fibres but that in 50-75 %of the times in which static y fibres innervate spindles the distribution isto bag fibres as well as to chain fibres. The interpretation to be put uponthis is uncertain. One possibility with which the results from peroneuslongus are consistent is that the bag fibres which are usually innervatedby static axons are the 'intermediate' bag fibres whose ultrastructure hasrecently been shown to resemble that of chain fibres.

INTRODUCTION

The evidence that static y fusimotor fibres may have their motor endingson nuclear bag intrafusal muscle fibres in addition to terminations onnuclear chain fibres is now strong. Evidence that static axons can be

* Fellow of the Multiple Sclerosis Society of Canada.

M. C. BROWN AND R. G. BUTLER

non-selective, and innervate chain and bag fibres comes from four separatelines of investigation: (1) Direction observation of contractions elicited inspindles by static axons (Bessou & Pages, 1973; Boyd, Gladden, McWilliam& Ward, 1973); (2) histological identification of intrafusal fibres yieldingspike or junction potentials (recorded with intracellular micro-electrodes)on static fibre stimulation (Barker, Bessou, Pages & Stacey, 1974a);(3) distribution of glycogen-depleted intrafusal fibres after stimulation ofstatic axons (Brown & Butler, 1973); and (4) silver staining of the onlysurviving static axon to one muscle (Barker, Emonet-Denand, Laporte,Proske & Stacey, 1973). These experiments are reviewed by Laporte &Emonet-D6nand (1973).Of course it could be the case that the gross morphological differences

between nuclear bag and chain intrafusal muscle fibres do not signifyanything of functional importance, so that non-selective innervationmay be then of no consequence. However, there is good evidence to showthat bags and chains not only differ in gross morphology but also histo-chemically and ultrastructurally (see, for example, review by Matthews,1972). It would be surprising if these differences did not underlie importantdifferences in physiological behaviour with significance for the functioningof the spindle. Furthermore, many static fusimotor fibres are specificallydistributed to nuclear chain fibres alone, and virtually all dynamicfusimotor fibres innervate only nuclear bag fibres (Bessou & Pages, 1973;Boyd et al. 1973; Brown & Butler, 1973). The non-selective static gammafibres which innervate both bag and chain fibres thus represent somethingof a puzzle. However, virtually all the recent evidence in favour of non-selectivity has come from 'small' muscles, especially the tenuissimus. Thisis a very long and extremely slender muscle (only twenty motor units,Boyd & Davey, 1968) which is very convenient experimentally, but it hasa very low ratio of y fibres to its spindle count (1-3:1, Boyd & Davey,1968) and has a reportedly high proportion of fi innervation (McWilliam,1974). It seemed possible that the low y fibre count, in association with thepresence of many dynamic , fibres, might indicate a shortage of appropri-ate y dynamic motor terminals for all the available sites on the bagintrafusal fibres, a situation which might lead to a significant amount ofnon-selective innervation of bag fibres by static y fibres.We have therefore looked again with the glycogen-depletion method at

the distribution of static y axons, using the peroneus longus muscle to seeif the 'availability' of the right sort of terminal affects the degree ofselectivity of connexions made. This is a medium sized muscle (ninety-fivemotor units, Boyd & Davey, 1968) and has a high y/spindle ratio (7.3:1,Boyd & Davey; 4-1:1, Barker, Stacey & Adal, 1970). It also has the prac-tical advantage that it is possible to partially separate off a small posterior

132

STATIC GAMMA FIBRE DISTRIBUTION 133

head in which the spindle to be activated by the static axon can be ap-proximately localized. This smaller piece of muscle is readily fixed forhistology, and also is much less time consuming to process than the wholemuscle.

METHODS

Ten adult cats anaesthetized with intraperitoneal sodium pentobarbitone wereused. The experimental methods were essentially those used by Brown & Butler(1973). A laminectomy was performed, followed by Ceiervation of the hind limbapart from the peroneus longus muscle. Static y fibres were found by stimulatingventral root filaments at 143/sec while recording from a spindle primary endingwhich had been roughly localized in the smaller more posterior portion of themuscle. Stretch of 5 mm at 8 mm/sec was applied to this part of the muscle duringthe ventral root stimulation. Having found a static action, this piece of ventral rootwas split until it contained only a single y action potential on stimulation of themuscle nerve. Great care was taken to ensure that only a single y fibre in the isolatedfilament was responsible for the spindle excitation.Glycogen depletion was produced using the same stimulus regime as Brown &

Butler (1973) and with the same method of intermittent occlusion of the bloodsupply. A total of 3 hr of intermittent y stimulation was used. We did not attemptto find out if a shorter period of stimulation would have been adequate (see Barker,Emonet-Denand, Harker, Jami & Laporte, 1974b).

Speed of histological fixation is vital for preservation of normal glycogen content.In pilot experiments we found that the whole peroneus longs muscle could not beadequately fixed for glycogen either by freezing or by injection and immersion inRossman's fixative. The smaller posterior head in which the activated spindle laywas completely cut away from the rest of the muscle at the end of the experimentwith the distal end and the very proximal end (which contain no spindles) cut off.Fixative (Rossman's fluid) was injected into the muscle followed by immersionovernight at 4 0° C. This procedure, especially when preceded by peeling awayand/or cutting through surface connective tissue, and an injection into themuscle of Hyalase solution in Ringer 10 min before final removal of the musclefrom the leg, gave excellent preservation of glycogen content in extrafusal andintrafusal fibres. The excision, trimming and injection of the muscle took less than20 sec.

This tissue was dehydrated in ethoxyethanol, embedded in ester wax andsectioned at 8 or 10 4am. Staining was as in Brown & Butler (1973). Serial recon-structions of spindles with glycogen-depleted fibres were made from drawings takenat 100 ,tm intervals.No control muscles from the other leg were taken. During the experiment the

number of afferents in the posterior segment of peroneus longus which the static yfibres excited could be determined. We found that the static fibres in peroneuslongus were not widely distributed amongst the spindles, often supplying only asingle spindle. Within the portion of experimental muscle there were therefore manyspindles upon which a particular static fibre had had no action, and in keeping withthis we usually found only one spindle which was depleted. This was assumed to bethe one on which the y had acted, the others with no depletion serving as thecontrols.The contrast with tenuissimus might be pointed out here. In that muscle the serial

arrangement of spindles enabled us (Brown & Butler, 1973) to remove for sectioningonly those parts of the tenuissimus which contained spindles which we knew had

M. C. BROWN AND R. G. BUTLERbeen activated by the fibre. We always expected to see depletions in the experi-mental material, and control sections through the contralateral muscle were neces-sary. In peroneus longus, where several spindles may lie parallel to one another,we could only infer that the spindle which showed depletion was in fact the oneupon which the y fibre had acted. As pointed out above this seems a particularlyreasonable conclusion on account of the correlation between the number of spindlesshowing depletion and the number known to have been excited.

RESULTSDepletion experimentsThe distribution of glycogen within individual intrafusal muscle fibres

in non-depleted spindles was generally that found in tenuissimus (Brown& Butler, 1973). We did not see the short pale segments in bag fibres ofcontrol peroneus longus spindles such as were seen in some tenuissimusspindles.

a b

.[ t .0-. ........

1 sec

Text-fig. 1. Excitatory action of the two static y fibres used to producedepletion ofglycogen in the spindles of PI. 1 (column a corresponds to P1. 1 aand b; column b corresponds to PI. Ic and d). Instantaneous frequencydisplay (vertical calibration bars 200 impulses/see), showing responses ofspindles to stretch of 5 mm for a, 4 mm for b; above, responses of passivespindles; below, responses during ys stimulation at 143/sec. Horizontal,time bar: 1 see; length record diagrammatic.

We investigated ten static y fibres which supplied eleven spindles in theperoneus longus muscle. Two examples of depletion are shown in PI. 1,and Texf-fig. 1 a and b show the typical static type of excitatory actionwhich these two y fibres had on their respective primary afferents. Oneof these (P1. 1 a and b) depleted only chain fibres-all five at one pole ofthe spindle and none at the other. The second (P1. 1 c and d) depleted onebag, fibre and four chain fibres at one pole, and the same bag fibre and twochains at the other pole.

134

STATIC GAMMA FIBRE DISTRIBUTION

The results for all eleven spindles are summarized in Text-fig. 2 inwhich the two poles of each spindle are shown, bag fibres represented withlarge circles, chains with small circles, glycogen-containing fibres as filledcircles, depleted ones as open circles. To the right of each spindle is giventhe total length of bag and chain fibres depleted in that spindle. Five outof the eleven spindles showed only chain fibres innervated by static iy's.In the remaining six, one bag fibre was also depleted in addition to a chainor chain fibres. All of these bag fibres were the larger of the two within thespindle and the regions of depletion were usually intracapsular. Three ofthe eleven spindles had chain fibres depleted at both poles. It is worthnoting that the one static fibre which supplied two spindles was specificallydistributed to chain fibres in one spindle, but to a bag and chain in theother.

Pole 1% I

Bags Chains

31 @0 Oges0 006*

29 @0 OOO@25 * 0 00000

21 *0 00000

19 @0 oooooee17 -4 * *& Odd

27 _ * *000@

25 @0 0-0921 * 00000

24 °°-00000

Pole 2

Bags

0@@00@@0@00@

@00@0@0@0@0

Chains

S...

e0g.0000

000000

00000

0000*@0

*000

00*0000

00000

e9g..000000

Total length depleted (pm)Bag Chain

320 960

0 1280

400 1440

0 1920

390 1790

300 3200

0 250

0 300

300 1100

0 4000

840 4560

Text-fig. 2. Summary of glycogen depletion data for static gamma fibres inperoneus longus. 0, glycogen-containing bag fibres; 0 glycogen-depletedbag fibres; *, glycogen-containing chain fibres; 0, glycogen-depletedchain fibres. The figures on the left are the conduction velocities of the '

fibres in m/sec. The figures on the right give the total length of bag andchain fibres depleted.

For seven of the eleven spindles, in which permanent records of theaction of the static fibre on its primary afferent were made, the extent ofthe chain fibre depletion has been plotted against the excitatory action(Text-fig. 3). From this figure it can be seen that there is a good correlation

135

136 M. C. BROWN AND R. G. BUTLER

between the two variables, so that powerful static fibres on the wholecaused more depletion (r = 0 91, P < 0.01).

Physiological experiments: combined stimulation of static and dynamicy fibres

There was no striking difference in excitatory action detectable betweenstatic fibres which subsequently turned out to have gone only to chains,and those which were non-selective. The dynamic index was reduced to alesser extent by the non-selective fibres, but numbers were too small forthis to be treated statistically.

U 200ain

.E 1500~~~alSOo2 0

100° *°

C 0 10 20 .3.0 40 5.0

Total length of chains depleted (mm)

Text-fig. 3. Correlation between excitatory action of static y fibres andthe length of depleted chain fibres, 0 results from static y fibres innervatingchain fibres only; * results from static y fibres innervating bag and chainfibres.

This failure to detect the effect of contraction of bag fibres excitedsimultaneously with chains by static y fibres which innervate both ispuzzling. Indeed it was the thought that such simultaneous activationwould lead to a 'mixed' response from the afferent, being dynamic undersome circumstances and static under others, that led some physiologists tofavour the idea that separate innervation of bag and chain fibres by dyna-mic and static fibres respectively, must occur. As Crowe & Matthews(1964) and Lennerstrand (1968) showed, the simultaneous activation atthe same stimulation rate of a static and dynamic fibre led to a hybridresponse, with the dynamic action being prominent during stretching,the static during release.

Lennerstrand (1968) has published (his Fig. 5) a picture showing that ifthe dynamic effect is not too strong, the static action can dominate the

STATIC GAMMA FIBRE DISTRIBUTION 137

afferent response even during stretching. We investigated this further bystimulating a static and a dynamic fibre simultaneously, but at differentrates, hoping thereby to mimic the behaviour of a non-selectively distri-buted static fibre which we envisaged from the present depletion studiesto have a much less strong action on bag fibres than chains. As can be seenin Text-fig. 2, such non-selective static fibres in peroneus longus alwaysdepleted at least a three times longer length of chain fibres than bagfibres.

YsO 50 100

YdO0

,. ~ ~ ~~~~~~~ .~..

25

50

< [ 1 ~~ ~~~sec ;

100

Text-fig. 4. Effect of separate and combined stimulation of a single staticand a single dynamic y fibre on the response of a spindle primary endingduring stretching and releasing. Frequencies of stimulation of static y fibreare given above each column of records, frequencies of stimulation ofdynamic y fibre are given beside each row of records. Calibrations: verticalbar for frequency meter, 200 impulseslsec; horizontal bar for time, 1 see;length of stretch, 4 mm; length record diagrammatic.

Text-fig. 4 illustrates the effects of separate but simultaneous stimulationof a single static and dynamic fibre at a variety of frequencies on the dis-charge of a single spindle primary ending. As the frequency of stimulationto the dynamic fibre is reduced it is found that the combined stimulationgives an excitatory action which becomes more or less indistinguishablefrom that of the static fibre alone, even though at that frequency the dyna-mic fibre on its own has a clear excitatory action.An alternative explanation, which takes into account the possible

occurrence of 'intermediate' type bag fibres (Barker & Stacey, 1970) withchain-like contraction properties, is given in the discussion to explain thefunctionally pure effect of a non-selective static y fibre.

M. C. BROWN AND R. G. BUTLER

DISCUSSION

The main finding is that static y fibre action in the peroneus longusmuscle is always associated with chain fibre contraction. But, as intenuissimus, there may be some innervation of bag fibres as well. In somespindles it is now possible to differentiate two sorts of bag fibres on histo-chemical and ultrastructural grounds (Barker & Stacey, 1970, rabbit;Banks & James, 1974, rabbit; Smith & Ovalle, 1972, cat and monkey;Barker et al. 1974b, cat; Barker et al. 1974a, cat) and this raises twoobvious questions for the present work. Are there two sorts of bag intra-fusal fibre in cat peroneus longus spindles (and if so in what proportion ofspindles) and if there are, to which sort, one or other or both, do the non-selective static y fibres go? It seems probable from the ultrastructural andother studies that the other kind of bag fibre (sometimes called 'inter-mediate' although the terminology on this point is confused) may havecontractile properties similar to those of chain fibres. These fibres possessM lines, have propagated action potentials; have a high mitochondrialvolume, a well-developed sarcoplasmic reticulum and probably havereasonably high activity for succinic dehydrogenase, alkaline ATPase andphosphatase enzymes. As the non-selective static fibres in peroneus longuswent to only a single bag fibre in each spindle, these could have been'intermediate' or chain-like bags. Unfortunately gross morphologicaldifferences, between true bag fibres and 'intermediate' bag fibres are notsufficiently clear for us to differentiate with certainty between the two inour PAS-stained preparations. However, all six bag fibres depleted in thepresent experiments on peroneus longus were definitely the larger indiameter of the two bag fibres in each spindle, and at present it does appearthat the 'intermediate' bag fibres are larger in diameter than true bagfibres in the polar regions (Barker & Stacey, 1970; Banks & James, 1974;Barker et al; 1974b).

Comparison of the present results with earlier workFour groups of workers using three different techniques have now in-

vestigated the distribution of static y fibres in muscle spindles of threedifferent hind-limb muscles, and in Table 1 we have summarized theirdata. The percentage of cases where the distribution is non-selective,i.e. not to chain fibres only, varies between 75 and 50 % (row 4 in Table 1),and the percentage of cases where the distribution is selective for chainsonly (row 1) varies between 50 and 25 %. There is no statistically signifi-cant difference between even the most extreme pairs of observations inrows 1 and 4, i.e. P > 0 05 comparing the data of Brown & Butler ontenuissimus (four specific, twelve non-specific) with those of Boyd et al.

138

STATIC GAMMA FIBRE DISTRIBUTION

-.

cd cec

~~ 0o0 ~~a ). b"5.

r-4 ;.4~0 ;4 -~$$5C4~I X 0

0 C o 0 0

139

M. C. BROWN AND R. G. BUTLER

on abductor digiti quinti medius (four specific, four non-specific) (prob-ability calculated using exact two-tailed test for 2 x 2 contingency table,Bailey, Statistical Methods in Biology, 1973, pp. 61-65).Rows 1 and 4 give the most 'pessimistic' view about the level of selec-

tivity displayed by static y fibres. As discussed above, the potentialpresence of a third type of intrafusal muscle fibre raises other possibilities.If it is assumed that this 'intermediate' bag fibre behaves physiologicallylike a chain fibre, that each spindle possessed only one such fibre, andthat where static y fibres innervate bag fibres their first priority is toinnervate the 'intermediate' fibre, then one can advance the most 'opti-mistic' view that can be taken about the selectivity of distribution ofstatic y fibres. This has been done in rows 5 and 6, where non-selectivity istaken to be present only where a static fibre innervates more than onebag fibre in a spindle, for only then could one be certain, given the aboveassumptions, that the distribution was to a true bag fibre. Using thesecriteria, both the present experiments on peroneus longus and those ofBoyd et al. (1973) show 100 % specificity. Data from Barker et al. (1973)on tenuissimus implies 87 % specificity and Brown & Butler's (1973)data, also from tenuissimus, implies 56 % specificity. Differences betweenthe data of the different groups in rows 5 and 6 are not statistically signi-ficant at the 1 % level, but the data of Brown & Butler (1973) on tenuissi-mus are significantly different at the 5 % level from the present data onperoneus longus (P = 0.013), and from the data of Boyd et al. on the footmuscle (P = 0.033). This may be explained by the fact that we deliber-ately chose particularly powerful static y fibres in our tenuissimus experi-ments, and these might have had more extensive connexions within thespindle, whereas in the other experiments there was no conscious selectionof static y fibres of any particular strength or other characteristic.The possibility of there being an 'intermediate' type of bag fibre also

helps to explain the disturbing finding of Barker et al. (1973) that 23 %of their static fibres went to bag fibres only (row 2, Table 1). Without thispossibility one has to assume either that in their experiments extensivesprouting of the sole remaining static fibre had occurred during thedegeneration time of the others, or that our whole concept of the way inwhich static and dynamic effects are produced would have to be changed(see below).The glycogen-depletion technique enables one to get some measure of

the extent of innervation of different sorts of intrafusal fibres by observingthe length of the depleted segments. Comparison of our data for bagfibres depleted by static axons from tenuissimus (Brown & Butler, 1973)with that from the present series is made in rows 8 and 9 of Table 1. Thetotal length of bag fibres depleted was less than 400 Itm in extent in ten

140

STATIC GAMMA FIBRE DISTRIBUTIONout of eleven peroneus longus spindles (five of course of these elevenspindles had no bag fibre depletion at all), but in tenuissimus it was greaterthan this amount in eleven out of the sixteen. This difference is significantat the 1% level (P = 0 003). In contrast the amount of chain fibredepletion was very similar in the two muscles. The average amount ofdepletion per chain fibre (total length of chain fibres depleted/total numberof chain fibres in all affected spindles) was 453 jtm in peroneus longus and410 jtm in tenuissimus.

ConclusionsOn the whole it seems that there are no major qualitative differences

between the tenuissimus, abductor digiti quinti medius and the peroneuslongus muscles as far as the distribution of static y fibres is concerned. Itseems highly probable, too, that the dynamic y fibres, which seem to bealmost exclusively distributed to bag fibres in tenuissimus (Bessou &Pages, 1973; Brown & Butler, 1973; Barker et al. 1974a) and in abductordigiti quinti medius (Boyd et al. 1973) will also be selective for bags inperoneus longus. Moreover it would seem likely that they should go totrue bag fibres, for if the arguments expressed above are correct, extensiveinnervation of intermediate bag fibres by dynamic y neurones might leadto a mixed physiological effect.

It should be noted, however, that recent evidence seems to point awayfrom mechanical explanations for spindle adaptation (frog, Kirkwood1972 and Ottoson & Shepherd, 1970; cat, Hunt & Ottoson, 1974). Toexplain the different action of static and dynamic y fibres one must thenassume that they induce preferential stretching of la terminals withdifferent differentiating properties. The most extreme view would simplyrequire separate access to such two sorts of 1 a terminals by making dif-ferent intrafusal fibres contract. Provided there was no sharing of intra-fusal fibres by static and dynamic fibres this scheme would work indepen-dently of the different contractile properties of different sorts of intrafusalfibres. Most significantly Bessou & Pages (1973) have discovered that nostatic fusimotor fibre ever caused contraction in the one nuclear bag fibrein each spindle which they found to be innervated by a dynamic fusimotorfibre, although other bag fibres were often made to contract. This is anextremely important observation. Further work will reveal whether thecompletely separate distribution is coupled with structural and chemicaldifferences in the fibres supplied by the two sorts of y neurone.The present experiments would be entirely consistent with there being

such differences. The accumulating evidence for the existence of a thirdtype of intrafusal muscle fibre thus rejuvenates the selective innervationhypothesis of Jansen and Matthews (1962) and may make unnecessary

141

M. C. BROWN AND R. C. BUTLER

alternative explanations, such as that based on our Text-fig. 4, for theclear functional distinction. of static and dynamic gamma fibres.

We would like to thank the Medical Research Council for a project grant in supportof this work. We are also grateful to Mr M. Hulliger and Dr D. R. Westbury forcritical comments on the paper, to Mr R. Barson for help during the experiments,and to Mr W. Laycock and Mr B. Howse for designing much of the equipment used.

REFERENCES

BAILEY, N. T. (1973). Statistical Methods in Biology. London: Methuen.BANKS, R. W. & JAMES, N. T. (1974). Rabbit intrafusal muscle fibres. Paper 13 of

the Anatomical Society meeting, April 1974. Symposium on Muscle Spindles.BARKER, D., BESSOU, P., PAGES, B. & STACEY, M. J. (1974a). Distribution of staticand dynamic y axones to cat intrafusal muscle fibres. Paper 24 of the AnatomicalSociety meeting, April 1974. Symposium on Muscle Spindles.

BARKER, D., EMONET-DANAND, F., HARKER, D. W., JAMI, L. & LAPORTE, Y.(1974b). Intrafusal glycogen depletion elicited by ,8 axones in cat spindles. Paper25 of the Anatomical Society meeting, April 1974. Symposium on MuscleSpindles.

BARKER, D., EMONET-DE'NAND, F., LAPORTE, Y., PROSKE, V. & STACEY, M. (1973).Morphological identification and intrafusal distribution of the endings of staticfusimotor axons in the cat. J. Physiol. 230, 405-427.

BARKER, D. & STACEY, M. J. (1970). Rabbit intrafusal fibres. J. Physiol. 210,70-72 P.

BARKER, D., STACEY, M. J. & ADAL, M. N. (1970). Fusimotor innervation in the cat.Phil. Trans. R. Soc. B 258, 315-346.

BEssou, P. & PAisS, B. (1973). Nature des fibres musculaire fusales activees par desaxones fusimoteurs statiques ou dynamiques chez le chat. C. r. hebd. Seanc. Acad.Sci., Pari8, series D, 277, 89-91.

BOYD, I. A. & DAVEY, M. R. (1968). Composition of Peripheral Nerves. Edinburghand London: Livingstone.

BOYD, I. A., GLADDEN, M. H., MCWILLIAM, P. N. & WARD, J. (1973). Static anddynamic fusimotor action in isolated cat muscle spindles with intact nerve andblood supply. J. Physiol. 230, 29-30 P.

BROWN, M. C. & BUTLER, R. G. (1973). Studies on the site of termination of staticand dynamic fusimotor fibres within muscle spindles of the tenuissimus muscle ofthe cat. J. Physiol. 233, 553-573.

CROWE, A. & MATTHEWs, P. B. C. (1964). Further studies of static and dynamicfusimotor fibres. J. Physiol. 174, 132-151.

EDSTROM, L. & KUGELBERG, E. (1968). Histochemical composition, distribution offibres and fatiguability of single motor units. J. Neurol. Neurosurg. Psychiat. 31,424-433.

HUNT, C. C. & OTTOSON, D. (1974). Dynamic and static responses of primary andsecondary endings of isolated mammalian muscle spindles. Paper 16 of theAnatomical Society meeting, April 1974. Symposium on Muscle Spindles.

JANSEN, K. S. & MATTHEWS, P. B. C. (1962). The central control of the dynamicresponse of muscle spindle receptors. J. Physiol. 161, 357-378.

KIRKWOOD, P. A. (1972). The frequency response of frog muscle spindles undervarious conditions. J. Physiol. 222, 135-160.

142

The Journal of Physiology, Vol. 247, No. 1 Plate 1

Y~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.....

a;.~ ':,:

M. C. BROWN AND R. G. BUTLER(Finp.43 (Facing p. 143)

STATIC GAMMA FIBRE DISTRIBUTION 143

LAPORTE, Y. & EMONET-DENAND, F. (1973). Evidence for common innervation ofbag and chain muscle fibres in cat spindles. In Advances in Behavioural Biology,vol. 7, Control of Posture and Locomotion, ed. STEIN, R. B., PEARSON, K. G.,SMITH, R. S. & REDFORD, J. B. New York and London: Plenum Press.

LENNERSTRAND, G. (1968). Position and velocity sensitivity of muscle spindles inthe cat. IV. Interaction between two fusimotor fibres converging on the samespindle ending. Acta physiol. scand. 74, 257-273.

MATTHEWS, P. B. C. (1972). Mammalian Muscle Receptors and their Central Actions.London: Arnold.

MCWILLIAM, P. N. (1974). A quantitative study of ft innervation of muscle spindlesin small muscles of the hind limb of the cat. J. Physiol. 239, 43 P.

OTTOSON, D. & SHEPHERD, G. M. (1970). Length changes within isolated frog musclespindle during and after stretching. J. Physiol. 207, 747-759.

SMITH, R. S. & OVALLE, W. (1972). Histochemical identification of three types ofintrafusal muscle fibres in the cat and monkey based on the myosin ATPasereaction. Can. J. Physiol. Pharmacol. 50, 195-202.

EXPLANATION OF PLATE

To illustrate the effect of stimulating single static fusimotor fibres on the glycogencontent of cat peroneus longus muscle spindles.

a, b, From the two poles of one spindle. There is depletion of all five chain fibres atone pole (a), and no depletion at the other (b).

c, d, From the two poles of another spindle in a different experiment. Here there isdepletion at both poles; in one bag and two chains in c, and in the same bag fibreand four chain fibres at the other pole.Bag fibres marked with arrows; length calibration bar, 25 /tm.All sections cut at 10 gim.