The anatomy and physiology of the muscle spindle,and its role in posture and movement: A review

Don Fitz-Ritson, BA, DC*

A detailed analysis of the anatomy of this precise senseorgan of muscle is reviewed. This includes the innervationof the nuclear bag and chain fibres, with an introductionto the B-system innervation. The physiology assesses theresponse of the primary (Ia) and secondary (II) afferentsalong with the responses occurring in the alpha a andgamma 'Y motoneurones. The integrative function of themuscle spindle is realized when the dynamic bag 1, staticbag 2 and nuclear chain fibres with their static gamma 'Yfibres is understood. This provides a basic background ofsome of the factors involved in the maintenance of postureand the generation and control of movement.

L'analyse detaillee de l'anatomie de cet organe sensitif dumuscle porte sur. l'innervation de l'enveloppe nucleaire etdes fibres catenaires et fournit une introduction 'a l'in-troduction du systeme B. La physiologie etudie la reactiondes conducteurs sensitifs primaires (Ia) et secondaires (II)ainsi que celle des motoneurones alpha (2) et gamma (y).La fonction integrative du fuseau musculaire peut etrevisualisee lorsque les enveloppes dynamiques 1, enveloppesstatiques 2 et les fibres de la chaine nucleaire, y compris lesfibres statiques gamma (Y), sont comprises. Ceci fournitun apercu de base de quelques uns des facteurs entrant enfonction dans le maintien de la posture et dans la genera-tion et le controle des mouvements.

AnatomyA number of points are pertinent when one considers

the embryology of the muscle spindle. According to Cua-junco,' it is not until the eleventh week of fetal develop-ment that there is differentiation of intrafusal from extra-fusal fibres. By the twelfth week the spindle can now berecognized even in the absence of its innervation.2

It is not until after the formation of the nuclear bag thatthe motor nerve supply reaches the spindle in the human. 'This confirms Zelena's2 findings that the sensory nervesupply is necessary for initial differentiation of musclefibre types.3

Although the spindle increases in length even afterbirth, the mature form of the organ appears to be attainedin the period between the 24th and 31st weeks. If neuro-muscular contact by sensory fibres is prevented in thedeveloping rat muscle by intrauterine severance of thesciatic nerve, no spindles are formed in the denervatedmuscles. 5The neuromuscular spindles (muscle spindles) are fusi-

form in shape and widely scattered in the fleshy bellies ofskeletal muscles. Each spindle consists of from 2-10 slen-der striated muscle fibres, enclosed within a thin connec-tive tissue capsule, and attached at both ends to the epi-mysium or ordinary striated muscle. These slender musclefibres, innervated by gamma (Y) fibres (3-7)u) are knownas intrafusal fibres, and they are tiny compared with theextrafusal fibres that produce contractile tension within amuscle. 6

Intrafusal muscle fibres are of two distinct sizes: one isof smaller diameter (10-12,u), is shorter in length (3-4mm)

and has a single chain of central nuclei. The second orlarger spindle fibres are about 2.5, in diameter, are 7-8mm in length and in the equatorial region are enlarged toaccommodate an area of numerous small nuclei (nuclearbag). The smaller intrafusal fibres are known as "nuclearchain fibres" and the larger fibres are designed "nuclearbag fibres". 7 The ends of the nuclear chain fibres areattached to the polar parts of the longer nuclear bagfibres. There are usually two of the longer fibres and fiveof the smaller fibres in each spindle, but these numbersare variable. A nuclear bag fibre with its capsule and asso-ciated sensory and motor nerve endings is shown.EXTRAFUSAI a-MOTONEURON Y-E4FFRENT GROUP & 11MUSCLE FIBRtS / 'FIFlRS Al//A[IRNT 11BRIS

-~~~~~.06~M '.; D D o','I I: ~I I**:L *I4**

INTRAFUSAL SUBCAPSULAR SPACE (APSULEMUSCLE FIBRES NUCLEAR SURROUNDING

NUCLEAR BAG SPINDLECHAIN

Figure 1: Scheme of mammalian muscle spindle innerva-tion. The spindle is embedded in the bulk of the muscle,made up of the large extrafusal muscle fibres and suppliedby a -motoneurons. (Adapted from Matthews 2' )

Two or more myelinated afferent fibres enter eachspindle. A thick primary afferent fibre forms a spiral,branching and reticulated ending within the nuclear bag

*Dr. Fitz-Ritson is a Research Fellow/lecturer at the Canadian MemorialChiropractic College, 1900 Bayview Ave., Toronto, Ontario.

144 The Journal of the CCA/Volume 26 No. 4/December 1982

The muscle spindle

area (primary, annulospiral or nuclear bag ending). (Fig. 1)The neuromuscular spindle is arrange parallel to the

extrafusal or contractile fibres of the muscle. Hence whentension on the spindle is relaxed, the afferent volleys fromthe annulo-spiral endings ceases, during active musclecontraction. The primary afferent fibre (Ia) is 12-20um indiameter. They conduct impulses at up to 120m/sec, andtheir central processes within the spinal cord participatesin the monosynaptic stretch (myotatic) reflex that regulatesmuscle tone.The myelinated secondary afferent fibres (Group II),

with diameters of 6-12,um conduct more slowly, their ter-minals are mainly in the less central region of the chainfibre, and form small rings, coils and spray-like variocosi-ties on both sides. These are called secondary (flower sprayor myotube) endings. Secondary endings are the principalsensory terminals associated with nuclear chain fibres. 8

Both primary and secondary endings are terminals of sen-sory fibres, for they degenerate after section of appropriatedorsal roots. Small fusimotor (efferent) fibres (gammaefferents 3 to 7gm in diameter) enter each spindle and ter-minate on intrafusal fibres. Two kinds of gamma fibreendings upon the intrafusal muscle fibres have beendescribed. Some end as diffuse, multiterminal "trailfibres" while others terminate in miniature "end plates".Barker 9 maintains that both nuclear bag and nuclearchain muscle fibres usually receive each type of gammamotor endings. Boyd8 maintains that nuclear bag intra-fusal fibres usually receive "trail endings". Physiologicalevidence indicates that the two types of gamma axon ter-minals subserve different spindle functions, and therebyalter the nerve impulses that are generated subsequentlyby primary and secondary afferent endings of the neuro-muscular spindle.

'Y-MOT

PRIMARY SECONDARYENDING ENDING

Figure 2: Diagram of the two intrafusal muscle fibre typesand their innervation. (Adapted from Matthews 21 )

Histochemically three types of intrafusal muscle fibresmay be recognized that correspond to glycolytic (Type A),oxidative (Type B) and oxidative glycolytic (Type C) extra-fusal muscle fibres.'0 The general conclusion from theabove study is that the chain fibres represent a homogo-nous category both with respect to their small size andtheir high succinic dehydrogenase (SD) and phosphorylase(p'ase) activity. The bag fibres, however, display heteroge-neity. Some resemble Type A (low SD, high p'ase activity)

The Journal of the CCA/Volume 26 No. 4/December 1982

and others, Type B (moderate SD, low p'ase activity) extra-fusal fibres, and the type/size relationship is retainedthough on a smaller scale. (See Fig. 2)

Barker and Stacey " were able to distinguish three typesof intrafusal muscle fibres, namely bag fibres, chain fibresand intermediate fibres. The difference between them con-cerned length, diameter, equatorial nucleation, ultrastruc-ture and histochemistry.More recently, Boyd'2 stated that mammalian intrafu-

sal muscle fibres are of three functional types. Mostspindles contain one slow nuclear bag fibre, one fastnuclear bag fibre, and four or five nuclear chain fibres.Tandem spindles have two or more encapsulated sensory

regions spaced along the length of the intrafusal bundle, incontrast to single spindle, which has only such regionssituated equatorially between two poles. The term"tandem" was introduced by Cooper'3 to describe suchspindles occurring in human neck and hand muscles,although they have been observed before in the muscles ofvarious mammals. The successive capsular regions arelinked together by one or two bag fibres which are themost common, while the chain fibres are restricted to eachcapsular unit in the series.

Barker '4 found that of 64 tandem spindles in cat rectusfemoris, 88% were double, 9% triple and 3% quintuple.The length of these tandem spindles ranged from 8.43-22.30mm (mean 13.0mm), the length of single spindles inthe same muscle ranged from 2.40-13.68mm (mean7.02mm). Some of the tandem spindles extend the entirelength of the extrafusal muscle bundles in this bipennatemuscle. The typical condition is for a large capsular unitreceiving a primary and one or more secondary endings tobe linked with one or two small capsules that receive aprimary ending only. (See Fig. 3)The distribution of spindles within a muscle follows that

of the major intramuscular nerve trunks and since the pat-tern of their distribution varies from muscle to muscle,concentration of spindles do not necessarily occur in"regions of nerve entry" or in the "belly" of a muscle ashas often been stated. "Nerve-entry" concentrations occuronly where there is rapid subdivision of the intramuscularsupply in this region, and "belly" concentration onlywhere there is a supply of one or more intramusculartrunks to this region. '5

There is some evidence that tonic muscles are equippedwith longer spindles than phasic muscles.'6'7

Barker,5 quoting from a paper by Gregor (1904) whichshowed that spindle density is highest in the hand, footand neck muscles, longest in the shoulder and thighmuscles, and medium in the more distal muscles of thearm and leg, with relatively high values for the rotatingmuscles such as pronator quadratus and popliteus. Gener-ally speaking, high spindle densities-characterize musclesinitiating fine movement (i.e. lumbricals, extraocularmuscles and small vertebral muscles) or maintaining pos-ture (i.e. soleus), low densities in those initiating grossmovement (i.e. gastrocnemius). Ultrastructural studies

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The muscle spinidle

claim that in addition to the somatic innervation, intra-fusal muscle fibres are innervated by postganglionic sym-pathetic nerve fibres. 18.19

PhysiologyThis section will be presented with reference to the spe-

cialized mechanorecptors of muscle which signal musclelength and tension. Two aspects are of importance: One isa description of the behaviour of the various receptors tothe appropriate natural stimuli in terms of impulse fre-quency in their afferent axons. Another concerns themechanisms underlying the receptor behaviour.

Muscle receptors responding to changes in musclelength or tension have properties seen in mechanorecep-tors generally. A mechanical deformation of the sensorynerve terminal leads to an alternation in the permeabilityof the nerve membrane which depolarizes the ending. Thedepolarization thus produced (the receptor or generatorpotential) at some point initiates impulse activity which isthen propagated along the axon. The sequence of eventsmay be outlined as follows:

Mechanical stimulusDeformation of receptor terminalsConductance change in terminalsReceptor (generator) potentialImpulse initiation

The physiology of the mammalian muscle spindle hasbeen studied extensively. Its input to the CNS, signallingmuscle length and rate of change of length, plays a majorrole in the reflex regulation of muscle movement, both atspinal and supraspinal levels. The mammalian spindle issubject to efferent control, mostly by neurons other thanthose innervating extrafusal muscle fibres. The reflexregulation of this spindle efferent system is an importantcomponent of the control of motor behaviour by the CNS.20

Fibre diameter of myelinated afferent fibres from limbmuscles show a trimodal distribution of external fibrediameter. Group I (12-22 ,m), group 11 (4-12 ,um) andgroup III (1-4tm). Since the external diameter of myelin-ated fibres is directly related to conduction velocity, thethree groups therefore have different conduction velocities.Group I contain fibres which terminate in muscle spindle(Ia) primary endings and tendon organ (Ib). Group IIfibres terminate as secondary endings. The division be-tween group Ia and group II fibres at 12pm, correspond-ing to a conduction velocity of 72m/sec. does not neces-sarily indicate a clear-cut separation of fibres from prim-ary endings and those conductive below 60m/sec are fromsecondary endings seems correct.

Figure 3: Diagram of spindle array in which five spindlesexist in tandem, sharing one common nuclear bag fibre.(Richmond F, Abrahams V. Morphology and distributionof muscle spindles in dorsal muscles of the cat neck. JNeurophysiol 1975; 38:1312-21.)

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The muscle spindle

4 12 20Fiber diameter ym11

24 72 120Conduction velocity m/sec.

Figure 4: Relationship between receptor type, fibre diame-ter and conduction velocity in muscle nerves. (Hunt CC,Kuffler S. Motor innervation of skeletal muscle: Multipleinnervation of individual muscle fibres and motor unitfunction. J Physiol 1954; 126:293-303.)

When rapid stretch is applied to a muscle and therebyto the spindles within it, a burst of impulses arises in bothtypes of sensory fibres. There is, however, a clear differ-ence in the characteristics of the discharges in the twoendings, the primary endings, connected to the largergroup Ia axons, are sensitive mainly to the rate of changeof stretch. The frequency of discharge is therefore maxi-mal during the dynamic phase, while stretch is increasing,and subsides to a lower, steady level while stretch is main-tained. The secondary endings connected to the smallergroup II behave differently; they are relatively unaffectedby the rate of stretch, but are sensitive to the steady levelof tension 2122.23 24 (See Fig. 4)

Muscle spindles are designed to enable the CNS tomodify or control the activity of the receptors they contain.This influence is exerted by way of the gamma fibres thatinnervate the intrafusal fibres in the spindle. The nuclearbag fibres consist of a central noncontractile region situ-ated between two contractile regions, each with its ownmotor innervation. Shortening of the contractile ends ofthe fibre stretches the central portion where the primaryreceptors are located and cause them to discharge. Sinceextension of the muscle and contraction of intrafusal fibresboth result in stretch of the primary receptor, the responseof this ending is determined by the additive effect of thesetwo kinds of stimuli. (See Fig. 5)By recording from alpha and gamma motor neurons

supplying the same muscle it has been shown that gammaactivity usually occurs in parallel with alpha activity inmost patterns of contraction. As a consequence, the intra-fusal fibres shorten as the muscle itself shortens. If intra-fusal contraction does not occur, the spindle receptorscease firing or slow down during a contraction, however,The Journal of the CCA/Volume 26 No. 4/December 1982

intrafusal contraction can maintain spindle firing duringcontraction. 25The elucidation of the motor control of mammalian

spindles followed investigation of the function of the smalldiameter motor fibres to skeletal muscle. Eccles and Sher-rington, 26 examined the diameter distribution of myelin-ated fibres in deafferented nerves to cat hind limb musclesand found it to be bimodal. The larger diameter or "A"group ranged from 9-9Zm and the smaller or " "(gamma) group from 3-8gm.

While it is clear that Y (gamma) fibres provide themotor innervation to most intrafusal fibres, the occurrenceof intrafusal fibres innervated by collaterals of axons ofalpha motoneurons has been more difficult to evaluate. Itmust be demonstrated that stimulation of a particularalpha motor axon causes an increase in discharge of aspindle afferent fibre which is due to intrafusal fibre con-traction and not to contraction in extrafusal fibres. Theproblem would be simple if extrafusal fibres were always"in parallel" with muscle spindles and on contractionalways unload the spindles. The first clear physiologicalevidence of innervation of intrafusal fibres by collaterals ofalpha motor axons came from studies of Bessou et al. 27Using the first lumbrical muscle of cat, which has only afew motor fibres, they found that some slowly conductingalpha (or B) fibres (conduction velocity about 60m/sec)

Tens ion

Secondary 1Ii iIIlIIIIIII I I I I 1-1111I1I1II

Pr ima ry 1aa I I IIIIjjIjjflfiI,-jjjj!H I II IIII1 _11 II0 0.1 0.2 0.3 0.4 sec.

Secondary 11

Tens ionI a 1- ~~~~~~~~~~~~~~~~~~~a-a

0 0.1 0.2 0.3 0.4 sec.

Figure 5: Differences in responses of Primary Ia and Sec-ondary II afferents, using different recording techniques.

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I

The muscle spindle

which caused extrafusal contraction also increased spindleafferent discharge. There was no relation between muscletension and discharge in the la fibre: As stimulus frequen-cy increased, the latter continued to increase after tetanictension produced by extrafusal fibres had become fusedand maximal.

Recently there has been demonstration of B-axons inmammals both physiologically and histologically. Repeta-tive stimulation of some motor axons supplying extrafusalmuscle fibres elicits an increase in the firing rate of spindleprimary endings, which can persist after the neuro-muscu-lar junctions of the fibres have been selectively blocked.Since excitation of spindles by extrafusal contraction isthereby eliminated, the increase in firing rate of the endingcan be ascribed to the contraction of intrafusal musclefibres and since it is elicited by stimulating single a -axons,it means that such axons give out collaterals to intrafusalmuscle fibres. 28

The stimulation of some individual motor axons at in-creasing frequencies elicits an acceleration of spindle dis-charge which continues to augment for stimulation fre-quencies above those producing maximal extrafusal tetaniccontraction. This further augmentation in the excitation ofthe spindle sensory ending cannot be due to extrafusalmuscle fibres since it occurs after the contraction of thesefibres has already reached a maximal value.29

Prolonged stimulation of some single motor axons elicitsglycogen depletion in both intrafusal and extrafusalmuscle fibres.Two types of B-axons, dynamic and static, have now

been identified. They differ by their conduction velocities,their action on primary endings and the types of intra- andextrafusal muscle fibres they innervate. Stimulation ofdynamic B-axons significantly enhance the dynamic sensi-tivity of primary endings.A0One may assume that B-motoneurones, like a -motor-

neurones, receive monosynaptic connections from homony-mous Ia and II afferent fibres, i.e. that B-motoneurons ofa given muscle are excited by impulses generated by thesensory endings of spindles in the same muscle. Thus theB-system appears to be part of a positive feedback system(B-motorneurones, spindle excitation by collaterals of B-axons, spindle afferent impulses exciting B-motorneuronesand a -motorneurones as well) which may be counteractedby the unloading effect on spindles of extrafusal contrac-tion whenever it produces muscle shortening. 29,31To better understand the integrated function of the

muscle spindle, I will analyse the functional components ofthe spindle mechanisms and their relationship to postureand movement.

Properties common to all intrafusal musclefibresContraction of all three types of intrafusal fibres is con-

fined to the capsular regions. Thus, contraction alwaysstretches the central region within the fluid space wherethe sensory endings lie. Secondly, all three types of fibreare unloaded by extrafusal contraction and intrafusal con-

148

traction is necessary if this unloading is to be prevented.Thirdly, the degree of the intrafusal contraction can befinely graded by varying the frequency of stimulation offusimotor axons, and single stimuli have, for the mostpart, a negligible effect. This is quite unlike extrafusalmuscle, in which a propagated action potential traversesthe length of the fibre, the whole fibre contracts, and asingle nerve impulse gives rise to a larger twitch of thefibre.

It follows that the input from both types of sensory end-ings, when the muscle is held at one particular length, canbe preset by the central nervous system by adjustment ofthe fusimotor output frequency. The sensitivity of themuscle spindle is thus under central control unlike that ofother receptors, such as those in the joints, which have nomotor innervation and whose sensitivity cannot be altered.StaticNuclearbag fiberSB2

-I

tstatic 'Ystatic 'Ytatic

Figure 6: The three types of intrafusal fibres in a typicalcat muscle spindle. Fusimotor innervation is from 'static'and 'dynamic' gamma 7 efferent fibres.(Boyd35.)

Properties of individual types of intrafusalfibresThe dynamic bag 1 fibre: When dynamic gamma 7

axons are activated the dynamic bag 1 fibre is depolarizedin the region of the endplate. This depolarization is localand decays electrotonically to zero along the length of thepole of the fibre without the production of an actionpotential. When the fibre is at constant length the mecha-nical response to a single stimulus is negligible. Repetitivestimulation at higher frequency, result in a maximal con-traction of the fibre, which is small and of slow timecourse near the endplate, and because of the proximity ofthe Ia afferent a discharge of slow frequency occurs.When the active fibre is stretched the pole exhibit a

greatly enhanced stiffness during the stretch so that amuch larger proportion of the extension of the spindleoccurs in the sensory spiral than was the case when thefibre was inactive. The peak Ia afferent frequency is thusgreatly increased. (See Fig. 6)The difference between the initial frequency and the

final adapted frequency at the extended length may begreater when the fibre is active than when it is not, indicat-ing some increase in the length sensitivity of the primarysensory ending when it is stretched from one stationaryposition to another. Such an increase is less obvious, how-ever, if the change in position is carried out slowly. Hence,the principal action of the dynamic bag 1 fibre is to in-crease markedly the length sensitivity of the primary sen-

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The nmuscle spinidle

sory ending during the movement. Incease in the velocityof movement also enhances the dynamic response of theprimary sensory ending, but any increase in the velocitysensitivity due to activity in the dynamic bag 1 fibre issmall compared with the increase in length sensitivity.

The static bag 2 fibre: When the static bag 2 fibre isactivated by a static 'Y axon, transient non-propagated de-polarizations are produced at the sites of the a endplatesas in the case of the dynamic bag 1 fibre and, again, singlestimuli produce a negligible mechanical response. Stimu-lation of the static bag 2 fibre at high frequencies (1OOHz),however, produces a powerful and relatively fast contrac-tion of the fibre. This causes the primary sensory spiralround the static bag 2 fibre to extend by a large amount,and the Ia frequency increases markedly, then decayswhen the active static bag 2 fibre is stretched. Theextension on the sensory spiral ending tends to be less thanthat which occurs when the fibre is inactive and both thedynamic response to stretch and the response to the main-tained length change are reduced or little altered. It seemsthat when the static bag 2 fibre spiral is already consider-ably extended by contraction of the fibre, its stiffness, orthat of the supporting connective tissue, is such that itcannot be readily extended further by stretch of thespindle. Thus, the principal action of the static bag 2 fibreis to increase the discharge of the primary sensory endingat any particular length of the muscle, without increasingits sensitivity to either the amplitude or the velocity of anychange in length.

Nuclear chain fibres: When a static gamma 'e axon tothe chain fibres is activated, the fibres are depolarized andaction potentials are produced at their motor endplates,with propagation of these potentials confined to the outerhalves of the poles of the chain fibres. Single stimuli pro-duce small local twitches of the chain fibres, which maytransmit very small twitch-like extension to the primarysensory spirals round these fibres. Contraction of the chainfibres is more rapid than that of either type of nuclear bagfibre, and such maximal contraction extends the smallprimary sensory spirals by about 20%.

Repetitive activation of chain fibres at low frequenciesresult in oscilatory contractions which 'drive' the primarysensory spirals mechanically so that they give rise to asingle Ia afferent impulse for every gamma 'Y efferentpulse. Chain fibres have a marked action on secondarysensory endings - group II. First, there is a marked in-crease in the group II afferent frequency when repetitivestimulation of a static gamma 'Y axon to the pole of thechain fibres containing the ending is commenced, whichreflects the large extension of the secondary sensory regionobserved in the isolated spindle. Second, the length sensi-tivity of the secondary ending to a maintained stretch isincreased, the difference between the initial frequency andthe adapted frequency at the final length being nearlytwice as great when the chain fibres are active as it is whenthey are inactive. The sensitivity to the dynamic phase ofthe movement is somewhat reduced.The Journal of the CCA/Volume 26 No. 4/December 1982

Role of the three intra/iisal systemsin movement and posture

Stretch of a muscle extends the sensory endings of itsmuscle spindles and this leads to an increase in the affer-ent input to the alpha a motoneurones by the way of thegroup Ia and group II afferent nerve fibres (Fig. 6). Morealpha at motoneurones are recruited and the muscle con-tracts. This capacity of the muscle to resist extension isknown as the 'stretch reflex'. Modification of this stretchreflex by excitatory or inhibitory impulses from the brainimpinging on the alpha a or gamma 'Y motoneurones inthe spinal cord is the basis of voluntary movement andpostural adjustment.32

Spindles are also involved in reflex pathways throughmotor centers higher up the brainstem. Information istransmitted from spindles to automatic motor controlcenters and to levels of consciousness, contributing toappreciation of limb movement and position. It should benoted that sensory receptors in muscle tendons, joints andskin are also involved in the control of movement. .34

There is evidence that in some voluntary movements inman, the alpha a and gamma 'Y motoneurones are acti-vated simultaneously (co-activation), however, it appearsthat in many movements in the cat, the alpha and gammasystems are activated independently of each other. Fur-ther, it seems that the brain in cat and man have at itsdisposal three distinct intrafusal systems. The choice ofwhich system to bring into action presumably dependsupon the particular task which the nervous system is calledupon to perform. Also, it is likely that it may use differentintrafusal systems in different phases of the same move-ment.

If the dynamic bag 1 fibre system is activated by way ofdynamic gamma 'Y motoneurones, then the spindle prim-ary sensory ending become extremely sensitive to changesin length during the movement. There will be a consequentdramatic increase in the 'gain' of the stretch reflex duringmovement, i.e., the output from the alpha a motoneuronesfor a given stretch of the muscle will be greatly increased.This increase in gain of the stretch reflex will decline andmay disappear when the muscle reaches the final position.Such a mechanism might be used to produce considerablecontraction of an antagonist muscle during a movementwhich would oppose the action of the agonist muscle pro-ducing the movement, but the opposing force would de-cline when the final position was reached. Unopposed con-traction of the agonist muscle could lead to a poorly con-trolled movement which would overshoot the desired finalposition.

If the static bag 2 fibre system is activated by way ofstatic gamma 'Y axons, then at any given muscle length,the input from the spindles will be increased and the forcedeveloped by the muscle will be greater, but there will belittle change in the gain of the stretch reflex. It may bethat this system is used to 'bias' the stretch reflex arc sothat it can contribute to postural adjustment.The role of the nuclear chain fibre system, with its curi-

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The muscle spindle

ous 'driving' action on the primary sensory ending isobscure. If activated it will tend to reduce the effectivenessof the spindle as a device that measures length, thoughthere may be substantial excitation of the alpha a moto-neurones. It seems likely that the action of nuclear chainfibres in providing a positive bias to the stretch reflex byway of the group II afferent pathway, and at the sametime increasing its gain because the length sensitivity ofsecondary endings is also increased, is important. Indeed,this seems to be the only intrafusal mechanism which con-sistently increases the gain of the stretch reflex when themuscle is at a constant length, the dynamic bag 1 fibredoing so primarily during movement. Such a mechanismwould enable a muscle to support a heavy load, since arelatively small degree of extension would generate enoughadditional tension to support the load.

Finally, it should be noted that the brain does not havecompletely separate control of the static bag II fibre andchain fibre intrafusal system. 5

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150 The Journal of the CCA/Volume 26 No. 4/December 1982