Morphological identification and intrafusal distribution of the endings of static fusimotor axons in the cat

There and back again: 50 years of wandering through terra incognita fusorum

This paper is in two parts: 'There', which is a review of some of the major advances in the study of spindle structure and function during the past 50 years, serving as an introduction to the symposium entitled 'Mechanotransduction, Muscle Spindles and Proprioception' held in Munich in July 2022; and 'And Back Again', presenting new quantitative morphological results on the equatorial nuclei of intrafusal muscle fibres and of the primary sensory ending in relationship to passive stretch of the spindle.

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Muscle spindle and fusimotor activity in locomotion

Mammals may exhibit different forms of locomotion even within a species. A particular form of locomotion (e.g. walk, run, bound) appears to be selected by supraspinal commands, but the precise pattern, i.e. phasing of limbs and muscles, is generated within the spinal cord by so-called central pattern generators. Peripheral sense organs, particularly the muscle spindle, play a crucial role in modulating the central pattern generator output. In turn, the feedback from muscle spindles is itself modulated by static and dynamic fusimotor (gamma) neurons. The activity of muscle spindle afferents and fusimotor neurons during locomotion in the cat is reviewed here. There is evidence for some alpha-gamma co-activation during locomotion involving static gamma motoneurons. However, both static and dynamic gamma motoneurons show patterns of modulation that are distinct from alpha motoneuron activity. It has been proposed that static gamma activity may drive muscle spindle secondary endings to signal the intended movement to the central nervous system. Dynamic gamma motoneuron drive appears to prime muscle spindle primary endings to signal transitions in phase of the locomotor cycle. These findings come largely from reduced animal preparations (decerebrate) and require confirmation in freely moving intact animals.

The innervation of the muscle spindle: a personal history

I present a brief review of current understanding of the innervation of the mammalian muscle spindle, from a personal historical perspective. The review begins with comparative studies on the numbers of spindle afferents and considers how their relative abundance may best be assessed. This is followed by an examination of the distribution and some functional properties of the motor innervation. The primary ending is the subject of the final section, in particular, I look at what can be learned from serial sectioning and volumetric reconstruction, and present new results on a model and simulations concerning sensory terminal deformation during stretch.

Alpha, beta and gamma motoneurons: functional diversity in the motor system's final pathway

Since their discovery in the late 19th century our conception of motoneurons has steadily evolved. Motoneurons share the same general function: they drive the contraction of muscle fibers and are the final common pathway, i.e., the seat of convergence of all the central and peripheral pathways involved in motricity. However, motoneurons innervate different types of muscular targets. Ordinary muscle fibers are subdivided into three main subtypes according to their structural and mechanical properties. Intrafusal muscle fibers located within spindles can elicit either a dynamic, or a static, action on the spindle sensory endings. No less than seven categories of motoneurons have thereby been identified on the basis of their innervation pattern. This functional diversity has hinted at a similar diversity in the inputs each motoneuron receives, as well as in the electrical, or cellular, properties of the motoneurons that match the properties of their muscle targets. The notion of the diverse properties of motoneurons has been well established by the work of many prominent neuroscientists. But in today's scientific literature, it tends to fade and motoneurons are often thought of as a homogenous group, which develop from a given population of precursor cells, and which express a common set of molecules. We first present here the historical milestones that led to the recognition of the functional diversity of motoneurons. We then review how the intrinsic electrical properties of motoneurons are precisely tuned in each category of motoneurons in order to produce an output that is adapted to the contractile properties of their specific targets.

Comparison of the effects of stimulating groups of static gamma axons with different conduction velocity ranges on cat spindles

In cat peroneus tertius muscles, static gamma axons were prepared in groups of three to four according to the conduction velocity of their axons (fast, intermediate, or slow). Effects of stimulating these groups (at 20, 30, and 50 Hz) on spindle ensemble discharges during sinusoidal stretch (peak-to-peak amplitude, 0.5 mm; frequency linearly increasing from 0.5 to 8 Hz in 10 s) were compared. Ensemble discharges were obtained by digital treatment of the discharges in afferent fibers from all the spindles in peroneus tertius as recorded from the muscle nerve. Stimulation of each group prevented ensemble discharges from falling to very low levels during shortening phases. However, this effect was clearly larger when the group of fast-conducting axons was stimulated. In view of the known effects of the activation of bag(2) and chain fibers (either separately or together) on single primary ending discharges during comparable sinusoidal stretches, this stronger effect supports the view that static gamma axons with faster conduction velocities are more likely to supply more bag(2) fibers than slower ones. Possibly the proportions of bag(2) and chain fibers activated during motor activity are determined by a recruitment of static gamma motoneurons related to their size.

Testing the classification of static gamma axons using different patterns of random stimulation

The possibility of using randomly generated stimulus intervals (with a Poisson distribution) to identify the type(s) of intrafusal fiber activated by the stimulation of single static gamma axons was tested in Peroneus tertius muscle spindles of anesthetized cats. Three patterns of random stimulation with different values of mean intervals [20 +/- 4. 47, 30 +/- 8.94, and 40 +/- 8.94 (SD) ms] were used. Single static gamma axons activating, in single spindles, either the bag2 fiber alone or the chain fibers alone or both types of intrafusal fiber were prepared. Responses of spindle primary endings elicited by the stimulation of gamma axons were recorded from Ia fibers in cut dorsal root filaments. Cross-correlograms between stimuli and spikes of the primary ending responses, autocorrelograms, interval histograms of responses, and stimulations were built. The characteristics of cross-correlograms were found to be related not only to the type of intrafusal muscle fibers activated but also to the parameters of the stimulation. Moreover some cross-correlograms with similar characteristics were produced by the activation of different intrafusal muscle fibers. It also was observed that, whatever the type of intrafusal muscle fiber activated, cross-correlograms could exhibit oscillations after an initial peak, provided the extent in frequency of the primary ending response was small; these oscillations arise in part from the autocorrelation of the primary ending responses. Therefore, cross-correlograms obtained during random stimulation of static gamma axons cannot be used for unequivocally identifying the type(s) of intrafusal muscle fiber these axons supply.

Functional consequences of bag2 and chain fiber coactivation by static gamma-axons in cat spindles

A study of the distribution in cat peroneus tertius spindles of 42 single static gamma-axons was recently carried out with a physiological method for identifying the intrafusal muscle fibers supplied by single gamma-axons. It was found that 35 axons (83%) supplied both slow-contracting bag2 fibers and fast-contracting chain fibers. The distribution of these axons generally varied from one spindle to another among all the spindles that each of them supplied (bag2 and chain fibers together, bag2 alone, chains alone). To find some functional consequences of this coactivation, responses of primary endings to sinusoidal stretch of the muscle (amplitude 0.5-1 mm, frequency linearly increasing from 0.6 to 8-9 Hz in 12 s) were recorded at different average muscle lengths (0.5, 1.0, and 1.5 mm shorter than maximal physiological length) in nembutalized cats during repetitive stimulation at 10, 20, and 30 Hz of single gamma-axons previously shown to supply bag2 and chain fibers in the spindles bearing the primary endings. These responses were compared with responses elicited in passive spindles and during activation of either bag2 fibers or chain fibers alone. Several records of discharge frequency were averaged. During stimulation at 30 Hz of gamma-axons coactivating bag2 and chain fibers, the averaged discharge of primary endings became continuous (that is, without interruption during each shortening phase as occurs in passive spindles) over the whole range of stretch frequencies. The modulation of the discharge was roughly sinusoidal, with an amplitude that increased with the stretch frequency. Stimulation at 30 Hz of gamma-axons activating bag2 fibers alone elicited a modulation of comparable shape and amplitude but only in the range of sinusoidal stretch from 0.6 to 3-4 Hz. Stimulation at 30 Hz of gamma-axons activating chain fibers alone elicited for each cycle in the range of 0.6 to 5-6 Hz a distorted modulation of large amplitude with a minimal frequency close to that of the stimulation. The average muscle length did not significantly influence these various responses. In summary, the coactivation of bag2 and chain fibers, at presumed physiological frequencies, enables primary endings to continuously signal changes of length over a large range of stretch velocities independently of the average muscle length.

Skeletofusimotor (beta) innervation of proximal and distal forelimb muscles of the cat

The innervation of muscle spindles by skeletofusimotor (beta) axons has been compared in two long digit extensor muscles with three elbow extensor muscles of the cat forelimb. The proportion of muscle spindle poles with p1-endplates was analysed in silver impregnated teased material. The proximal and distal muscles displayed a significant difference in their proportion of muscle spindle poles with p1-endplates. The distal muscles had an estimated proportion of more than 70% beta innervation of their muscle spindles, the proximal muscles of 41-47%. This difference is discussed with respect to the different circuitry of the motoneurones. We suggest that the high proportion of beta-innervated spindles in distal muscles obtained with the absence of a recurrent inhibitory system in the innervating motoneurones [18] serves the execution of manipulative movements.

Incidence of non-driving excitation of Ia afferents during ramp frequency stimulation of static gamma-axons in cat hindlimbs

1. The aim of this investigation was to identify static gamma-axons which do not drive any Ia afferents at any stimulus frequency in any spindle which they supply, and to determine their occurrence in various hindlimb muscles (peroneus tertius, brevis, longus and tenuissimus). 2. Ia responses to static gamma stimulation were classified as 'non-driven' when the discharge did not follow the stimulation frequency, or its subharmonics, at any time during a linear increase in stimulus frequency up to 150 Hz lasting 2-3 s, and when tested at two muscle lengths--except in the tenuissimus muscle. In almost all experiments, cross-correlograms were used in addition to evaluate the percentage of these 'non-driven' responses in which a time-locking of discharge to stimulus pulses was obscured by irregularity of the Ia discharge. 3. In 104 spindles, out of 347 responses to stimulation of single static gamma-axons 332 (93%) could be characterized, and of these, 57% (183) were of the non-driven type. The mean number of static gamma effects characterized per spindle was 4.1 (fourteen experiments). In the large majority of spindles (79%, 82 out of 104) at least one response was of the non-driven type. 4. Of the static gamma-axons studied 16% were called 'non driving' ('ndr' gamma s-axons) because they elicited non-driven effects, and since they had the same qualitative effect consistently in all spindles whose discharge was modulated by stimulating them they were called specific 'ndr' axons. If axons with non-driven effects, but acting on one spindle were included in the 'non-driving' category the proportion was 23%. Of spindles tested 63% were innervated by at least one 'ndr' axon. 5. Absence of Ia driving during ramp frequency stimulation of gamma s-axons has been equated with selective bag2 contraction. All the non-driven responses identified in this study cannot be attributed to exclusive bag2 involvement because the total number of 'ndr' responses was too high. In fact, in the isolated spindle preparation bag2 and chain co-contraction were shown to elicit non-driven responses, so chain contraction is not detected reliably in all experimental conditions. Possibly chain fibre contraction is sometimes too weak to dominate the response, or can be of a non-driving character.(ABSTRACT TRUNCATED AT 400 WORDS)

Sprouting of fusimotor neurones after partial denervation of the cat soleus muscle

Normally, gamma motoneurones innervate only the intrafusal fibres of muscle spindles. This is a report of sprouting of gamma motoneurones to innervate extrafusal muscle fibres following partial denervation of the soleus muscle of kittens. In eight newborn animals, the L7 ventral root was cut on one side under anaesthesia and the animals were then allowed to recover. At approximately 100 days of age animals were reanaesthetised and a study made of mechanical properties of motor units whose axons ran in the S1 ventral root and supplied the partially denervated soleus muscle. Evidence was obtained for sprouting of all surviving alpha motoneurones. In addition, in four experiments axons conducting within the gamma range, on stimulation, produced measurable tension. In one experiment, stimulation of one such gamma axon also produced specific fusimotor effects on four afferents identified as coming from primary endings of muscle spindles. The gamma axon was therefore a fusimotor axon. The effect observed on stimulation of the gamma axon suggested a largely dynamic action. Other examples of gamma axons were encountered that on stimulation produced tension, but which could not be specifically associated with spindles. In addition, a number of gamma axons that did not develop tension were shown, on stimulation, to have fusimotor effects that were static in action. It is concluded that in extensively denervated muscles gamma motoneurones may sometimes sprout to innervate extrafusal fibres. The mechanical properties of the extrafusal fibres innervated by such gamma axons were similar to those of ordinary alpha motor units.

The distribution of static gamma-axons in the tenuissimus muscle of the cat

1. The distribution of static gamma-axons within and among muscle spindles of the tenuissimus muscle has been studied in the anaesthetized cat, on the basis of the effects on the responses of primary endings when bag2 or chain fibres or both are activated by static gamma-stimulation. 2. Locations of spindles were marked for subsequent histological analysis using teased, silver-impregnated preparations. 3. Static effects were classified into: (i) biassing; (ii) driving; or (iii) indeterminate categories. 4. Critical correlations established that the biassing type was produced by bag2 activity, either alone or in combination with chain fibres, whereas the driving type was produced by chain fibres active alone. Indirect evidence suggested that indeterminate effects were produced by bag2 and chain fibres active together. 5. The static gamma-axons showed some differential distribution according to their conduction velocities: faster-conducting axons were likely to be more widely distributed among spindles but less likely to innervate chain fibres alone than were more slowly conducting axons. 6. The results are discussed in terms of their possible functional and developmental significance.

Extrafusal and intrafusal motor units in the kitten

In the lateral flexor digitorum longus muscle (FDL) of anaesthetized kittens aged 3-21 days, alpha (alpha) and gamma (gamma) motoneurone conduction velocities were measured together with motor unit tensions. Conduction velocity lay in the range 8-27 msec-1 for alpha motoneurones and 2-10 msec-1 for gamma motoneurones. Motor unit tetanic tensions were 3-47 mN with the largest units being 2-2.2% of whole muscle tension. The hypothesis was tested that motor axons conducting within the gamma range and presumed to be innervating the intrafusal fibres of muscle spindles, on stimulation, produced measurable tension. Stimulating gamma motoneurones in the adult produces no tension. The hypothesis was based on histological observations that while in the adult, intrafusal fibres were about one third of the size of extrafusal fibres, in kittens the two were nearly equal, both in length and diameter. It was shown by means of signal averaging during stimulation of ventral root filaments that whenever tension was recorded in the muscle an impulse could be detected in the muscle nerve conducting at alpha motoneurone tempo. It is concluded that in the kitten, despite the fact that intrafusal and extrafusal fibres are of similar size, stimulating single gamma motoneurones does not develop measurable tension.

Individual differences in multiple-bag spindles of cat superficial lumbrical muscles

A total of 791 spindle poles was analysed with regard to intrafusal fibre composition in the first and second superficial lumbrical muscles from the right and left hindfeet of 9 male and 5 female adult cats. Bag and chain muscle fibres were identified by their myofibrillar ATPase staining profile in the B region, after either acid or alkaline preincubation. A high proportion of the spindle pole population (43.2%) was observed to contain three or more (up to 5) bag fibres; those poles were classified as multiple-bag spindle poles. In the 334 muscle spindles in which both poles were studied, 42 bag fibres (12.6%) were found to be of the 'mixed' type, that is a fibre in which the two poles differ in their ATPase staining profile (either bag1/bag2 or bag/chain). The variability of the intrafusal fibre content observed in spindles of these muscles has been studied in relation to individual characteristics such as sex, weight and side of the animal. In general, multiple-bag spindles are more frequent in male than in female cats and in right as compared to left side muscles. Nearly all 'mixed' bag intrafusal fibres (38 out of 42) were observed in spindles containing 3 or more bag fibres. In 3-bag spindles the proportion of 'mixed' bag spindles is approximately the same in male and female cats. The ratio of 'dynamic' (mean polar bag1 content) to 'static' (mean polar bag2 plus chain fibre content) intrafusal effectors per muscle tends to increase in spindles of right side muscles and to decrease in the heaviest animals. The quantitative and qualitative differences in fibre content of spindles observed in first lumbrical muscles of different animals suggest that the spindle fibre composition, especially that of the 'dynamic' bag1 fibre, may be related to individual predetermined and/or acquired factors.

Factors that determine the form of neuromuscular junctions of intrafusal fibers in the cat

The form of terminations of fusimotor (gamma) and skeletofusimotor (beta) axons on intrafusal fibers was analyzed in serial sections of 20 spindles of the cat tenuissimus muscle. Seven synaptic features were assessed either qualitatively or quantitatively from electron micrographs of transverse sections of 184 intrafusal and 30 extrafusal endings. Features were compared among endings that were terminations of gamma or beta axons on different types of intrafusal fiber at different distances from the spindle equator. These comparisons indicated that interactions of several factors, and not the motor axon alone, determine the form of motor endings. Intrafusal muscle fiber type is dominant to the motor axon in regulation of the number and depth of postsynaptic folds. Separation of the influence of the motor axon from the muscle fiber was less clear with respect to the size of ending. Complete expression of the muscle fiber-motor axon interaction reflected by the form of motor endings is dependent upon location of the ending relative to the sensory region. Both depth of the primary synaptic cleft and size of the soleplate of motor endings increased with increasing distance of the ending from the spindle equator. A system of classification of cat intrafusal motor endings that reflects the multiplicity of factors that determine the form of endings, and one that simplifies the current terminology, is proposed.

The innervation of tandem muscle spindles in the cat neck

Patterns of innervation were examined in tandem muscle spindles teased from silver-stained muscles of the cat neck. Each tandem spindle was composed of two or more encapsulated receptors linked in series by a shared bag2 fiber. In most tandem spindles, two different types of encapsulation were identified according to differences in their intrafusal fiber content. One type, the b1b2c unit, contained typical bag1, bag2, and chain fibers and was structurally similar to single spindles described in other cat muscles. Each b1b2c unit contained a single primary sensory ending and 1-6 secondary endings. Fusimotor innervation was supplied by many axons. Some fusimotor axons ended in trail ramifications on bag2 and chain fibers, others ended in plates on the bag1 or long chain fiber. The other type of tandem encapsulation, the b2c unit, had only bag2 and chain fibers in its intrafusal fiber bundle. The b2c unit was usually supplied by only one sensory axon that ended on the nucleated part of the intrafusal fiber bundle. This single ending had a more variable terminal morphology than the primary ending in b1b2c units. A few b2c units (3/49) were also supplied by a secondary ending. The fusimotor innervation of the b2c unit was relatively simple. A single pole of the b2c unit was usually supplied by only one to three axons, all ending in trail ramifications. No plate endings were found in b2c units. These morphological specializations suggest that b1b2c and b2c units in tandem spindles differ in both their transductive and fusimotor mechanisms. Thus, the tandem spindle is a specialized structure that may provide additional proprioceptive information beyond that available from single muscle spindles.

Form and classification of motor endings in mammalian muscle spindles

The presynaptic features of 234 motor endings supplied to cat hindlimb muscle spindles have been studied in teased, silver preparations, and the postsynaptic features of a further 27 endings have been studied in serial, 1 micron thick, transverse sections. In the presynaptic study motor endings received by the three types of intrafusal muscle fibre were compared with the endings supplied to spindles by the various functional categories of motor axon. Three forms of motor ending were found that had significantly different presynaptic features. These forms correspond closely to those previously identified in the literature as p1 (beta), p2 (dynamic gamma) and trail (static gamma). The results of the postsynaptic study showed that the degree of indentation of the intrafusal muscle fibres by motor axon terminals increases with greater distance from the primary ending, irrespective of muscle-fibre type. We conclude that the postsynaptic form of intrafusal motor endings is determined by distance from primary ending and muscle-fibre type. It is not determined by type of motor axon, and cannot be correlated with presynaptic form so as to produce a unified classification of intrafusal motor endings.

Two types of cat static fusimotor neurones under separate central control?

The effect on the fusimotor system of an injection of the gamma-aminobutyric acid (GABA) antagonist picrotoxin into the reticular part of the substantia nigra (SNR) was studied in muscle spindle primary and secondary endings of pretibial flexors in cats anaesthetized with ketamine. Changes in fusimotor action after drug administration were deduced from changes in bias and sensitivity to sinusoidal stretching of the receptor-bearing muscles when compared to the deefferented preparation. Intranigral injection of picrotoxin (2 micrograms) removed a tonic static fusimotor action from flexor muscle spindle primary endings without affecting the secondary endings in the same muscles. It is concluded that static gamma motoneurones of cat flexors are of more than one functional type, and that the central nervous control of the static fusimotor input to cat flexor muscle spindles has selective access to static bag2 fibres and nuclear chain fibres.

Nonselective motor innervation of nuclear bag1 intrafusal muscle fibers in the cat

The motor nerve supply to cat nuclear bag1 intrafusal muscle fibers was reconstructed from light and electron microscopy of serial transverse sections of spindles in the tenuissimus muscle. Twenty-six of thirty poles of bag1 fibers that were examined received motor innervation. Every innervated bag1 pole received at least one (range 1-3) selective motor axon that supplied this fiber type only. Four of the innervated bag1 poles (15%) received additional motor supply from a nonselective motor axon that also innervated one nuclear chain fiber in the same spindle pole. The chain fibers co-innervated with bag1 fibers were among the longest chain fibers although they were shorter than two long chain fibers also present in the spindle poles. In cross-sections stained with toluidine blue they displayed 1-3 equatorial nuclei side by side, and there were fewer intermyofibrillar granules in their polar regions than in most of the other chain fibers. The endings of nonselective motor axons on the bag1 and chain fibers were morphologically and ultrastructurally dissimilar. It is suggested that instances of common innervation of the (dynamic) bag1 fiber and a (static?) chain fiber represent an integral and, presumably, functionally meaningful part of the motor pattern in some cat spindles.

Glycogen depletion elicited in tenuissimus intrafusal muscle fibres by stimulation of static gamma-axons in the cat

In this study the experimental conditions used to elicit glycogen depletion in tenuissimus intrafusal muscle fibres were different from those used by Barker, Emonet-Dénand, Harker, Jami & Laporte (1976): the tenuissimus was left in situ; several (4-6) static gamma-axons were stimulated together; the blood flow through the muscle was not reduced during the periods of gamma stimulation except in two experiments; very much longer periods (up to 9 h) of intermittent stimulation by bursts at 50-500/s were used. Bag1 and bag2 fibres were identified by their different ATPase activities in the B region. In two experiments with normal circulation, test responses of several primary endings to short periods of stimulation at 50-100/s were still very strong after stimulation of several static gamma-axons for 5 and 9 h, respectively. Glycogen depletion was observed in a large number of chain and bag2 poles but in only one of nineteen bag1 poles examined. In two other experiments with normal circulation, there was a very pronounced reduction of the test responses after stimulation of several static gamma-axons for 7 and 9 h, respectively. Out of twenty-four bag1 poles examined, nineteen exhibited zones of depletion. In an experiment in which stimulation was conducted as in Barker et al. (1976), i.e. with reduction of muscle blood flow during 1 min periods of stimulation at 50-100/s, the primary endings still gave a strong response after fifteen periods of stimulation in contrast with the marked 'fatigue' that was constantly observed in the former study. No depleted intrafusal fibres were found in the spindles of this muscle. In a last experiment, after an initial pattern of stimulation similar to that described above, the new pattern of stimulation, but with periodical reduction of blood flow, was applied, leading to a 'fatigue' of the test responses in 2 h. In the spindles of this muscle three out of ten bag1 poles were depleted. The variability of glycogen depletion in bag1 fibres appears to be linked to the degree of spindle 'fatigue' which may develop after static gamma stimulation. It seems that in 'fatigued' spindles some factor or factors liberated by the contraction of neighbouring fibres may deplete glycogen in bag1 fibres by a non-neural mechanism. When, in spite of a prolonged stimulation of static gamma-axons, no fatigue of the test responses develops, zones of depletion in bag1 fibres--possibly of neural origin--are very rare, although a large proportion of bag2 and chain fibres are depleted.

The ultrastructure of cat fusimotor endings and their relationship to foci of sarcomere convergence in intrafusal fibres

1. Six muscle spindle poles, five from experiments in which foci of sarcomere convergence had been observed during stimulation of fusimotor axons, were serially sectioned for light and electron microscopy. Every somatic motor terminal was studied in ultrathin sections at several levels.2. In all six poles static gamma axons, or presumed static gamma axons, supplying the static bag(2) fibre and/or chain fibres had no terminations on the dynamic bag(1) fibre. In five poles, the dynamic bag(1) fibre was selectively innervated by dynamic gamma or beta axons save in one case where a dynamic gamma axon also innervated one chain fibre.3. Seventy-seven motor endings were of four distinct ultrastructural types: ;m(a) plates' lay superficially on the surface of static bag(2) or chain fibres; ;m(b) plates' were deeply indented into dynamic bag(1) fibres; in ;m(c) plates', found on chain fibres only, the muscle surface was thrown into projecting fingers between which the axon terminals were embedded; one type ;m(d) plate' was found, fully indented into a long chain fibre. A few plates of intermediate form (m(ab)) were variants of m(a) and m(b) plates.4. The muscle membrane beneath both m(a) and m(b) plates was smooth, or had a few wide, shallow folds; m(c) plates usually had wide, shallow subjunctional folds; numerous deep, narrow folds were characteristic of the m(d) plate. The length of unmyelinated pre-terminal axon or the number of sole plate nuclei were not useful diagnostic features.5. Obvious foci of sarcomere convergence in the capsular sleeve region of dynamic bag(1) and static bag(2) fibres coincided with the location of motor plates. Additional contraction foci were observed in the extracapsular region of dynamic bag(1) fibres where there was no motor innervation; contraction occurs principally in the outer half of these fibres. No foci of contraction or motor plates were observed in the extracapsular region of static bag(2) fibres; contraction in these fibres is typically mid-polar.6. In some poles local contraction of chain fibres centred on the location of m(c) plates. In others, very localized contraction occurred distal to the sites of m(a) plates. Both m(a) and m(c) plates were never found on the same pole of a chain fibre.7. Dynamic gamma or beta axons end in m(b) plates, probably equivalent to p(2) plates. The concept of distinctly different p(1) and p(2) plates on dynamic bag(1) fibres, supplied by dynamic beta and gamma axons, respectively, is not supported by ultrastructural evidence.8. Some static gamma axons end in multiple m(a) plates which correspond with ;trail endings', or in single large m(a) plates, on static bag(2) or chain fibres. The m(c) plates are the terminations of other static gamma, or occasionally dynamic gamma, axons on chain fibres. Static beta axons probably end in m(d) plates on long chain fibres which may correspond with p(1) plates.9. It is proposed that there are two types of static gamma motoneurone, one terminating in m(a) plates and the other in m(c) plates, possibly directed preferentially towards static bag(2) fibres and chain fibres, respectively.

A study of motor nerve terminals on cat nuclear bag1 intrafusal muscle fibers using the ChE staining technique

Muscle spindles were traced in serial transverse sections of cat tenuissimus muscles. Histochemical staining for "myofibrillar" adenosine 5'-triphosphatase was employed to identify nuclear bag1 intrafusal muscle fibers. Staining for cholinesterases (ChE) was used to demonstrate the termination sites of motor axons along the fibers. Several types of ChE deposits could be distinguished along the bag1 fibers based on intensity of staining and morphological characteristics. Most ChE deposits could be classified as either the "pale" or the "nonpale" plates. Some ChE active areas fitted neither of these two categories. Among 328 ChE "plates" encountered on 192 bag, fiber poles, 197 (60%) were of the "pale" and 27 (8%) of the "nonpale" type with 104 (32%) remaining unclassified. These histochemical observations are discussed with regard to the current structural and functional concepts of motor innervation of the nuclear bag1 fiber. It is suggested that the histochemical (ChE staining intensity) and morphological (length and form) characteristics of bag1 fiber motor endings are not determined solely by the nature of the corresponding motor axons.

A histological study of the motor innervation of the cat's muscle spindle

The motor innervation of four muscle spindles from the tenuissimus muscle of the cat was demonstrated using reconstructions of 1 micrometer thick, serial transverse sections. Analysis of the results clearly indicates that the bag intrafusal muscle fibre usually does not receive a static fusimotor input via trail innervation. In contrast to the highly selective innervation of bag fibres, almost half the axons supplying bag or chain fibres branched to terminate on both types of fibre. The significance of these results is discussed in relation to previous studies on fusimotor innervation and to their functional implications. The presence of autonomic innervation is a further complication that appears to have led to erroneous conclusions concerning the nature of the trail innervation of chain fibres in a recent study of the distribution of cholinesterase activity in the spindle.

Histochemical profiles of cat intrafusal muscle fibers and their motor innervation

Muscle spindles were examined histochemically in serial transverse sections of cat tenuissimus muscles. The myofibrillar adenosine triphosphatase (ATPase) staining reaction was used to identify nuclear bag1, bag2 and nuclear chain intrafusal muscle fibers. Regional differences in ATPase staining occurred along the bag1 and bag2 fibers but not along the chain fibers. All intrafusal fiber types displayed regional variability in staining for nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR). Motor nerve terminals were demonstrated along the poles of bag1, bag2 and chain fibers by staining for cholinesterase (ChE). There was no consistent spatial correlation between the intensity of regional ATPase staining along the bag fibers and location, number or type of motor endings. However, most ChE deposits occurred in intrafusal fiber regions that displayed the greatest NADH-TR variability. Some fiber poles or whole intrafusal fibers were devoid of any ChE deposits but their ATPase and NADH-TR content was comparable to that of fibers bearing ChE deposits. The observations suggested that motor nerve fibers per se may not play a major role in determining the histoenzymatic content of intrafusal fibers.

Evolving views on the internal operation and functional role of the muscle spindle

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Glycogen depletion of bag1 fibers elicited by stimulation of static gamma axons in cat peroneus brevis muscle spindles

1. The distribution of static fusimotor axons to intrafusal muscle fibres in cat peroneus brevis spindles has been studied with the glycogen depletion method. 2. In each of six experiments three to seven static axons were stimulated. The muscle was subsequently quick-frozen and cut in serial transverse sections that were stained for glycogen. In each muscle, nearly all the spindles were examined for depletion. 3. Intrafusal muscle fibres displaying zones of complete glycogen depletion were observed in fifty-two whole spindles and seven half-spindles. Chain fibres were depleted in forty-eight (92%) of the whole spindles, bag2 fibres in thirty-five spindles (67%) and bag1 fibres in nineteen spindles (36%). 4. Seven other experiments were performed to test whether small amplitude sinusoidal stretching (30--100 Hz) of the muscle might produce glycogen depletion in bag1 fibres. Two hundred and seven spindles were examined. In nearly all of them the glycogen content of the intrafusal muscle fibres was normal. Two muscles had limited atrophic portions within which a few depleted spindles were found. 5. These experiments show that the intrafusal distribution of static gamma axons is not restricted to chain and bag2 fibres but that in a significant number of spindles the bag1 fibres are also supplied by static gamma axons.

Identifications of the intrafusal endings of skeletofusimotor axons in the cat

Direct identification of the endings of skeletofusimotor (beta) axons has been made in muscle spindles deprived for their gamma innervation by degeneration. Hindlimb muscles were prepared in which 1--5 fast-conducting motor axons were left intact while the rest of the motor supply was cut and allowed to degenerate for a period of 7 days. In 3 experiments a single beta axons survived supplying tenuissimus, and in 2 experiments beta axons were among 4 or 5 surviving axons that supplied superficial lumbrical and abductor digiti quinti medius muscles. Motor endings identified as p1 plates were found in teased, silver preparations of all experimental muscles, a total of 35 such plates being located in 15 spindles. The plates were all supplied to bag1 fibres. The experiments show that if a spindle innervated by a beta axon is deprived of its gamma supply by degeneration the motor endings that remain intact are p1 plates.

Motor innervation of the cat muscle spindle studied by the cholinesterase technique

Muscle spindles were traced in serial transverse sections of cat tenuissimus muscles. "Myofibrillar" adenosine triphosphatase staining reaction was used to identify nuclear bag1, nuclear bag2 and nuclear chain intrafusal muscle fibers. Typical chain fibers and long chain fibers were distinguished, the latter extending for more than 1,000 micron beyong the termination of the spindle capsule. Simple "rim" and more elaborate "plate" deposits were demonstrated histochemically along the poles of the typical chain fibers in staining for cholinesterases. They were considered to correspond, respectively, to the trail and plate motor nerve terminals. Most long chain fibers and the majority of nuclear bag fibers had their motor innervation limitd to "plate"-type endings. In addition, faint diffuse cholinesterase staining occurred along the spindle capsule and the surface of some intrafusal fibers. These histochemical observations are discussed with regard to the current concepts concerning the morphological and functional organization of the motor innervation of the cat muscle spindle.

A new grouping of intrafusal muscle fibers based on developmental studies of muscle spindles in the cat

The development of muscle spindles was studied using the tenuissimus muscle of the cat. Observations show that the intrafusal muscle fibers develop as two separate groups: one group represented by a single nuclear bag fiber while the second group comprises the second nuclear bag fiber in association with all the nuclear chain fibers. This grouping is most pronounced in the fetus and is clearly seen in neonatal kittens (i.e., up to 2 weeks of age). As the intrafusal fibers begin to separate from each other, the groupings become less noticeable, although this basic pattern is often retained in the adult. The pattern of intrafusal fiber grouping is most noticeable in the equatorial regions of the spindle and least noticeable in the polar regions. This is not the grouping of fibers which would have been expected from a consideration of existing reports on muscle spindles. The implications for spindle form and function are considered.

Identification of intrafusal muscle fibres activated by single fusimotor axons and injected with fluorescent dye in cat tenuissimus spindles

1. Intrafusal muscle fibres of cat tenuissimus spindles have been injected with the fluorescent dye Procion Yellow and identified histologically after recording their changes in membrane potential during 1/sec stimulation of single static or dynamic gamma axons. 2. Thirteen intrafusal muscle fibres innervated by static gamma axons were identified as eight bag2 and five chain fibres. The fact that none proved to be a bag1 fibre is not regarded as significant, for reasons given in the Discussion. 3. In one spindle Procion Yellow was injected into two intrafusal muscle fibres activated by the same static gamma axon; they were identified as a bag2 and a chain fibre. 4. Nine intrafusal muscle fibres innervated by dynamic gamma axons were identified as seven bag1 fibres, one bag2 fibre, and one long chain fibre. 5. In one spindle two bag fibres were injected, one activated by a dynamic gamma axon, the other by a static gamma axon; the former proved to be a bag1 fibre, the latter a bag2 fibre. 6. Stimulation of static gamma axons elicited junctional potentials in seven bag2 fibres and one damaged chain fibre, and action potentials in one bag2 and four chain fibres. In the whole sample of impaled intrafusal muscle fibres (identified and unidentified) activated by static axons, junctional potentials were recorded from twenty-three (62.2%), and action potentials from fourteen (37.8%). Stimulation of dynamic gamma axons always elicited junctional potentials. 7. In a number of instances it was possible to examine the ultrastructure of motor endings belonging to the stimulated gamma axon. The myoneural junctions of trail endings supplied by static gamma axons to bag2 and chain fibres were both smooth and folded; the deepest and most regular folding occurred on chain fibres. The terminals of p2 plates supplied to bag1 fibres by dynamic gamma axons had smooth myoneural junctions.

On the subdivision of static and dynamic fusimotor actions on the primary ending of the cat muscle spindle

1. Using large ramp and triangular stretches a survey has been made of the effect of stimulating single gamma fusimotor fibres on primary endings of muscle spindles in the peroneus brevis to see whether 'intermediate' types of fusimotor action could be recognized, falling between the well known static and dynamic types. 2. Responses were classified into six groups, as detailed on pp. 844-846, ranging from apparently 'pure' dynamic action (category I) to apparently 'pure' static action (category IV). Models for a putative mixed action were produced by combining the stimulation of a static and of a dynamic fibre to the same spindle. The clearest sign of static action was firing on the releasing phase of the stretch. The essential sign of dynamic action, which survived combination with the more dominant static action, was the low adaptive decay of firing with a time constant of about 0-5 sec that occurs on the plateau of the ramp stretch. 3. Out of 153 responses, each elicited from a primary ending on stimulation of a single fusimotor fibre, 67% were apparently 'pure' examples of dynamic and static action. The remaining 33% of responses were to some degree suggestive of an admixture, in various proportions, of static and dynamic actions. For only 18% of them was there firm indication of such admixture. 4. When a given fibre was tested on more than one ending then, with one exception out of thirty-six instances, its action always proved to be either predominantly static or predominantly dynamic. There was no special tendency for an axon with a mixed action on one spindle to have a similarly mixed action on other endings so that individual fusimotor fibres were best classified as static or dynamic without intermediate grades. 5. Simultaneous stimulation of two fusimotor fibres eliciting apparently 'pure static and dynamic actions, could mimic all the intermediate types of action. 6. The results are discussed in relation to recent studies, especially those based on glycogen depletion. It was concluded that dynamic action arises from activation of the bag1 intrafusal muscle fibre, and that static action arises from the bag2 and chain fibres, whether acting individually or collaboratively. The intermediate actions are suggested to arise from an overlap of motor innervation to contrasting types of intrafusal muscle fibre. 7. On the basis of effects on the regularity of the afferent discharge the findings support the view that a given static action axon can innervate bag2 and chain fibres in various proportions in different spindles, so that they do not provide separable effector pathways. 8. Responses to large amplitude sinusoidal stretching were also studied in relation to our classification.

The action of piperidine on muscle spindles in the rat

Piperidine hydrochloride induced discharge in primary and secondary endings of muscle spindles in rat tailbase muscle preparations. The threshold concentration for primary endings (2.0 X 10(-5) g/ml) was below that for secondary endings (4.0 X 10(-5) g/ml). The effect of piperidine was antagonised by tubocurarine but not by atropine. The action of piperidine on primary and secondary endings responding to 'ramp-and-hold' stretch was predominantly on the static component of excitation, and usually in the absence of any concomitant enhancement of the dynamic effect. This effect was opposite to that induced by succinylcholine. A possible action of piperidine at cholinoceptive sites of the nicotinic type and associated with gamma-trial fusimotor terminals is discussed.

Control of dynamic and static nuclear bag fibres and nuclear chain fibres by gamma and beta axons in isolated cat muscle spindels

1. The behaviour of nuclear bag and nuclear chain intrafusal fibres in isolated cat muscle spindles with a blood supply, during stimulation of dynamic gamma axons, dynamic beta axons, or static gamma axons in ventral root filaments was observed and recorded on still and moving film. 2. Most spindles were controlled by one dynamic gamma axon (sometimes a beta axon) and three static gamma axons, one of which was often non-selective in distribution. A large majority of fusimotor axons controlled one pole of the spindle only. 3. Dynamic gamma and beta axons produced focal contraction in only one of the two nuclear bag fibres in any spindle and this fibre was never activated by static gamma axons. Maximal tetanic contraction was attained slowly and the primary sensory spiral on this fibre was stretched by a small amount only. This fibre has been named the 'dynamic nuclear bag fibre'. 4. Static gamma axons produced either: (a) focal contraction in the second of the two nuclear bag fibres only; (b) local contraction in the bundle of nuclear chain fibres only; or (c) contraction in one nuclear bag fibre and the nuclear chain fibres together. Maximum tetanic contraction of this nuclear bag fibre stretched its primary sensory spiral considerably and the time to plateau was relatively short. This fibre has been named the 'static nuclear bag fibre'. 5. 'Driving' of the Ia afferent discharge could always be produced by non-selective static gamma axons, frequently by static gamma axons controlling nuclear chain fibres alone, and was probably due to mechanical oscillation in nuclear chain fibres. It was never produced by dynamic gamma axons and on one occasion only by a static gamma axon controlling a nuclear bag fibre alone. 6. The conduction velocities of dynamic gamma and static gamma axons overlapped extensively, though dynamic gamma axons were absent from the lower end, and static gamma axons innervating nuclear chain fibres only were absent from the upper end, of the range of velocities. 7. The observations are correlated with spindle structure and histochemistry. Dynamic and static nuclear bag fibres are shown to correspond with 'bag1 fibres' and 'bag2 fibres', respectively (Ovalle & Smith, 1972). 8. The possible origin of the dynamic and static actions of fusimotor axons and the role of the dynamic and static intrafusal systems in motor control are discussed.

Distribution of fusimotor axons to intrafusal muscle fibres in cat tenuissimus spindles as determined by the glycogen-depletion method

1. The distribution of fusimotor axons to bag1, bag2 and chain muscle fibres in cat tenuissimus spindles has been studied using a modification of the glycogen-depletion technique of Edstrrom & Kugelberg (1968). Single fusimotor axons were stimulated intermittently at 40-100/sec for long periods (30-90 sec) during blood occlusion. Portions of muscle containing the activated spindles were quick-frozen, fixed in absolute ethanol during freeze-substitution, and then embedded in paraffin wax. Serial transverse sections were stained for glycogen using the periodic acid-Schiff method, and examined for depletion. 2. Dynamic gamma axons (i.e. those that increase the dynamic index of primary-ending responses to ramp stretches of large amplitude) depleted bag1 fibres almost exclusively. 3. Static gamma axons (i.e. those that reduce or abolish the dynamic index) depleted both bag and chain fibres. Bag1 and bag2 fibres were depleted about equally. 4. A single static gamma axon may activate both bag and chain fibres in one spindle (the most common pattern), chain fibres only in another, and bag fibres only in a third spindle. 5. Static gamma axons with conduction velocities less than 25 m/sec also had a non-selective distribution, but no depletion was observed in bag2 fibres. 6. The zones of depletion produced by dynamic gamma axons were distributed more or less equally in the intra- and extracapsular parts of spindle poles, whereas those produced by static gamma axons were mainly intracapsular. 7. The results are compared with the glycogen-depletion studies of Brown & Butler (1973, 1975) and our own study of the distribution of static gamma axons to spindles in which all other motor axons had degenerated (Barker, Emonet-Dénand, Laporte, Proske & Stacey, 1973). The implications of the finding that both static gamma and dynamic gamma axons activate bag1 fibres are discussed.

Cinematographic analysis of contractile events produced in intrafusal muscle fibres by stimulation of static and dynamic fusimotor axons

1. Muscle spindles with an intact blood supply and uninterrupted connexions with ventral and dorsal spinal roots (Bessou & Pagés, 1967, 1972) have been prepared in cat's tenuissimus muscles with the aim of cinephotographically recording intrafusal movements induced by the stimulation of single static or dynamic gamma axons; the time cours of these movements and the morphological kind of activated intrafusal muscle fibres have been established. 2. Displacements of spindle guiding marks in the equatorial region elicited by stimulating single static gamma axons are 4-20 times greater in amplitude than the ones elicited by stimulating dynamic gamma axons at the same frequency. 3. The dynamic gamma axons induced a contraction only in nuclear bag fibres which, in addition, never received any static gamma innervation. The static gamma axons evoked contractions either in nuclear bag fibres alone, or in nuclear chain fibres alone, or in both types of intrafusal fibres. Two thirds of static gamma axons supplied nuclear bag fibres. For various reasons, one half only of static gamma axons innervating nuclear bag fibres could be shown to simultaneously innervate nuclear chain fibres. Consequently, about one third of static gamma axons supplied both nuclear bag fibres and nuclear chain fibres, but it is highly probable that this latter figure is an underestimate. One third of static gamma axons produced contraction in nuclear chain fibres only. In this work, the distribution of fusimotor axons has been established in only one muscle spindle of the cluster of muscle spindles that each fusimotor axon is generally innervating. 4. Generally speaking, a static gamma axon elicits contraction of several intrafusal fibres whereas a dynamic gamma axon innervates only one intrafusal fibre and frequently only one pole of the fibre. 5. One third of static gamma axons evoked contractions in nuclear chain fibres that seemed to involve the whole pole. The other static gamma axons and all dynamic gamma axons produced, in the intrafusal fibres that they supplied, one or several foci of localized contractions. 6. The nuclear chain fibres contract and relax faster than nuclear bag fibres. The contractions of nuclear bag fibres supplied by static gamma axons are stronger and faster than those of nuclear bag fibres innervated by dynamic gamma axons. Nearly all nuclear bag fibres innervated by static gamma axons, like the nuclear chain fibres, show transient contractions at each pulse of a stimulation at low frequency (2-20/sec). 7. The results are discussed taking into account the available anatomical and physiological data on the muscle spindle. Their consequences with regard to intrafusal working are briefly considered.

Skeleto-fusimotor axons in the hind-limb muscles of the cat

1. Motor axons supplying various hind-limb muscles of the cat (flexor hallucis lingus, peroneus brevis, peroneus digiti quinti, tibialis anterior, soleus and tenuissimus) were identified as skeleto-fusimotor or beta axons because their repetitive stimulation elicited both the contraction of extrafusal muscle fibres and an increase in the rate of discharge of spindle primary endings which perisited after selective blockade of extrafusal neuromuscular junctions. 2. The conduction velocity of these axons ranged from 39 to 92 m/sec. 3. Of seventy-six beta axons, seventy-two had a dynamic action on the sensitivity to velocity of stretching of primary endings, four had a static action. 4. The dynamic action of six beta axons was observed only after the contraction of extrafusal muscle fibres was selectively suppressed. 5. Tendon organs can be activated by beta motor units.