Types Of Intrafusal Muscle Fibre Research Articles

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.

Fibre typing of intrafusal fibres.

The first descriptions of muscle spindles with intrafusal fibres containing striated myofibrils and nervous elements were given approximately 150 years ago. It took, however, another 100 years to establish the presence of two types of intrafusal muscle fibres: nuclear bag and nuclear chain fibres. The present paper highlights primarily the contribution of Robert Banks in fibre typing of intrafusal fibres: the confirmation of the principle of two types of nuclear bag fibres in mammalian spindles and the variation in occurrence of a dense M-band along the fibres. Furthermore, this paper summarizes how studies from the Umeå University group (Laboratory of Muscle Biology in the Department of Integrative Medical Biology) on fibre typing and the structure and composition of M-bands have contributed to the current understanding of muscle spindle complexity in adult humans as well as to muscle spindle development and effects of ageing. The variable molecular composition of the intrafusal sarcomeres with respect to myosin heavy chains and M-band proteins gives new perspectives on the role of the intrafusal myofibrils as stretch-activated sensors influencing tension/stiffness and signalling to nuclei.

Fine structural study of the innervation of muscle spindles in the internal oblique muscle of the abdominal wall in the adult mouse

We examined by electron microscopy the innervation of muscle spindles in the internal oblique muscle of the mouse abdominal wall. In the equatorial region, in addition to the sensory innervation on individual intrafusal muscle fibers, sensory cross terminals were often observed between nuclear chain fibers. In the area from the juxtaequatorial region to the polar region, nuclear bag fibers were supplied by trail and plate-type motor endings, while nuclear chain fibers were innervated by sensory endings, being probably secondary sensory endings. From these findings, it is clear that the innervation patterns differ between two types of intrafusal muscle fibers.

Chapter 10 Why are there Three Types of Intrafusal Muscle Fibers?

Publisher Summary This chapter describes a working scheme for the use of the three intrafusal fiber types. The bag 1 fibers provide control of the stretch sensitivity of primary afferents. Rapid changes in dynamic fusimotor output would be pointless because of the relative slowness of response of the bag 1 fibers and their inability to effectively oppose unloading. The bias levels of primary and secondary afferents could be set by bag 2 activity, but the slowness of bag 2 fiber contraction means that quick changes could not be made in this way. Gamma axons supplying bag 2 fibers alone can be expected, therefore, like dynamic axons, to fire tonically rather than phasically. Finally, the γ motoneurons that would be expected to show rapid modulation during movements should be those jointly supplying bag and chain fibers. The proposed function for the dynamic system is already well supported by spindle recordings in natural movements, but the crucial test for a dual function within the static system is to look for two distinct patterns of natural activity among γ s axons. The prediction is that one group, directed mainly to bag fibers, will fire tonically while the other, directed to chain or to bag 2 and chain fibers, will be modulated roughly in parallel with the active muscle contractions.

The transverse tubular system of cat intrafusal muscle fibres

A modified staining technique for transverse tubular and sarcoplasmic reticular systems was used to investigate their occurrence in different types of intrafusal muscle fibres in cat tail spindles. Intrafusal muscle fibres can be divided into three basic regions, namely, the periaxial space (A-region), intracapsular area (B-region) and the extracapsular area (C-region); the components of these systems were seen to vary in structure and distribution. The occurrence of these systems also varied among the different types of intrafusal muscle fibres, namely, the bag1, bag2 and chain fibres. In bag1 fibres components were sparse in the A-region, increased slightly in the B-region, but were most developed in the C-region; triads were consistently located at the border between A- and I-bands. In bag2 fibres membrane components were noted in the A-region but were more abundant in the B-region where some tubular components showed transverse and longitudinal branches linked together in the form of a network; membrane systems diminished towards the C-region. The majority of triads were located within the A-bands. In chain fibres the membrane systems occurred more commonly in the A-region, while in the B- and C-regions, the transverse tubular system possessed numerous transverse and longitudinal branches forming irregularly distributed tubular networks. Some tubular branches were dilated, while other branches terminated as sacs among arrays of the sarcoplasmic reticular system in I-bands. Some transverse tubules bifurcated into two branches with numbers of dilated sarcoplasmic reticular cisternae lying on either side, or between, the branches. Triads sometimes occurred between A- and I-bands, but, generally, were situated well within A-bands.

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.

Histochemical evidence for the existence of skeletofusimotor (beta) innervation in the primate.

A total of ten alpha motor axons which innervated the peroneus brevis muscle were isolated in two cynomolgus monkeys. In each experiment, the isolated alpha axons were stimulated collectively to deplete glycogen from their muscle units. The muscle was then frozen quickly , cut serially, and stained for glycogen. Of the 52 muscle spindles that were examined, zones of glycogen depletion were found in the intrafusal fibres of 32 spindles. The glycogen-depleted motor units included both fast-twitch and slow-twitch types. Depleted zones were observed in all three types of intrafusal muscle fibres. It was concluded that skeletofusimotor (beta) efferents were among the stimulated motor axons. This finding constitutes the first anatomical evidence for the existence of beta innervation in the primate.

The isolated mammalian muscle spindle

As a sensory receptor the muscle spindle parallels the eye in its physiological and anatomical complexity. For example, it contains three types of intrafusal muscle fibre and is innervated by sensory and motor fibres which can also be further differentiated and classified. But how does the spindle work? In this article L A. Boyd provides some clear answers to what is technically a very complicated question.

Changes in ATPase and SDH reactions of the rat extrafusal and intrafusal muscle fibres after preincubations at different pH

The dependence of adenosine-triphosphatase (ATPase) and succinic dehydrogenase (SDH) histochemical reactions on the pH of the preincubation medium was studied in serial cross sections of 1- to 6-month-old rat extensor digitorum longus (EDL) and soleus (SOL) muscles. The use of a wide spectrum of pH values confirmed the previous results showing that: (1) according to their ATPase and SDH reactions 3 types of extrafusal muscle fibres, i.e., fast-twitch glycolytic (FG), fast-twitch oxidative-glycolytic (FOG) and slow-twitch oxidative (SO) and 3 types of intrafusal muscle fibres, i.e. typical and intermediate nuclear bag fibres and nuclear chain fibres were observed; (2) only acid preincubation (pH 4.35) is necessary to demonstrate the reversal of the ATPase reaction; while (3) alkali preincubation (pH 10.4) does not provide any new important information as compared with ATPase without preincubation. Furthermore, it was shown that: (4) fast-twitch muscle fibres exhibited high ATPase activity on preincubations at pH 4.9 to 10.4, slow-twitch fibres had very high ATPase activity on preincubation at pH 4.3 and 4.5; (5) after preincubation at pH 4.5 two types of FOG fibres were observed, differing in their ATPase activity; (6) in both muscles there were fibres with intermediate ATPase activity both after acid and/or alkali preincubations; (7) the intrafusal muscle fibres exhibited some specific characteristics when compared with extrafusal fibres. In contrast to the ATPase reactions, SDH activity was decreased equally, in both extra- and intrafusal fibres, with increasing acidity and alkality of the preincubation medium.

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.

Histochemical study of normal human muscle spindle. Histochemical classification of intrafusal muscle fibers and intrafusal nerve endings.

Normal muscle spindles of human skeletal muscle were studied histochemically. 1) Four histochemical types of intrafusal muscle fibers were classified by ATPase stain: Bag I fiber, Bag II fiber, Chain I fiber and Chain II fiber. Moreover, two types of nuclear bag fibers were classified by NADH Tetrazolium Reductase stain and PAS stain: Bag I fiber and Bag II fiber. 2) Three kinds of fusimotor endings were verified by the cholinesterase technic: en plaque, en grappe and diffuse endings. 3) Two kinds of fusisensory endings were verified by NADH TR stain and also electron-microscopically: primary and secondary sensory endings.

Quantitative studies on the distribution of myofilaments in intrafusal muscle fibres.

Muscle spindles contain two types of intrafusal muscle fibre, nuclear bag fibres and nuclear chain fibres. The intrafusal fibres of rabbit and guinea pig spindles have been studied using quantitative stereological techniques at the ultrastructural level. The crosssectional areas occupied by myofilaments have been measured in the polar and equatorial regions of both types of intrafusal fibre. There are considerably fewer myofilaments in the equatorial regions of both types of fibre compared with their polar regions.

Histochemical identification of three types of intrafusal muscle fibers in the cat and monkey based on the myosin ATPase reaction.

Serial frozen section of muscle spindles in the lumbrical muscles of the cat and the monkey were tested with a recently described modification of the myosin ATPase reaction (Guth and Samaha 1970. Exp. Neurol 28, 365). This reaction is able to detect acid-stable, alkaline-labile myosin ATPase and acid-labile, alkaline-stable myosin ATPase. The reactions were carried out by preincubation of the sections at pH 10.4 or 4.35 with incubation in a substrate at pH 9.4. In both the cat and the monkey three types of intrafusal muscle fibers could be detected on the basis of their reaction for myosin ATPase. Nuclear bag fibers could be divided into two types: those containing the acid-stable form of myosin ATPase only, and those containing ATPase detectable under both acid and alkaline conditions. Nuclear chain fibers contained the alkaline-stable form of ATPase only.

Fine structure of rat intrafusal muscle fibers. The polar region.

An ultrastructural comparison of the two types of intrafusal muscle fibers in muscle spindles of the rat was undertaken. Discrete myofibrils with abundant interfibrillar sarcoplasm and organelles characterize the nuclear chain muscle fiber, while a continuous myofibril-like bundle with sparse interfibrillar sarcoplasm distinguishes the nuclear bag muscle fiber. Nuclear chain fibers possess well-defined and typical M bands in the center of each sarcomere, while nuclear bag fibers contain ill-defined M bands composed of two parallel thin densities in the center of the pseudo-H zone of each sarcomere. Mitochondria of nuclear chain fibers are larger and more numerous than they are in nuclear bag fibers. Mitochondria of chain fibers, in addition, often contain conspicuous dense granules, and they are frequently intimately related to elements of the sarcoplasmic reticulum (SR). Striking differences are noted in the organization and degree of development of the sarcotubular system. Nuclear bag fibers contain a poorly developed SR and T system with only occasional junctional couplings (dyads and triads). Nuclear chain fibers, in contrast, possess an unusually well-developed SR and T system and a variety of multiple junctional couplings (dyads, triads, quatrads, pentads, septads). Greatly dilated SR cisternae are common features of nuclear chain fibers, often forming intimate associations with T tubules, mitochondria, and the sarcolemma. Such dilatations of the SR were not encountered in nuclear bag fibers. The functional significance of these structural findings is discussed.

Fusimotor innervation in the cat

The motor innervation of cat spindles was examined in hindlimb muscles using a variety of techniques employed in light and electron microscopy. Observations were made on teased, silver preparations of 267 spindles sampled from the peroneal, flexor hallucis longus, and soleus muscles, hereafter referred to as the PER/FHL/SOL series. The γ innervation. Trail endings are almost invariably present, and innervate both bag and chain muscle fibres. Trail fibres accounted for 64.6 to 74.8% of the total fusimotor supply to samples of spindle poles in the PER/FHL/SOL series, the mean number of fibres per pole varying from 2.7 to 5.0 in the different muscles, and the mean number of ramifications (areas of synaptic contact) per fibre being 3.7. By contrast, the p₂innervation of a spindle pole generally consists of a single fibre supplying only one plate. In the above samples p(2) fibres accounted for 4.1 to 28.0% of the total fusimotor supply, and the mean number of fibres per pole varied from 0.3 to 1.2 in the different muscles. Ninety per cent of p(2) plates innervate bag fibres. The α innervation. The structure of p₁plates as seen in both light and electron microscopy compares very closely with that of extrafusal plates. After nerve section p₁plates degenerate at the same time as extrafusal plates, being the first of the three types of fusimotor ending to disappear. The frequency of the p₁innervation is similar to that of the p₂innervation. In the same samples of PER/FHL/SOL spindle poles as above p₁ fibres accounted for 6.0 to 28.8% of the total fusimotor supply, the mean number of fibres per pole varying from 0.25 to 2.1 in the different muscles. The majority of p₁ fibres enter a pole to terminate in one plate only. Seventy-five per cent of the plates innervate bag fibres. The three types of fusimotor ending are thus not selectively distributed to the two types of intrafusal muscle fibre. All three types of fusimotor fibre may branch within the spindle so as to innervate both bag and chain fibres. Bag fibres receive both types of plate ending as well as trail endings. Most chain fibres receive trail endings only; the rest receive either a p₁or a p₂plate innervation in addition, 25% of the p₁and 10% of the p₂innervation being distributed to chain fibres. The significance of this nonselective innervation is interpreted as indicating that the type of contraction elicited by stimulating a fusimotor fibre depends upon the type of ending initiating it rather than upon the type of muscle fibre executing it. Reasons are given for concluding that the dynamic response is controlled via the p₁and p₂plates, and that the static response is controlled by the trail endings. The participation of the α fibres in mammalian fusimotor innervation, previously regarded as a vestigial feature, proved to be widespread in the muscles studied and more prevalent in fast muscles (FHL, peroneus digiti quinti) than slow (soleus). A low frequency of p₁innervation is offset by a high frequency of p₂(as in peroneus longus), and vice versa (as in FHL). It is unlikely that collaterals from slow α fibres innervating type B muscle fibres are wholly responsible for the high frequency of the p₁innervation in FHL, and it is suggested that collaterals may also be derived from fast α fibres innervating type C muscle fibres. The possibility of there being some motor fibres of α conduction velocity and with an exclusively fusimotor distribution is also taken into account.

A second order mechanical model of muscle spindle primary endings.

With reference to experimental data and the failure of earlier proposed first order linear models of mammalian muscle spindles, a second order mechanical model of de-efferented primary endings is studied. The model takes into account the presence of two different types of intrafusal muscle fibres in a complete spindle organ. It further allows the incorporation of different gain of the mechano-electric conversion into a depolarization of the sensory terminals innervating the two types of fibres. It is shown that a closer approximation to the behaviour of the biological prototype is obtained if the transducer gain of the branch corresponding to the nuclear bag intrafusal fibres is chosen significantly higher than that corresponding to the nuclear chain branch. The marked nonlinear behaviour of muscle spindle primary endings as recently reported by Matthews and Stein (1968, 1969) is interpreted as a saturation effect of the high gain mechano-electric transducer of the nuclear bag branch. The saturation is considered to reflect a condition of complete depolarization of these sensory terminals. If a higher transducer gain actually is present, a complete depolarization of these terminals would occur at a lower degree of mechanical deformation than for the nuclear chain terminals. The mechano-electric transducer system of the nuclear chain fibres might thus behave approximately linearly within a larger range of input amplitudes. The greatly reduced gain of the primary endings at large emplitudes of imposed muscle vibrations as observed experimentally (Matthews and Stein, 1968, 1969) may thus be accounted for by the transducer gain of the nuclear chain fibres alone.

The differentiation of two types of intrafusal fibres in rabbit muscle spindles. An electron microscopic study.

1. The ultrastructure of muscle spindles in deep lumbrical muscles of adult rabbit has been investigated. Contrary to findings obtained by previous authors at the light microscope, indicating morphologic uniformity of intrafusal fibre type in rabbit spindles, two populations of intrafusal muscle fibres have been detected. 2. The first type of fibres is made by myofibrils which have a thick Z line and lack a distinct M line. These fibres are also characterized by the apparent fusion of fibrillar fields due to the paucity of the sarcoplasm, which contains very little glycogen. The mitochondria are scattered and of small size. The sarcoplasmic reticulum is poorly developed and the triads are quite rare. These fibres resemble the nuclear bag fibres of cat muscle spindles. 3. The second type of fibres is made by myofibrils which have a thin Z line and a prominent M line. There is a great separation of the fibrillar fields as a result of abundant sarcoplasm which contains large accumulation of glycogen and ribosomes. The mitochondria are densely distributed and of large size. The sarcoplasmic reticulum is well developed and the triads are rather numerous. These fibres resemble the nuclear chain fibres of cat muscle spindles. 4. The demonstration of two types of intrafusal muscle fibres in rabbit spindles may account for the duality of physiological responses obtained from these receptor organs in this animal species.

Muscle-spindle histochemistry.

Reduced nicotinamide adenine dinucleotide tatrazolium reductase is abundant in cat intrafusal muscle fibers, whereas in the toad its activity is equal to that in extrafusal fibers. Spindles of both species contain little fat. In sections stained for adenosine triphosphatase bound to myofibrils, two types of intrafusal muscle fibers appear in spindles of both the cat and toad.

CONTRACTION IN INTRAFUSAL MUSCLE FIBRES OF XENOPUS LAEVIS FOLLOWING STIMULATION OF THEIR MOTOR NERVES.

AbstractSmith, K. S. Contraction in intramural muscle fibres of Venous leaves following stimulation of their motor nerves. Acta physiol. scand. 1964. 62. 195–208. — The contractile behaviour of amphibian intramural muscle fibres following stimulation of their motor nerves was investigated by cinephotography of the changing sarcomere length. Two types of intrafusal muscle fibre were distinguished: large and small. Small intrafusal muscle fibres were supplied by small diameter motor nerves and had a longe: lasting contraction than the large intrafusal fibres which were supplied by large diameter motor nerves. Contraction was a propagated event in both large and small muscle fibres following single nerve stimuli. At high stimulation frequencies the contraction was not propagated through the complete length of the large muscle fibres. Evidence was obtained that the large muscle fibres contained, in the region of sensory contacts, zones which were less stiff than the resting striated muscle fibre and that the small muscle fibres did not contain such zones. Some observations were also made on spontaneous contraction in these muscle fibres.

The structure and innervation of the nuclear bag muscle fibre system and the nuclear chain muscle fibre system in mammalian muscle spindles

1. The structure and innervation of muscle spindles from normal, de-afferented and de-efferented muscles of the cat hind limb were studied. The spindles were either completely isolated by microdissection, or were serially sectioned transversely. 2. All spindles contain two distinct types of intrafusal muscle fibre, ‘nuclear bag fibres’ and ‘nuclear chain fibres’, which differ in structure and innervation. 3. Nuclear bag muscle fibres, usually two per spindle, are less than half the diameter of extrafusal fibres, and each contains numerous large nuclei packed together in the equatorial region of the spindle. Nuclear bag fibres practically never branch. The fibres contain numerous myofibrils uniformly distributed in cross-sections, and relatively little sarcoplasm; they atrophy very slowly after the ventral spinal roots are cut. Several small motor nerve fibres (y, fibres) enter each spindle and terminate in a number of discrete motor end-plates on the nuclear bag muscle fibres. These y x end-plates lie in a group at each spindle pole and long lengths of nuclear bag fibre are free of motor innervation. 4. Nuclear chain muscle fibres, usually four per spindle, are about half the length and diameter of nuclear bag fibres in spindles in the leg muscles. The nuclear chain fibres in spindles from the small muscles of the foot may, however, equal the nuclear bag fibres in length, and in diameter beyond the ends of the lymph space. Each nuclear chain fibre contains a single row of central nuclei in the equatorial region; the fibres occasionally branch, but often none of them do so. They contain fewer myofibrils per unit area, irregular in size and distribution, and relatively more sarcoplasm, than nuclear bag fibres. Nuclear chain fibres atrophy nearly as rapidly as extrafusal fibres after the ventral roots are cut. A number of very fine motor nerve fibres fibres) enter each spindle and terminate in a network of fine axons and small nerve endings (the network’) situated on the nuclear chain muscle fibres in most regions other than the nuclear region. 5. All spindles receive both y 1 xand y 2 innervation, fibres forming slightly more than half of the total number of motor fibres which varies from seven in simple spindles in phasic muscles to twenty-five in the most complex spindles in tonic muscles. Both y 1 and y 2 fibres remain intact after dorsal root transection and degenerate following ventral root transection. The histological evidence supports the view that the yj and y2 nerve fibres at the spindles are derived from two types of stem fibre, neither of which belongs to the a group. 6. Each spindle has one primary sensory nerve ending, supplied by one group 1 a afferent nerve fibre, and from zero to five secondary sensory nerve endings, each supplied by one group II afferent nerve fibre. The primary sensory terminations lie on both nuclear bag and nuclear chain muscle fibres. The secondary sensory terminations lie predominantly on the nuclear chain muscle fibres. In spindles with several secondary sensory endings, their terminations may lie on the same region of nuclear chain fibres as motor endings of the y 2 network. 7. In general, spindles in tonic muscles have more secondary sensory endings and motor nerve fibres and endings than those in other muscles. Nuclear chain intrafusal fibres are probably functionally ‘slower’ than nuclear bag intrafusal fibres, while both types are ‘slower’ than extrafusal fibres. Both nuclear chain fibres and nuclear bag fibres, however, probably show a gradation in activity related to the nature of the muscle in which they lie. The reader is advised to study figure 33 and its legend first, at the same time studying the plate figures to which reference is made in figure 33 b , then to read the portions of the Results in italics consecutively followed by the Discussion, finally studying the detailed Results. Further details of many of the illustrations and tables are available for reference in the Archives of the Royal Society.