Histochemical profiles of cat intrafusal muscle fibers and their motor innervation

Expression of Myosin heavy chain isoforms in rat soleus muscle spindles after 19 days of hypergravity

The aim of this study was to determine whether a period of 19 days in hypergravity was long enough to induce changes in the expression of myosin heavy chain (MyHC) isoforms in the muscle spindles. The soleus muscle of 10 male Wistar rats (control: CONT, n=5; hypergravity: HG, n=5) was frozen, cut into serial sections, and labeled with antibodies against MyHCs: I, IIA, IIA + IIX + IIB, slow-tonic, and alpha-cardiac. Forty CONT and 45 HG spindles were analyzed. The results from HG spindles compared to CONT showed that there was no change in the cross-sectional area of intrafusal fibers. However, along the entire length of B1 fibers, the expression of both MyHC I and alpha-cardiac was increased significantly, whereas the labeling against MyHC IIA and MyHC slow-tonic was decreased. In B2 fibers, the labeling against MyHC IIA (region A), slow-tonic (region A), and fast myosins (regions A-C) was statistically decreased. In chain fibers, the labeling against both MyHC IIA and fast MyHC was reduced significantly. We conclude that hypergravity has a real impact on the MyHC content in the muscle spindles and induces some inverse changes of those observed in hypogravity for MyHCs I, alpha-cardiac, and slow-tonic.

[Histochemical study of the sensory endings of muscle spindles in rat longissimus muscles]

Most studies concerning the structure and function of muscle spindles have utilized the hind limbs of experimental animals. However, little is known about muscle spindles of the back muscles. The purpose of this study was to investigate the sensory innervation of muscle spindles of the paravertebral muscle in the rat. The subjects were 10 normal male rats. The longissimus muscles were isolated and frozen in cooled isopentane (-160 degrees C), and serial transverse sections were made with a cryostat. Histochemical preparations were then made using nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR) stain and modified Gomori-trichrome stain. The muscle spindles in each segment were identified microscopically by observing the equatorial and polar regions. NADH-TR staining was employed to distinguish nuclear bag1, nuclear bag2, and nuclear chain intrafusal muscle fibers. A total of 20 spindle poles were surveyed. The mean polar length of intrafusal fibers as well as that of each region (A, B, and C) were measured. NADH-TR staining also demonstrated the terminal sites of sensory fibers along intrafusal fibers. All spindle poles surveyed were innervated by secondary sensory fibers in addition to primary sensory fibers. Eight spindle poles were intermediate type muscle spindles that were innervated by one primary sensory fiber and one secondary sensory fiber. Twelve spindle poles were complex type muscle spindles that were innervated by one primary sensory fiber and multiple secondary sensory fibers. The mean length of the A region was 223.1+/-37.9 microm (n=8) for intermediate type spindles and 493.8+/-157.0 microm (n=12) for complex type spindles. The length of the A region was significantly longer in the complex type spindles than in the intermediate type spindles (p<0.001). The results suggest that the innervations of secondary sensory fibers were well developed in the longissimus muscle spindles in the rat. The morphological features of muscle spindles of the longissimus muscle may represent the structural basis for qualitatively different afferent discharges that relate to the characteristic types of locomotion served by paravertebral muscles.

Expression of myosin heavy chain isoforms along intrafusal fibers of rat soleus muscle spindles after 14 days of hindlimb unloading

Morphological, contractile, histochemical, and electrophoretical characteristics of slow postural muscles are altered after hindlimb unloading (HU). However, very few data on intrafusal fibers (IFs) are available. Our aim was to determine the effects of 14 days of hindlimb unloading on the morphological and immunohistochemical characteristics of IF in rat soleus muscle. Thirty-three control and 32 unloaded spindles were analyzed. The number and distribution of muscle spindles did not appear to be affected after unloading. There was no significant difference in number, cross-sectional area, and histochemical properties of IF between the two groups. However, after unloading, a significant decrease in slow type 1 MHC isoform and a slight increase in slow-tonic MHC expression were observed in the B and C regions of the bag1 fibers. The alpha-cardiac MHC expression was significantly decreased along the entire length of the bag2 fibers and in the B and C regions of the bag1 fibers. In 12 muscle spindles, the chain fibers expressed the slow type 1 and alpha-cardiac MHC isoforms over a short distance of the A region, although these isoforms are not normally expressed. The most striking finding of the study was the relative resistance of muscle spindles to perturbation induced by HU.

Expression of myosin heavy chain isoforms and myogenesis of intrafusal fibres in rat muscle spindles

This review concerns the pattern of expression and regulation of myosin heavy chain (MHC) isoforms in intrafusal fibres of rat muscle spindles detected by immunocytochemistry. The three types of intrafusal fibres--nuclear bag1, nuclear bag2, and nuclear chain fibres--are unique in co-expressing several MHCs including special isoforms such as slow tonic and alpha cardiac-like MHC and isoforms typical of muscle development, such as embryonic and neonatal MHC. The distinct intrafusal fibre types appear sequentially during rat hind limb development, the nuclear bag2 precursors being first identifiable at 17-18 days in utero as the only primary myotubes expressing slow tonic MHC. Sensory innervation is required for the expression of "spindle-specific" MHC isoforms. Motor innervation contributes to the diversity in distribution of the different MHCs along the length of the nuclear bag fibres. It is suggested that unique populations of myoblasts are destined to become intrafusal fibres during development in the rat hind limb muscles and that the regional heterogeneity in MHC expression is related both to sensory and motor innervation and to the properties of the myoblast lineages. These distinct features make intrafusal fibres an attractive in situ model for investigating myogenesis, myofibrillogenesis, and the mechanisms regulating MHC expression.

The avian muscle spindle

The literature on the morphology and physiology of the avian muscle spindle is reviewed, with emphasis placed on the period from 1960 to 1991. Traits similar to or different from mammalian spindles are recognized. Apart from receptors with low intrafusal fiber counts, bird spindles contain two or three types of intrafusal fiber. Unlike that of mammals, the equatorial fiber structure in birds does not lend itself to classification into nuclear bag and nuclear chain types. Avian intrafusal fibers are separable into types based on differences in myosin heavy chain composition and motor innervation, but apportionment of these fiber types to individual spindles is more variable in birds than in mammals. There is morphological evidence in birds for the existence of both gamma and beta innervation; however, confirmation of these systems by physiological experiments is at best sketchy. A general lack of physiological data is currently the greatest drawback to a better understanding of how the avian receptor works, and what role it plays in sensorimotor integration.

Morphometry and histoenzymology of the hamster tenuissimus and its muscle spindles

Muscle spindles and extrafusal fibers in the tenuissimus muscle of mature golden Syrian hamsters were studied morphologically and quantitatively using several light microscopic techniques. Muscle spindles were identified in serial-transverse frozen-sections of whole muscles stained with hematoxylin and eosin. Five tenuissimus muscles were examined from origin to insertion, and the locations of individual receptors were plotted in camera-lucida reconstructions. Spindles were found in proximity to the main neurovascular bundle in the central core of each muscle. A range of 16-20 receptors was noted per muscle. The mean muscle spindle index (the total number of spindles per gram of muscle weight) was 503 and the average spindle length was 7.5 mm. Oxidative enzyme and myosin adenosine-triphosphatase (ATPase) staining profiles were also evaluated in the intrafusal and extrafusal fibers in each muscle. Even numbers of type I and type IIA extrafusal fibers were distributed homogeneously throughout all muscle cross-sections. Histochemical staining patterns varied along the lengths of the three intrafusal fiber types. Nuclear chain fibers possessed staining properties similar to the type IIA extrafusal fibers and exhibited no regional variations. Bag1 fibers displayed staining variability, particularly when treated for myosin ATPase under acid preincubation conditions. Some spindles were isolated under darkfield illumination and then either treated with 7-nitrobenz-2-oxa-1,3-diazole (NBD)-phallacidin to detect filamentous actin by fluorescence microscopy, or prepared for conventional scanning electron microscopy (SEM). By fluorescence microscopy, a registered actin banding-pattern was observed in the sarcomeres of the intrafusal fibers, and variations in the intensity of banding were noted amongst different fibers. SEM revealed punctate sensory nerve endings that adhered intimately to the surfaces of underlying intrafusal fibers in the equatorial and juxtaequatorial regions. By transmission electron microscopy (TEM) these endings appeared crescent-shaped and were enveloped by external laminae. Each profile contained numerous mitochondria and cytoskeletal organelles. The high spindle density observed in this muscle suggests that the hamster tenuissimus may function in hindlimb proprioception.

Presence in chicken tibialis anterior and extensor digitorum longus muscle spindles of reactive and unreactive intrafusal fibers after incubation with monoclonal antibodies against myosin heavy chains

Cross and longitudinal sections from the encapsulated portions of chicken tibialis anterior and extensor digitorum longus muscle spindles were examined to determine whether their intrafusal fibers were a structurally homogeneous or heterogeneous population. The techniques used were the histochemical actomyosin (mATPase) reaction, and fluorescence immunohistochemistry employing two monoclonal antibodies, CA-83 and CCM-52, that are specific for myosin heavy chains. After incubation with antibody CCM-52, intrafusal fibers fluoresced either strongly or weakly to moderately. Antibody CA-83 was even more selective. In addition to identifying the strongly reactive category, it clearly separated the remaining fibers into unreactive and moderately reactive groups. As a whole, after incubation for mATPase, pH 9.6 preincubation, unreactive fibers stained darker than strongly reactive fibers. Moreover, the cross-sectional area of the unreactive fibers was significantly larger than that of the strongly reactive fibers. In the average-size muscle spindle with six intrafusal fibers, there were four unreactive fibers and two strongly reactive fibers. In about one-third of the receptors examined, one moderately reactive fiber was present. Taken together, the data indicate that intrafusal fibers of chicken tibialis anterior and extensor digitorum longus muscles are not structurally homogeneous. The observed variations can be better explained in terms of different fiber types than of continuous gradients within one type of fiber.

Histochemical profile of muscle spindles of rat's sural muscles

The histochemical profile of intrafusal fibres of muscle spindles and muscle spindle capsule were analysed in normal sural muscles of a rat. Weak activity of oxidative enzymes (NADH diaphorase, succinic dehydrogenase, lactate dehydrogenase) and creatine phosphokinase were found in bag1 and bag2 fibres, but the oxidative enzyme activity was moderate in chain fibres. High phosphorylase activity was demonstrated in bag fibres as well as in chain fibres. No differences could be detected in adenosine desaminase activity between these various intrafusal fibres. In the outer capsule of muscle spindles, high amount of type III collagen and elastin, but only small amount of type I collagen and fibronectin could be observed.

Reactivity of rat and rabbit intrafusal fibers with monoclonal antibodies directed against myosin heavy chains

Serial cross and longitudinal sections from the intracapsular portions of intrafusal fibers of rat and rabbit tibialis anterior muscles were examined by fluorescence microscopy with a library of monoclonal antibodies directed against different epitopes on myosin heavy chains. Intrafusal fiber types were identified with the histochemical reactions for acid-stable and alkali-stable actomyosin ATPase. Three antibodies, known to react with avian heart and slow-tonic myosins, produced fluorescent staining in intrafusal fibers. Nuclear bag2 fibers reacted with all three antibodies, chain fibers with two, and nuclear bag1 fibers with only one. These results indicate that in rat and rabbit tibialis anterior muscle spindles nuclear bag2 fibers and chain fibers contain more than one myosin isoform. They also demonstrate that, in addition to the histochemical actomysin ATPase reaction, nuclear chain fibers and the two types of nuclear bag fibers can be identified by the selective reactivities of their myosin heavy chains.

Contours and distribution of sites that react with antiacetylcholinesterase in chicken intrafusal fibers

Serial cross and longitudinal sections of intrafusal fibers from the intracapsular portions of chicken tibialis anterior muscle spindles were incubated with a monoclonal antibody specific for chicken acetylcholinesterase (AchE) and examined by immunofluorescence for the presence of the enzyme on presynaptic and postsynaptic membranes of neuromuscular junctions. The midequatorial sensory region which lacks organized sarcomeres was negative, but immediately distal to it faintly staining regions of AchE localization were observed on intrafusal fibers. In cross sections at the juxtaequator, the outlines of areas that were positive for AchE were either thin and crescentlike or thick and compact. The distribution of both types of localization continued into the polar region. Toward the more distal polar region, the intensity of sites on the postsynaptic membrane that reacted with the anti-AchE progressively increased. In longitudinal sections, AchE localization was largely limited to two configurations. One was elongate, while the other was more round or oval and often also smaller. Both types might occur on the same, or on different, intrafusal fibers. Examination of silver-impregnated sections revealed the presence of platelike and of traillike axon terminals. The variety of shapes observed on presynaptic and postsynaptic membranes warrants further study to determine whether chicken muscle spindles are innervated by more than one type of motor neuron.

Nonuniform expression of myosin heavy chain isoforms along the length of cat intrafusal muscle fibers

The expression of four myosin heavy chain (MHC) isoforms, avian slow-tonic (ATO) or neonatal-twitch (ANT) and mammalian slow-twitch (MST) or fast-twitch (MFT) in intrafusal fibers was examined by immunocytochemistry of spindles in the tenuissimus muscle of adult cats. The predominant MHCs expressed by nuclear bag fibers were ATO and MST, whereas the MHCs prevalent in nuclear chain fibers were ANT and MFT. The expression of these isoforms of MHC was not uniform along the length of intrafusal fibers. In general, both bag and chain fibers expressed avian MHC in the intracapsular region and mammalian MHC in the extracapsular region. The nonuniform expression of MHCs observed along the length of bag and chain fibers implies that different genes are activated in myonuclei located in the intracapsular and extracapsular regions of the same muscle fiber. Regional differences in gene activation might result from a greater effect of afferents on myonuclei located near the equator of intrafusal fibers then on myonuclei outside the spindle capsule.

Noncoordinate expression of M band proteins in slow and fast embryonic chick muscles

The expression of the myosin-associated M band proteins myomesin and M protein in differentiating muscle fibers in the anterior latissimus dorsi (ALD) and posterior latissimus dorsi (PLD) muscles during embryonic chicken development was examined by immunocytochemistry using monoclonal antibodies. Early in the embryonic development of both muscles, both myomesin and M protein are expressed in primary and secondary myotubes. However, beginning at 10 days in ovo, M protein is gradually suppressed first in primary, then in secondary, presumptive slow-tonic type 3 fibers. M protein is transiently suppressed in presumptive fast-twitch type 2 fibers derived from primary myotubes but continuously expressed in those derived from secondary myotubes. Thus, initially all myotubes have a common intrinsic M band composition with respect to myomesin and M protein, whereas at later stages the expression of M protein is fiber-type specific. Intrafusal spindle fibers, which are segregated from extrafusal fibers around 14 days in ovo, have a heterogeneous M band composition atypical of extrafusal fibers.

Immunohistochemical demonstration of embryonic myosin heavy chains in adult mammalian intrafusal fibers

Serial cross sections of rat, rabbit and cat intrafusal fibers from muscle spindles of normal adult hindlimb muscles were incubated with a monoclonal antibody against embryonic myosin heavy chains. Intrafusal fiber types were identified by noting their staining patterns in adjacent sections incubated for myofibrillar ATPase after acid or alkaline preincubation. In rat and rabbit muscle spindles dynamic nuclear bag1 fibers reacted strongly at the polar and juxtaequatorial regions. Static nuclear bag2 fibers reacted weakly or not at all at the polar region, but showed a moderate amount of activity at the juxtaequator. At the equatorial region both types of nuclear bag fibers displayed a rim of fluorescence surrounding the nuclear bags, while the areas occupied by the nuclear bags themselves were negative. Nuclear chain fibers in rat and rabbit muscle spindles were unreactive with the specific antibody over their entire length. In cat muscle spindles both types of nuclear bag fibers presented profiles which resembled those of the nuclear bag fibers in the other two species, but unlike in rat and rabbit spindles, cat nuclear chain fibers reacted as strongly as dynamic nuclear bag1 fibers.

The effect of tenotomy and immobilization on muscle spindles and tendon organs of the rat calf muscles. A histochemical and morphometrical study

The morphological and histochemical alterations in the muscle spindles and Golgi tendon organs of tenotomized or immobilized calf muscles of rats (m. soleus, m. gastrocnemius) were studied in 54 animals. The intact contralateral feet served as controls. There was no change in the number of mechanoreceptors in either the tenotomized or the immobilized muscles. The diameter of the intrafusal fibers was increased somewhat after 1 week, but decreased by 14% to 40% in 3 weeks. The periaxial space of muscle spindle had diminished or disappeared by 2 weeks from the beginning of the procedures. The thickness of the outer capsule of the spindles had markedly increased, especially in the polar region. In the Golgi tendon organs the capsule had also thickened significantly and the internal space diminished. The alterations were similar following tenotomy or immobilization, but more marked after tenotomy than after immobilization.

Sharing of sensory terminals between the dynamic bag1 and static bag2 fibers in the rat muscle spindle

The nuclear bag1 intrafusal fiber mediates the dynamic (velocity) sensitivity, whereas the nuclear bag2 and nuclear chain fibers mediate the static (length) sensitivity of muscle spindles to stretch. The pattern of branching of primary and secondary afferents, the distribution of their terminals to the 3 types of intrafusal fibers, and the incidence of sensory cross-terminals were determined by reconstruction of 4 spindles from serial 1-micron and ultrathin transverse sections of rat extensor digitorum longus muscles. A single primary afferent supplied each spindle, and secondary afferents innervated intrafusal fibers in 3 spindles. Only static intrafusal fibers shared cross-terminals of the secondary afferents. In contrast, the dynamic bag1 and static bag2 fibers of each spindle shared at least one terminal of the primary afferent. Cross-terminals shared by the dynamic bag1 and static bag2 fiber parallel the presence of fusimotor (gamma) axons which coinnervate these types of intrafusal fibers in muscle spindles of rats. Consequently, the greater degree of overlap of elements comprising the dynamic and static systems of spindles of the rat relative to that of the cat reduces the probability of generating a purely dynamic or purely static response to an applied stretch.

Postnatal maturation of spindles in deafferented rat soleus muscles

Whether the motor innervation can direct the morphological and histochemical differentiation of developing muscle spindles in the absence of sensory innervation was investigated by deafferentation of the soleus muscle in immature rats. Dorsal root ganglia containing the cell bodies of afferents from the soleus muscle were removed surgically at a stage of postnatal development when spindles already contain the full complement of intrafusal fibers innervated by both afferents and efferents, but when the fibers are histochemically and structurally immature. Experimental soleus muscles were excised one year after deafferentation and sectioned frozen at a thickness of 8 micron. Sections were stained for enzymes indicative of types of muscle fibers and sites of neuromuscular junctions, and were examined by light microscopy. Spindles of muscles that matured in the absence of sensory innervation were abnormal. They lacked the periaxial fluid space and contained fewer intrafusal fibers than did normal spindles. The morphological and histochemical profiles of the encapsulated fibers present in the deafferented spindles more closely resembled extrafusal rather than intrafusal muscle fibers. These observations suggest that deafferentation of the immature spindles induces disintegration of some intrafusal fibers and alters maturation of others. Moreover, motor axons terminated less frequently along muscle fibers in deafferented spindles than on intrafusal fibers of normal spindles. Thus, maintenance of a full complement of intrafusal fibers in the developing spindle, emergence of histochemical profiles typical of normal intrafusal fibers, and development of adult pattern of fusimotor innervation require intact sensory innervation.

A comparative study of muscle spindles in slow and fast neonatal muscles of normal and dystrophic mice

Muscle spindles from the slow-twitch soleus and the fast-twitch extensor digitorum longus (EDL) muscles of genetically dystrophic mice of the dy2J/dy2J strain were compared with age-matched normal animals at neonatal ages of 1-3 weeks according to histochemical, quantitative, and ultrastructural parameters. Intrafusal fibers in both the soleus and EDL exhibited similar regional differences in myosin ATPase activity, and conformed to those noted previously in various adult species. In distal polar regions, all nuclear bag fibers resembled extrafusal fibers of the type 1 variety, whereas in capsular zones they could be divided into two subtypes. Nuclear chain fibers possessed a staining pattern similar to type 2 extrafusal fibers, and in contrast to the bag fibers they exhibited no regional variations. These features were consistently observed in both the normal and dystrophic muscles at all ages. Spindles varied only slightly in their number and distribution in the two types of muscle, and their location followed the neurovascular branching pattern in each. Irrespective of age or genotype, spindles in the soleus were more homogeneously dispersed, but those in the EDL were concentrated along the dorsal aspect of the muscle. No significant differences were noted in the total number of spindles between normal and dystrophic muscles. In addition, no dramatic differences were observed in the muscle spindle index for soleus and EDL. The first obvious disease-related changes were noted in extrafusal fibers of the soleus of 3-week-old mice, and spindles were often located close to these areas of fiber degeneration. Despite alterations in the surrounding tissue, however, spindles appeared morphologically unaltered in dystrophy. These observations indicate that intrafusal fibers of spindles in neonatal mice appear enzymatically and histologically unaffected in incipient stages of progressive muscular dystrophy.

Morphology and histochemistry of the ambiens muscle of the red-eared turtle (Pseudemys scripta)

Six fiber types have been described in the ambiens muscle of red-eared turtles. These include one slow oxidative type, two fast oxidative types, two fast oxidative and glycolytic types, and one fast glycolytic type. Fiber types are non-randomly distributed throughout cross sections of the muscle. There is a decreasing gradient of oxidative staining and an increasing gradient of glycolytic staining along an axis from the superficial to deep regions of the muscle. The slow oxidative fibers are predominantly located within one or two fascicles of the superficial surface of the muscle. The fast glycolytic fibers are predominant in deep fascicles. In contrast to previous reports of histochemically monotypic intrafusal fibers in turtle muscle, ambiens muscle spindles have been observed containing one to eleven intrafusal fibers, including two fiber types. Fiber diameter and area are consistently smaller than observed in most extrafusal fibers. Spindles are predominantly located in superficial and cranial fascicles of the ambiens muscle and are located in regions characterized by extrafusal fibers with high oxidative activity.

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.

Motor innervation of intrafusal fibers in rat muscle spindles: incomplete separation of dynamic and static systems

Distributions of 53 motor axons to different types of intrafusal fibers were reconstructed from serial 1-micron-thick transverse sections of 13 poles of spindles in the rat soleus muscle. The mean number of motor axons that innervated a spindle pole was 4.1. Approximately 60% of motor axons lost their myelination prior to or shortly after entry into the periaxial fluid space of spindles. Motor innervation to the juxtaequatorial portion of nuclear bag fibers (particularly the bag1) consisted of groups of short, synaptic contacts that were terminations of thin, unmyelinated axons. In contrast, motor endings on both the bag1 and bag2 fibers were platelike in the polar intracapsular region. Chain fibers had a single midpolar platelike ending. The ratio of motor axons that innervated the bag1 fiber exclusively to axons that innervated bag2 and/or chain fibers was 1:1. However, one-fourth of motor axons coinnervated the dynamic bag1 fiber in conjunction with static bag2 and/or chain fibers. Thus the complete separation of motor control of the dynamic bag1 and static bag2 intrafusal systems observed in cat tenuissimus spindles is neither representative of the pattern of motor innervation in all other species of mammals nor essential to normal spindle function.

Ultrastructure of extrafusal and intrafusal terminals of a (dynamic) skeletofusimotor axon in cat tenuissimus muscle

Terminals of a histologically identified beta skeletofusimotor axon in cat tenuissimus muscle were examined in semi-thin (1 micrometer thick) and ultrathin transverse sections. The beta axon supplied one extrafusal endplate and two motor endings on the nuclear bag1 intrafusal muscle fiber. The staining properties of the co-innervated bag1 and extrafusal fibers were dissimilar. The beta intrafusal and extrafusal terminals were also dissimilar by both light and electron microscopy , differing in length, mitochondrial content of the sole-plate, and folding of the post-synaptic membrane.

Histological identification of (static) skeletofusimotor innervation to a cat muscle spindle

Two skeletofusimotor (beta) axons were identified histologically in cat tenuissimus muscle. The axons supplied the distal poles of 3 intrafusal muscle fibers of the nuclear chain type and two extrafusal fibers of the fast twitch type. The observations strengthen the case for the existence of static beta-innervation to cat muscle spindles.

Multiple-bag-fiber muscle spindles in tenuissimus muscles of the cat

Over 300 complete and incomplete cat muscle spindles were examined in serial transverse sections of tenuissimus muscles in search of spindles with more than two nuclear bag intrafusal muscle fibers. Several histochemical and histological stains were used to identify the intrafusal fibers and assess their motor and sensory innervation. About 13% of the spindles contained either three or four bag fibers rather than the usual two. Every multiple-bag-fiber spindle possessed at least one nuclear bag1 and one nuclear bag2 fiber. The supernumerary bag fibers were either another bag1 and/or bag2 fiber, or a mixed bag fiber. The extra bag fibers had the usual morphologic and histochemical properties of cat nuclear bag fibers. All multiple-bag spindles received primary sensory innervation, and most had secondary sensory endings in addition. Their motor pattern was similar in the number, appearance and disposition of intrafusal motor endings to that of the usual two-bag-fiber spindles. Bag fibers of the same kind shared motor nerve supply in three multiple-bag spindles in which tracings of individual motor axons were obtained histologically. It is unclear whether any functional advantage is conveyed to a muscle spindle by its having more than one bag1 and one bag2 fiber.

Histological study of an unusual cat muscle spindle deficient in motor innervation

An unusual muscle spindle innervated by only one somatic motor axon and one primary sensory axon was encountered in a cat tenuissimus muscle cut in serial transverse sections and examined by light and electron microscopes. The motor axon branched to supply the distal poles of the nuclear bag2 and nuclear chain intrafusal muscle fibers through motor terminals of several types. The proximal poles of the bag2 and chain fibers and both poles of the nuclear bag1 fiber were devoid of motor endings. In spite of the limited motor nerve supply of the bag 1, bag2 and chain fibers exhibited the usual morphologic characteristics in terms of the appearance of the equatorial region, relative fiber lengths and diameters, and the number of associated elastic fibers. It appears that motor axons may play only a limited role, if any, in the differentiation and maintenance of the three types of intrafusal muscle fiber in the cat.

The topography of long nuclear chain intrafusal fibers in the cat muscle spindle

Muscle spindles were studied histochemically in serial transverse sections of specimens of the cat tenuissimus muscle. The nuclear chain intrafusal muscle fibers were separated into three subtypes, called long, intermediate and typical. The long chain and intermediate chain fibers tended to assume a particular position within the axial bundle of intrafusal fibers. The fibers were usually located in that layer of chain fibers that was positioned farthest away from the bag2 fiber. Furthermore, they were usually situated adjacent to the bag1 fiber throughout much of the extent of the spindle pole. Some long chain and intermediate chain fibers had several fiber nuclei abreast at the equator rather than a single row of central nuclei, as in most nuclear chain fibers. The relative position of intrafusal fibers within the cat spindle may reflect their order of formation during development, with the fibers retaining, to a variable degree, their association with the bag2 fiber which acted as template. Thus, the axial position of long chain and intermediate chain fibers suggests that they are among the first nuclear chain fibers to form. This may play a role in the known preferential innervation of these chain fibers by skeleto-fusimotor axons.

One-bag-fiber muscle spindles in tenuissimus muscles of the cat

Over 150 complete and 139 incomplete single muscle spindles were examined in serial transverse sections of cat tenuissimus muscles in search for spindles lacking one of the two types of nuclear bag intrafusal fiber. Several histochemical reactions were used to type the intrafusal muscle fibers and assess the spindle motor and sensory innervation. One complete spindle lacked a bag1 fiber, and another spindle lacked a bag2 fiber. Several incomplete spindles also lacked bag1 fibers. In addition, ten double tandem spindles contained one capsular unit each that lacked the bag1 fiber, and one triple tandem spindle had two such capsules. All one-bag-fiber spindles had primary sensory innervation, but none had secondary sensory innervation. Their motor innervation was similar to that of the usual two-bag-fiber spindles in the number and disposition of intrafusal motor endings. It is unclear whether the one-bag fiber spindles, either single or tandem-linked, are products of an aberrant spindle development or represent a true anatomical and functional subcategory of the cat muscle spindle.

Histochemical study of an unusual cat intrafusal muscle fiber

A cat tenuissimus muscle spindle that contained two long chain intrafusal fibers in its distal pole is described. One of the fibers (lc1) had a histochemical profile (ATPase, NADH-TR, ChE reactions) of the kind which is characteristic for long chain fibers. The other fiber (lc2) consisted of two separate segments. The inner lc2 segment included the sensory equatorial region and was histochemically normal. The outer lc2 segment carried a motor plate, and did not stain for NADH-TR in the same way as the inner lc2 segment and the lc1 fiber. It is suggested that the unusual enzyme staining properties of the outer lc2 segment stemmed from its lack of sensory innervation, a situation which may have permitted the full expression of influences mediated by its motor nerve supply.