Formation of the circulatory bed of the neuromuscular spindles in forearm and hand muscles]

The data are presented on the formation and main constitutional principles in the blood supply of the neuromuscular spindles in the human forearm and hand during embryogenesis and early postnatal life. It has been stated that the neuromuscular spindles posses their own microcirculatory bed which is formed by the vessels of the surrounding muscular tissue, tends to separate in the course of development and subdivides into two parts: extracapsular and intracapsular. The vessels of the extracapsular part form dense capillary nets on the external surface of the capsule and follow extracapsular parts of the intrafusal muscular fibres. The intracapsular vessels either cover the internal surface of the capsule, or adjoin the intrafusal muscular fibres, or else run in the free subcapsular space.     

Morphometric characteristics of neuromuscular spindles in hypertrophied skeletal muscle

Skeletal muscle hypertrophy in young male rats was found to be accompanied by adaptive changes in neuromuscular spindles. The changes consisted in connective capsule thickening, increased diameter of NMS and intrafusal muscle fibers, expanded afferent and efferent nerve terminals, increased microcirculatory bed capacity. The quantitative and qualitative shifts observed in NMS structure are morphologically equivalent to the rise in their functional potential, which forms the basis for the functional changes in conditions of increasing skeletal muscle hypertrophy.     

Changes in the respiratory muscles and their microcirculatory bed during exposure of the body to chronic hypoxia and its aftereffects

Changes of the respiratory muscles have been studied under the effect of a rarefied atmosphere when the pressure is 560 mm Hg (2,500 m), 405 mm Hg (5,000 m), 286 mm Hg (7,500 m) and during the period of their aftereffect. The experimental group consists of 260 and the control group--of 130 white rats. Adaptation of the respiratory muscles to the effect of the rarefied atmosphere at the pressure of 560 mm Hg takes place mainly at the expense of certain functional changes of the microcirculatory bed and hypertrophy of the muscle fibers. The period of aftereffect is characterized with normalization of these phenomena. Adaptation of the respiratory muscles at the pressure of 405 mm Hg takes place at the expense of hypertrophy of the muscle fibers, small destructive changes and a complex rearrangement of the microcirulatory bed. During the period of aftereffect, by the 40th day no destructive changes are noted in the muscle tissue. The capillary bed undergoes some rearrangements by the 42d -56th day with increasing quantitative indices per area unit. This results in improvement of the muscles nutrition. In the respiratory muscles at the pressure 286 mm Hg, atrophic changes in the muscle fibers take place at certain stages of the experiment. Essential destructive changes are observed, rearrangement of the microcirculatory bed with decreasing convolution of the longitudinal capillaries and a decreasing number of the transversal capillaries are noted by the end of the experiment. During the period of aftereffect, by the 56th day, the external and internal intercostal muscles completely restore their structure, and in the diaphragm the destructive changes remain. By the same time, the microcirculatory bed becomes more rare at the expense of decreasing capillarization of the muscle fibers and a decreasing number of the transversal capillaries.     

Ultrastructure of Muscle Spindles (Mechanoreceptors) of Rat m. soleus after Support Unloading and Its Combination with Hypergravity

We used electron microscopy to evaluate the effect of support unloading of m. soleus in adult Wistar rats (restrained in antiorthostatic position for 23–24 h/day within 24 days) on the ultrastructure of the intrafusal fibers and motor neuromuscular junctions of the muscle spindles, as well as the efficiency of intermittent hypergravity (+2G_Z; 1 h/day for 19 days in a centrifuge in hypokinetic cages) as a countermeasure used in conditions of support unloading of this muscle. In the absence of support on the hind limbs, most of intrafusal fibers of m. soleus preserved the typical ultrastructure, while the axon terminals of the neuromuscular junctions accumulated a lot of synaptic vesicles (including large vesicles); the coated vesicles were absent due to unloading of the muscle and its muscle spindles (no contractions of the intrafusal fibers). A short-term effects of hypergravity at the background of support unloading of m. soleus mostly induced static loading of the muscle inducing different responses of the intrafusal fibers in different regions of the muscle spindles: local lysis of myofilaments was observed in single intrafusal fibers of the equatorial and intracapsular motor regions, while myofibrils remained intact in most fibers in the intra- and extracapsular regions of the spindles. The revealed adaptive response of the intrafusal fibers is, on the one hand, due to their specific innervation and ultrastructure and, on the other hand, to positive effect of hypergravity on the motor and extracapsular regions of the muscle spindles. Hypergravity decreased the number of synaptic vesicles and induced appearance of the coated vesicles in the axon terminals of the neuromuscular junctions of the intrafusal fibers in the animals restrained in antiorthostatic position (support unloading of m. soleus), which is due to increased functional load of the muscle. The ultrastructure of the muscle spindles adequately reflected the functional status of the postural m. soleus both during support unloading and support unloading combined with hypergravity load.     

Muscle spindle formation and differentiation in regenerating rat muscle grafts

Muscle spindle development and function are dependent upon sensory innervation. During muscle regeneration, both neural and muscular components of spindles degenerate and it is not known whether reinnervation of a regenerating muscle results in reestablishment of proper neuromuscular relationships within spindles or whether sensory neurons may exert an influence upon differentiation of these spindles. Muscle spindle regeneration was studied in bupivacaine-treated grafts of rat extensor digitorum longus (EDL) muscles. Three types of EDL graft were performed in order to manipulate the extent to which regenerating spindles might be reinnervated: (1) grafts reinnervated following severance of their nerve supply (standard grafts); (2) grafts in which intact nerve sheaths appear to facilitate reinnervation (nerveintact grafts); and (3) grafts in which reinnervation was prevented (nonreinnervated grafts). Complete degeneration of muscle fibers occurred in all grafts prior to regeneration. Initial formation of spindles in regenerating EDL grafts is independent of innervation; intrafusal muscle fibers degenerate and regenerate within spindle capsules that remain intact and viable. The extent of spindle differentiation was evaluated in each type of graft using criteria that included nucleation and ATPase activity, both of which have been shown to be regulated by sensory innervation, as well as the number of muscle fibers/spindle and morphology of spindle capsules.While most spindles contained normal numbers of muscle fibers, most of these fibers were morphologically and histochemically abnormal. Alterations of ATPase activity occurred in all spindles, but were least severe in nerve-intact grafts. While fully differentiated nuclear bag and chain fibers were not observed in regenerated spindles, large, vesicular nuclei, similar to those of normal intrafusal fibers, were present in a small number of spindles in nerve-intact grafts. Sensory nerve terminations were observed only in those spindles that also contained the distinctive nuclei. This study suggests that a specific neurotrophic influence is necessary for regeneration of normal intrafusal muscle fibers and that this influence corresponds to the properly timed sensory neuron-muscle interaction which directs muscle spindle embryogenesis. However, the infrequent occurrence of characteristics unique to intrafusal muscle fibers indicates that reinnervation of regenerating muscle grafts by sensory neurons is inadequate and/or faulty.     

Role of nerve and muscle factors in the development of rat muscle spindles

The soleus muscles of fetal rats were examined by electron microscopy to determine whether the early differentiation of muscle spindles is dependent upon sensory innervation, motor innervation, or both. Simple unencapsulated afferent-muscle contacts were observed on the primary myotubes at 17 and 18 days of gestation. Spindles, encapsulations of muscle fibers innervated by afferents, could be recognized early on day 18 of gestation. The full complement of spindles in the soleus muscle was present at day 19, in the region of the neuromuscular hilum. More afferents innervated spindles at days 18 and 19 of gestation than at subsequent developmental stages, or in adult rats; hence, competition for available myotubes may exist among afferents early in development. Some of the myotubes that gave rise to the first intrafusal (bag2) fiber had been innervated by skeletomotor (alpha) axons prior to their incorporation into spindles. However, encapsulated intrafusal fibers received no motor innervation until fusimotor (gamma) axons innervated spindles 3 days after the arrival of afferents and formation of spindles, at day 20. The second (bag1) intrafusal fiber was already formed when gamma axons arrived. Thus, the assembly of bag1 and bag2 intrafusal fibers occurs in the presence of sensory but not gamma motor innervation. However, transient innervation of future bag2 fibers by alpha axons suggests that both sensory and alpha motor neurons may influence the initial stages of bag2 fiber assembly. The confinement of nascent spindles to a localized region of the developing muscle and the limited number of spindles in developing muscles in spite of an abundance of afferents raise the possibility that afferents interact with a special population of undifferentiated myotubes to form intrafusal fibers.     

Proportions of slow myosin heavy chain-positive fibers in muscle spindles and adjoining extrafusal fascicles, and the positioning of spindles relative to these fascicles.

Chicken leg muscles were examined to calculate the percentages of slow myosin heavy chain (MHC)-positive fibers in spindles and in adjacent extrafusal fascicles, and to clarify how the encapsulated portions of muscle spindles are positioned relative to these fascicles. Unlike mammals, in chicken leg muscles slow-twitch MHC and slow-tonic MHC are expressed in intrafusal fibers and in extrafusal fibers, suggesting a close developmental connection between the two fiber populations. In 8-week-old muscles the proportions of slow MHC-positive extrafusal fibers that ringed muscle spindles ranged from 0-100%. In contrast, proportions of slow MHC-positive intrafusal fibers in spindles ranged from 0-57%. Similar proportions in fiber type composition between intrafusal fibers and surrounding extrafusal fibers were apparent at embryonic days 15 and 16, demonstrating early divergence of extrafusal and intrafusal fibers. Muscle spindles were rarely located within single fascicles. Instead, they were commonly placed where several fascicles converged. The frequent extrafascicular location of spindles suggests migration of intrafusal myoblasts from developing clusters of extrafusal fibers toward the interstitium, perhaps along a neurotrophic gradient established by sensory axons that are advancing in the connective tissue matrix that separates adjoining fascicles.     

Histological study of motor innervation to long nuclear chain intrafusal fibers in the muscle spindle of the cat

Several muscle spindles of the cat tenuissimus muscle were cut in serial, 1-micron thick transverse sections and stained with toluidine blue in search for long nuclear chain intrafusal muscle fibers. Five complete poles of the long chain fibers were located. Each fiber pole displayed one plate-type motor ending situated in the extracapsular fiber region. The endings were supplied by myelinated motor axons that originated from intramuscular nerve fascicles containing motor axons to extrafusal muscle fibers. One of the endings was innervated by a collateral from a motor axon that supplied an extrafusal end-plate. Ultrastructurally, the long chain endings resembled extrafusal end-plates. They were more complex, in terms of prominence of sole-plate and degree of post-junctional folding, than any other intrafusal ending present in the spindles. The motor endings of the long chain fibers were assumed to be the terminals of static (fast) skeletofusimotor axons, which preferentially innervate the longest nuclear chain fibers of cat muscle spindles.     

Myosin heavy chain composition of muscle spindles in human biceps brachii.

Data on the myosin heavy chain (MyHC) composition of human muscle spindles are scarce in spite of the well-known correlation between MyHC composition and functional properties of skeletal muscle fibers. The MyHC composition of intrafusal fibers from 36 spindles of human biceps brachii muscle was studied in detail by immunocytochemistry with a large battery of antibodies. The MyHC content of isolated muscle spindles was assessed with SDS-PAGE and immunoblots. Four major MyHC isoforms (MyHCI, IIa, embryonic, and intrafusal) were detected with SDS-PAGE. Immunocytochemistry revealed very complex staining patterns for each intrafusal fiber type. The bag(1) fibers contained slow tonic MyHC along their entire fiber length and MyHCI, alpha-cardiac, embryonic, and fetal isoforms along a variable part of their length. The bag(2) fibers contained MyHC slow tonic, I, alpha-cardiac, embryonic, and fetal isoforms with regional variations. Chain fibers contained MyHCIIa, embryonic, and fetal isoforms throughout the fiber, and MyHCIIx at least in the juxtaequatorial region. Virtually each muscle spindle had a different allotment of numbers of bag(1), bag(2) and chain fibers. Taken together, the complexity in intrafusal fiber content and MyHC composition observed indicate that each muscle spindle in the human biceps has a unique identity.     

The absence of support loading on the rat m. soleus leads to an increase in the number of intrafusal muscle fibres in muscle spindles

The ultrastructure of muscle spindles (incapsulated mechanoreceptors of stretch of extrafusal muscle fibres) of m. soleus in adult Wistar rats after repeated unloading of support on hind limbs with preservation of support loading on fore limbs has been studied by transmissing electron microscopy. It was shown that, along with muscle spindles with the ordinary number of intrafusal muscle fibres (four), m. soleus contains spindles with an increased number of intrafusal fibers (five to six). It was assumed that the increase in the number of intrafusal muscle fibers is due to the proliferation of their satellite cells.     

On Sympathetic Innervation of Muscle Spindles of the Frog Rana temporaria

The work is concerned with solution of a questionable problem, whether there exists a direct sympathetic innervation of muscle spindles. On the isolated total preparations of muscle spindles, with the aid of a specific histochemical fluorescent method using glyoxylic acid, revealed is the sympathetic innervation of these structures: adrenergic terminal axons were found both on the intrafusal muscular fibers and the blood vessels supplying the receptor.     

The effect of neonatal deafferentation or deefferentation on myosin heavy chain expression in intrafusal muscle fibers of the rat

Summary Muscle spindles were either deafferented or deefferented by selectively severing the sensory or motor nerve supply to neonatal soleus muscles of rats at a time when spindles are formed but when intrafusal muscle fibers are structurally and immunocytochemically immature. Experimental muscles wereexcised two months after nerve section. Control and experimental spindles were examined using monoclonal antibodies specific for myosin heavy chains of slow-tonic (ALD58) and fast-twitch (MF30) chicken muscles. Only intrafusal fibers bound these antibodies in intact soleus muscles. The deefferented spindles exhibited a pattern of ALD58 and MF30 binding similar to that of normal adult intrafusal fibers, whereas deafferented intrafusal fibers were unreactive with the two antibodies. Thus intact sensory innervation is essential for myosin heavy chain expression in intrafusal muscle fibers during postnatal development of rat spindles.     

The effect of neonatal deafferentation or defferentation on myosin heavy chain expression in intrafusal muscle fibers of the rat

Muscle spindles were either deafferented or deefferented by selectively severing the sensory or motor nerve supply to neonatal soleus muscles of rats at a time when spindles are formed but when intrafusal muscle fibers are structurally and immunocytochemically immature. Experimental muscles were excised two months after nerve section. Control and experimental spindles were examined using monoclonal antibodies specific for myosin heavy chains of slow-tonic (ALD58) and fast-twitch (MF30) chicken muscles. Only intrafusal fibers bound these antibodies in intact soleus muscles. The deefferented spindles exhibited a pattern of ALD58 and MF30 binding similar to that of normal adult intrafusal fibers, whereas deafferented intrafusal fibers were unreactive with the two antibodies. Thus intact sensory innervation is essential for myosin heavy chain expression in intrafusal muscle fibers during postnatal development of rat spindles.     

Prolonged degeneration of muscle spindles in the masseter muscle after treatment of developing mice with the local anesthetic lidocaine hydrochloride

The effect of lidocaine-HCl on muscle spindles in the masseter muscle of developing mice was investigated. Repeated injections of mice with anesthetic in the short term decreased the diameters of primary endings, intrafusal muscle fibers and outer capsules in the equatorial regions of muscle spindles, and caused a drop in the succinic dehydrogenase activity in intrafusal muscle fibers of the muscle spindles. In addition, the diameters did not recover to the control value even after about 10 weeks following cessation of anesthetic treatment. Thus, the present results suggest that repeated use of lidocaine-HCl in developing animals may cause dysfunction of the skeletal muscles.     

Unloading of the soleus in tail-suspended rats increases the number of intrafusal fibers in muscle spindles

Transmission electron microscopy was used to study the ultrastructure of muscle spindles (encapsulated stretch receptors) in m. soleus of adult Wistar rats after repeated hindlimb unloading. It was shown that the unloaded soleus contained not only spindles with a typical number of intrafusal fibers (four) but also spindles with five or six fibers. The increase in the number of intrafusal fibers in muscle spindles of the unloaded animals is likely to be caused by the proliferation of their satellite cells (myoblasts).     

Structure of the neuromuscular spindles in the tongue of human fetuses

The dynamics in development of some components--of the neuromuscular spindles in the human fetus and newborn tongues have been studied by means of certain general and neurohistological methods with elements of morphometry. During the whole prenatal period of the human life, there is a certain synchronism in the development of the lingual proper muscles and the neuro-muscular spindles. Certain integration in the development of the neuro-muscular spindles is observed in 4-6-month-old fetuses; in 8-9-month-old fetuses and in newborns it is substituted for heterochronicity. By the time of birth, the muscle spindle contractile elements are supplied with a more differentiated efferent innervation. The latter actively effects the morphological state of the intrafusal muscle fibers and forms a base for functional activity of the tongue.     

Morphology and morphometry of motor endings on macaque intrafusal fibers

The ultrastructural studies have shown three types of motor endings in the macaque intrafusal fibers: 1) unindented axon terminals with smooth or shallowly folded postsynaptic membrane; 2) indented terminals with few postsynaptic folds; and 3) indented terminals with heavily folded postsynaptic membrane. The terminals on bag 1 and chain fibers were generally more indented than those on the bag 2 fibers. Deeply indented terminals with highly folded postsynaptic membranes were noticed on the bag 1 and chain endings in spindles from lumbrical but not the biceps muscle. In the individual intrafusal fibers from the biceps and lumbrical spindles, the degree of indentation did not correlate with the extent of postsynaptic folding (P greater than .01). Endings on bag 1 and chain fibers in the lumbrical spindles showed a positive correlation between indentation of terminals and their distance from the primary sensory endings (P less than .01), whereas the lumbrical bag 2 endings and the biceps intrafusal endings did not (P greater than .01). The shape of the intrafusal motor endings thus is independent of their location but dependent on the type of intrafusal fibers.     

Slow-tonic MHC expression in paralyzed hindlimbs of fetal rats.

Whether nerve activity and active contraction of myotubes are essential for the assembly and initial differentiation of muscle spindles was investigated by paralyzing fetal rats with tetrodotoxin (TTX) from embryonic day 16 (E16) to E21, prior to and during the period when spindles typically form. TTX-treated soleus muscles were examined by light and electron microscopy for the presence of spindles and expression of myosin heavy chain (MHC) isoforms by the intrafusal fibers. Treatment with TTX did not inhibit the formation of a spindle capsule or the expression of a slow-tonic MHC isoform characteristic of intrafusal fibers, but did retard development of spindles. Spindles of TTX-treated E21 muscles usually consisted of one intrafusal fiber (bag2) only rather than two fibers (bag1 and bag2) typically present in untreated (control) E21 spindles. Intrafusal fibers of TTX-treated spindles also had only one sensory region supplied by multiple afferents, and were devoid of motor innervation. These features are characteristic of spindles in normal E18-E19 muscles. Thus, nerve and/or muscle activity is not essential for the assembly of muscle spindles, formation of a spindle capsule, and transformation of undifferentiated myotubes into the intrafusal fibers containing spindle-specific myosin isoforms. However, activity may promote the maturation of intrafusal bundles, as well as the maturation of afferent and efferent nerve supplies to intrafusal fibers.     

Fiber-type compositions in postnatal chicken muscle spindles with low intrafusal fiber counts and their developmental significance

The fiber-type composition of postnatal chicken leg muscle spindles with from one to four intrafusal fibers was examined in sections incubated with monoclonal antibodies against fast and slow myosin heavy chains. In monofibral spindles the lone intrafusal fiber was almost always fast. In duofibral spindles usually one slow and one fast fiber were present. Trifibral spindles most often displayed two fast and one slow fiber, whereas quadrofibral receptors characteristically contained two slow and two fast fibers. Earlier results showed that the primary intrafusal myotube in nascent spindles has almost always a fast myosin heavy chain profile and that the proportion of slow myotubes and fibers increases as intrafusal fiber bundles grow in size. Data from postnatal chicken leg muscles collected here suggest that up to the first four fibers this proportional increase can be largely accounted for if consecutive intrafusal fibers arise in a fast-slow-fast-slow sequence. The late recognition during myogenesis of primary intrafusal myotubes and their fast myosin heavy chain profiles warrant exploring if nascent chicken muscles spindles are first seeded by fast fetal myoblasts. © 1995 Wiley-Liss, Inc.     

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.