Intrafusal muscle fibers of duck muscle spindles

Serial transverse paraffin sections of intrafusal muscle fibers of spindles from the extensor pollicis and the extensor digitorum communis of ducks show that only one type of intrafusal muscle fiber exists, based on the mid-equatorial nucleation pattern, diameter, and length. Although the overall range in fiber diameter at the mid-equatorial region is between 4.2-20.0 microns, the average caliber is 10.4 +/- 3.18 microns (S.D.) for spindles of the extensor pollicis and 9.3 +/- 2.11 microns (S.D.) for spindles of the extensor digitorum communis muscles. The range in spindle length for the extensor pollicis is 290-2,090 microns, average 1,120 +/- 569 microns (S.D.), and for the extensor digitorum communis 1,160-2,500 microns, average 1,745 +/- 367 microns (S.D.). The range in number of fibers per spindle for the extensor pollicis muscle is 5-12, average 8.2, and for the extensor digitorum muscle it is 1-11. In the extensor digitorum communis, there appear to be two groups, based on fiber number. Spindles of one group have a range of 5-11 fibers per spindle with an average of 7.2, whereas the second group has a range of 1-4 with an average of 2.7 fibers per spindle. The second group of spindles constitutes 52.5% of the 40 spindles studied, and of these 7.5% were monofibril spindles, 15.0% difibril, 17.5% trifibril, 12.5% quadrifibril spindles.     

Studies on the distribution of calcitonin gene-related peptide-like and substance P-like immunoreactivities in rat hind limb muscles

Summary The tibialis anterior, extensor digitorum longus and soleus muscles in the rat were examined with respect to the presence of calcitonin gene-related peptide-like as well as substance P-like immunoreactivity. In some of the motor endplates in these muscles, identified by staining for acetylcholinesterase activity, calcitonin gene-related peptide-like immunoreactivity was detected, but in others it was not. Calcitonin gene-related peptide-like immunoreactivity was found to coexist with substance-P-like immunoreactivity in nerve fibres located outside and inside the capsule of the muscle spindles, as well as in nerve fibres located in nerve fascicles. These fibres presumably represent sensory nerve fibres. Calcitonin gene-related peptide-like immunoreactivity, but not substance P-like immunoreactivity, was also detected, in cap-like structures located on the surface of the intrafusal muscle fibres in the polar regions of the spindles, structures which are likely to correspond to motor plate endings. The observations suggest that calcitonin gene-related peptide is heterogeneously present in the endplates of rat hind limb muscles, and gives for the first time immunohistochemical evidence for the presence of calcitonin gene-related peptide and substance P in the innervation of muscle spindles.     

Expression of nestin,desmin and vimentin in intact and regenerating muscle spindles of rat hind limb skeletal muscles

We describe the expression and distribution patterns of nestin, desmin and vimentin in intact and regenerating muscle spindles of the rat hind limb skeletal muscles. Regeneration was induced by intramuscular isotransplantation of extensor digitorum longus (EDL) or soleus muscles from 15-day-old rats into the EDL muscle of adult female inbred Lewis rats. The host muscles with grafts were excised after 7-, 16-, 21- and 29-day survival and immunohistochemically stained. Nestin expression in intact spindles in host muscles was restricted to Schwann cells of sensory and motor nerves. In transplanted muscles, however, nestin expression was also found in regenerating “spindle fibers”, 7 and 16 days after grafting. From the 21st day onwards, the regenerated spindle fibers were devoid of nestin immunoreactivity. Desmin was detected in spindle fibers at all developmental stages in regenerating as well as in intact spindles. Vimentin was expressed in cells of the outer and inner capsules of all muscle spindles and in newly formed myoblasts and myotubes of regenerating spindles 7 days after grafting. Our results show that the expression pattern of these intermediate filaments in regenerating spindle fibers corresponds to that found in regenerating extrafusal fibers, which supports our earlier suggestion that they resemble small-diameter extrafusal fibers.     

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.     

Histochemical fiber type distribution and epinephrine sensitivity in normal and cross-transplanted rat muscles

The metabolic integrity of fully regenerated transplants was investigated by measuring induced changes in glycogen concentration. The extensor digitorum longus and the soleus muscles were cross transplanted: the extensor digitorum longus into the soleus muscle bed (SOLT) and the soleus muscle into the extensor digitorum longus bed (EDLT). The histochemical fiber type distribution of the regenerated muscles was determined and was found to transform in cross-transplanted EDLT and SOLT. After transplantation and regeneration, both muscles had initially low glycogen concentrations. However, the EDLT glycogen concentration was not significantly different from that of the contralateral extensor digitorum longus control muscle after 60 days. In the SOLT, glycogen gradually increased but remained less than in the contralateral soleus control muscle. SOLT and control soleus muscles responded with a significant glycogen depletion to an epinephrine dose two orders of magnitude less than the lowest dose affecting glycogen levels in EDLT and extensor digitorum longus muscles. These results indicate that transplanted muscles are capable of regenerating normal glycogenolytic responses and that the sensitivity of the response observed depends on the site of transplantation and is related to the type of innervation and histochemical fiber type.     

Formation of muscle spindles in regenerated avian muscle grafts

Summary Pigeon muscles lacking muscle spindles were grafted into sites which normally have a muscle containing spindles. The reciprocal transplantations were also made. After two to eight months, the graft of the donor muscle without spindles had regenerated into a muscle containing muscle spindles. The reciprocal grafts, muscles containing spindles transplanted to a site lacking spindle innervation, had neither muscle spindles nor remnants of the spindles. These experiments demonstrate that 1) the innervation is required for formation of the spindle; 2) the original spindles do not survive transplantation; and 3) parts of the original spindle are not required for spindle regeneration.This work was supported in part by NSF grants PCM 77-15960 and PCM 79-16540     

Responses of muscle spindles of fast and slow muscles to mechanical stimulation during hypokinesia

Activity of muscle spindles of fast (extensor digitorum longus) and slow (soleus) muscles was studied in cats during hypokinesia of the limb immobilized in a plaster cast. Spontaneous activity of muscle spindles of the fast and slow muscles was unchanged during hypokinesia. Spontaneous activity of primary and secondary endings evoked by passive stretching of the muscle exceeded normal. During stretching of the muscles at different speeds and of different amplitudes, the discharge frequency of the primary and secondary endings was much greater than normally during both the dynamic and the static phase of stretching. These changes were more marked in the slow muscles.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 186–192, March–April, 1978.     

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.     

Skeletal muscle fiber composition of the English sparrow (Passer domesticus)

Substrate utilization by English sparrow skeletal muscle has been extensively studied in our lab. However, there are few published studies on the muscle fiber composition of English sparrow wing and gastrocnemius muscles. The objective of the present study was to examine the fiber type composition of a variety of muscles in the English sparrow. The classification of a muscle fiber as fast glycolytic, slow oxidative, or fast oxidative glycolytic provides insight into the physiological function of muscles. Therefore, we completed mATPase and NADH stains on four muscles of the sparrow wing, as well as the gastrocnemius muscle, to characterize these muscle fiber types. Results show that the fibers of extensor digitorum communis, extensor metacarpi ulnaris, and extensor metacarpi radialis are homogeneous fast oxidative. The fibers of the supinator are homogeneous fast oxidative in 62.5% of samples, and heterogeneous (45.2% fast oxidative, 54.8% fast nonoxidative) in 37.5% of samples. Whereas the gastrocnemius muscle fibers are heterogeneous (10% fast oxidative, 64% fast nonoxidative, 26% slow oxidative) in all muscles examined.     

Effects of Latrodectus spider venoms on sensory and motor nerve terminals of muscle spindles

The effects of the venoms of the spiders Latrodectus mactans tredecimguttatus (black widow) and Latrodectus mactans hasselti (red back) on sensory nerve terminals in muscle spindles were studied in the mouse. A sublethal dose of venom was injected into tibialis anterior and extensor digitorum longus muscles of one leg. After survival from 30 minutes to 6 weeks muscles were examined in serial paraffin sections impregnated with silver or by electron microscopy. Sensory endings became swollen, some within 30 minutes, while over the next few hours there was progressive degeneration of annulospiral endings. By 24 hours every spindle identified by light or electron microscopy was devoid of sensory terminals. Degenerated nerve endings were taken up into the sarcoplasm of intrafusal muscle fibres. Regeneration of sensory axons began within 24 hours, new incomplete spirals were formed by 5 days and by 1 week annulospiral endings were almost all normal in appearance. Intrafusal motor terminals underwent similar acute degenerative and regenerative changes. These experiments show that intrafusal sensory and motor terminals are equally affected by Latrodectus venoms. Sensory nerve fibres possess a capacity for regeneration equal to that of motor fibres and reinnervate intrafusal muscle fibres close to their original sites of innervation.     

Effect of beta agonists on protein turnover in isolated chick skeletal and atrial muscle

Various beta-adrenergic agonists were found to inhibit rates of protein degradation and net protein breakdown in isolated chick extensor digitorum communis (EDC) and atrial muscles. Rates of protein synthesis were not altered by these compounds. The beta-agonist cimaterol inhibited rates of protein degradation in EDC muscles incubated with or without amino acids and insulin. Cimaterol also inhibited the increased proteolysis induced by injury to muscle or by incubating muscles at body temperature (42 degrees C) versus 37 degrees C. Thus, beta-agonists may help promote skeletal muscle accretion in vivo even under conditions of severe negative nitrogen balance by slowing muscle proteolysis.     

Glucose transporter expression in English sparrows (Passer domesticus)

Patterns of glucose transporter expression have been well-characterized in mammals. However, data for birds is currently restricted to isolated cells, domestic chickens and chicks, and ducklings. Therefore, in the present study, protein and gene expression of various glucose transporters (GLUTs) in English sparrow extensor digitorum communis, gastrocnemius and pectoralis muscles as well as heart, kidney, and brain tissues were examined. The hypothesis is that the expression pattern of avian GLUTs differs from mammals to maintain the high plasma glucose levels of birds and insulin insensitivity. Our studies failed to identify a GLUT4-like insulin responsive transporter in sparrows. GLUT1 gene expression was identified in all tissues examined and shares 88% homology with chicken and 84% homology with human GLUT1. Compared to the rat control, GLUT1 immunostaining of sparrow extensor digitorum communis muscle was weak and appeared to be localized to blood vessels whereas immunostaining of gastrocnemius muscles was comparable to rat muscle controls. Gene expression of GLUT3 was identified in all tissues examined and shares 90% gene sequence homology with chicken embryonic fibroblast and 75% homology with human GLUT3. Protein expression of GLUT3 was not determined as an avian antibody is not available. Moreover, the C-terminus of the mammalian GLUT3 transporter, against which antibodies are typically designed, differs significantly among species. The predominant difference of chicken and sparrow GLUT expression patterns from that of mammals is the lack of an avian GLUT4. The absence of this insulin responsive GLUT in birds may be a contributing factor to the observed high blood glucose levels and insulin insensitivity.     

Organization of skeletal muscle in the urodele Triturus cristatus: muscle fibre types and motor units

Four muscle fibre types are described in the biceps and extensor digitorum communis muscles of the newt forelimb. The histological criteria forming the basis for the distinctions include differential staining with p-phenylenediamine and succinate dehydrogenase histochemistry and electron microscopy. In addition, three distinctive motor unit types are described for the biceps muscle. These are fast units, slow units and intermediate units. The structure of muscle fibre and the physiological characteristics of muscle fibres belonging to each motor unit, have been correlated by using iontophoretic passage of Lucifer yellow into muscle fibres belonging to physiologically characterized motor units and their subsequent histological identification by the succinate dehydrogenase reaction. The three motor unit types correspond to slow muscle fibres, intermediate muscle fibres and two classes of fast muscle fibres.     

Migration of myogenic cells in the rat extensor digitorum longus muscle studied with a split autograft model

Summary The ability of myogenic cells to migrate perpendicular to the long axis of freely autografted muscles was examined. Rat extensor digitorum longus muscles were divided, and one half was devitalized by repeated freezing in liquid nitrogen while the other half was kept viable in physiologic saline. The halves were reunited with sutures and grafted back into the original muscle bed. At intervals between 5 and 25 days the grafts were removed and examined histologically for the presence of myotubes within the devitalized region. Myotubes were first seen in the devitalized half 10 days postgrafting with the maximum number of myotubes observed after 12 to 15 days. These results indicate that myogenic cells are capable of migration perpendicular to the long axis of the muscle fibers in an autograft.     

Estimation of the effective static moment arms of the tendons in the index finger extensor mechanism

A novel technique to estimate the contribution of finger extensor tendons to joint moment generation was proposed. Effective static moment arms (ESMAs), which represent the net effects of the tendon force on joint moments in static finger postures, were estimated for the 4 degrees of freedom (DOFs) in the index finger. Specifically, the ESMAs for the five tendons contributing to the finger extensor apparatus were estimated by directly correlating the applied tendon force to the measured resultant joint moments in cadaveric hand specimens. Repeated measures analysis of variance revealed that the finger posture, specifically interphalangeal joint angles, had significant effects on the measured ESMA values in 7 out of 20 conditions (four DOFs for each of the five muscles). Extensor digitorum communis and extensor indicis proprius tendons were found to have greater MCP ESMA values when IP joints are flexed, whereas abduction ESMAs of all muscles except extensor digitorum profundus were mainly affected by MCP flexion. The ESMAs were generally smaller than the moment arms estimated in previous studies that employed kinematic measurement techniques. Tendon force distribution within the extensor hood and dissipation into adjacent structures are believed to contribute to the joint moment reductions, which result in smaller ESMA values.     

Denervated muscle fibers explain the deficit in specific force following reinnervation of the rat extensor digitorum longus muscle

The authors tested the hypothesis that, after denervation and reinnervation of skeletal muscle, observed deficits in specific force can be completely attributed to the presence of denervated muscle fibers. The peroneal nerve innervating the extensor digitorum longus muscle in rats was sectioned and the distal stump was coapted to the proximal stump, allowing either a large number of motor axons (nonreduced, n = 12) or a drastically reduced number of axons access to the distal nerve stump (drastically reduced, n = 18). A control group of rats underwent exposure of the peroneal nerve, without transection, followed by wound closure (control, n = 9). Four months after the operation, the maximum tetanic isometric force (Fo) of the extensor digitorum longus muscle was measured in situ and the specific force (sFo) was calculated. Cross-sections of the muscles were labeled for neural cell adhesion molecule (NCAM) protein to distinguish between innervated and denervated muscle fibers. Compared with extensor digitorum longus muscles from rats in the control (295 +/- 11 kN/m2) and nonreduced (276 +/- 12 kN/m2) groups, sFo of the extensor digitorum longus muscles from animals in the drastically reduced group was decreased (227 +/- 15 kN/m2, p < 0.05). The percentage of denervated muscle fibers in the extensor digitorum longus muscles from animals in the drastically reduced group (18 +/- 3 percent) was significantly higher than in the control (3 +/- 1 percent) group, but not compared with the nonreduced (9 +/- 2 percent) group. After exclusion of the denervated fibers, sFo did not differ between extensor digitorum longus muscles from animals in the drastically reduced (270 +/- 20 kN/m2), nonreduced (301 +/- 13 kN/m2), or control (303 +/- 10 kN/m2) groups. The authors conclude that, under circumstances of denervation and rapid reinnervation, the decrease in sFo of muscle can be attributed to the presence of denervated muscle fibers.     

Regulation of protein metabolism by a physiological concentration of insulin in mouse soleus and extensor digitorum longus muscles. Effects of starvation and scald injury.

1. Although high concentrations of insulin affect both synthesis and degradation of skeletal-muscle protein, it is not known to what extent these effects occur with physiological concentrations. The effects of a physiological concentration of insulin (100 mu units/ml) on muscle protein synthesis, measured with [3H]tyrosine, and on muscle protein degradation, measured by tyrosine release in the presence of cycloheximide, were studied in mouse soleus and extensor digitorum longus muscles in vitro. 2. Insulin significantly stimualated protein synthesis in both muscles, but an inhibition of degradation was seen only in the extensor digitorum longus. 3. Starvation for 24 h decreased the rate of protein synthesis and increased the rate of breakdown in the extensor digitorum longus. Sensitivity to insulin-stimulation of proteins synthesis in the soleus was increased by starvation. 4. ;a 20%-surface-area full-skin-thickness dorsal scald injury produced a fall in total protein content in soleus and extensor digitorum muscles, maximal on the third day after injury. Soleus muscles 2 days after injury showed an impairment of protein synthesis; degradation was unaffected and neither synthesis nor degradation in vitro was significantly affected in the extensor digitorum longus. 5. The advantages and limitations of studies of protein metabolism in vitro are discussed.     

Lactate sensitive cells in newt kidneys

Summary The blood supply of duck muscle spindles from the extensor pollicis and the extensor digitorum comminis wing muscles has been studied by light and electron microscopy. Capillaries usually accompany the nerve bundle that innervates the spindle, approaching at an oblique angle around the midequatorial region. Capillaries may run for some distance along the surface of the outer capsule, or penetrate partially into the outer capsule and lie between layers of the capsule cell processes. Some capillaries also penetrate the outer capsule, running into the periaxial space and continuing further towards the polar region. They have been shown to be in close contact with intrafusal muscle fibres, from the juxta-equatorial to the polar region, but have not been encountered among sensory terminals in the mid-equatorial region.     

Identifying tasks to elicit maximum voluntary contraction in the muscles of the forearm

Maximum voluntary contractions (MVCs) are often used for the normalisation of electromyography data to enable comparison of signal patterns within and between study participants. Recommendations regarding the types of tasks that are needed to collect MVCs for the muscles of the forearm have been made, specifically advocating the use of resisted moment tasks to get better estimates of forearm MVCs. However, a protocol detailing which specific tasks to employ has yet to be published. Furthermore, the effects of limb dominance on the collection of MVCs have not been considered previously. Muscle activity was monitored while 23 participants performed nine isometric, resisted tasks. The tasks that are likely to elicit MVC in the flexor carpi ulnaris, flexor carpi radialis, flexor digitorum superficialis, extensor carpi ulnaris, extensor carpi radialis, extensor digitorum communis, and pronator teres were identified. Thus, targeted protocols can be designed to mitigate against fatigue. Hand dominance had limited effect, with differences being found only in the finger flexors and extensors (p< 0.03). Thus, use of the contralateral flexor digitorum superficialis and extensor digitorum communis muscles to obtain baselines for activation levels and patterns may not be appropriate.     

The effects of cutting or of stretching skeletal muscle in vitro on the rates of protein synthesis and degradation.

Rates of protein synthesis were significantly lower in the cut soleus and extensor digitorum longus muscles than in their uncut counterparts. Rates of protein degradation were significantly higher in cut soleus muscles, but not in cut extensor digitorum longus muscles as compared with their uncut controls. Concentrations of ATP and phosphocreatine were significantly lower in cut soleus and extensor digitorum longus muscles after incubation in vitro in contrast with respective control uncut muscles. These data indicate that cutting of muscle fibres alters rates of protein synthesis and degradation, in addition to altering concentrations of high-energy phosphates. Since these findings stressed the importance of using intact muscles to study protein metabolism, additional studies were made on intact muscles in vitro. Stretched soleus muscles had higher concentrations of high-energy phosphates at the end of an incubation period than did unstretched muscles. However, the length of the soleus, extensor digitorum longus and diaphragm muscles during incubation did not affect rates of protein degradation.U