Ends Of Muscle Fibers Research Articles

Detection of epimuscular myofascial forces by Golgi tendon organs

Skeletal muscles embed multiple tendon organs, both at the proximal and distal ends of muscle fibers. One of the functions of such spatial distribution may be to provide locally unique force feedback, which may become more important when stresses are distributed non-uniformly within the muscle. Forces exerted by connections between adjacent muscles (i.e. epimuscular myofascial forces) may cause such local differences in force. The aim of this exploratory study was to investigate the effects of mechanical interactions between adjacent muscles on sensory encoding by tendon organs. Action potentials from single afferents were recorded intra-axonally in response to ramp-hold release (RHR) stretches of a passive agonistic muscle at different lengths or relative positions of its passive synergist. The tendons of gastrocnemius (GAS), plantaris (PL) and soleus (SO) muscles were cut from the skeleton for attachment to servomotors. Connective tissues among these muscles were kept intact. Lengthening GAS + PL decreased the force threshold of SO tendon organs (p = 0.035). The force threshold of lateral gastrocnemius (LG) tendon organs was not affected by SO length (p = 0.371). Also displacing LG + PL, kept at a constant muscle–tendon unit length, from a proximal to a more distal position resulted in a decrease in force threshold of LG tendon organs (p = 0.007). These results indicate that tendon organ firing is affected by changes in length and/or relative position of adjacent synergistic muscles. We conclude that tendon organs can provide the central nervous system with information about local stresses caused by epimuscular myofascial forces.

New data of topographic anatomy of semilunar line (semilunar line by A. Spiegel)

Background. Today, a special place in surgical practice is the treatment of patients with spigelian hernias, which can be subcutaneous, interstitial, prepperitoneal, and make up 1% of all external hernias of the abdomen. The difficulty in diagnosing of spigelian hernias can lead to a serious complication – infringement of the hernia, which will require emergency surgical treatment. Therefore, the study of the typical, sexual and variant anatomy of the Spigelian line region is important in improving the diagnosis and treatment of such patients.The aim. To study the features of typical, sexual and variant anatomy of the Spigelian line region.Materials and methods. We examined 42 non-fixed corpses of persons of both sexes without signs of pathology of the anterior abdominal wall. Among them were 26 (54.2%) men and 22 (45.8%) women. At autopsy, we performed anatomical dissection of the semilunar line region. We measured the width of the aponeurotic stretch from the end of the transverse abdominal muscle fibers to the lateral edge of the rectus abdominis muscle at level corresponding to d. bicostarum, umbilical ring and d. bispinarum.Results. We identified four clusters corresponding to the variations in the shape of the semilunar line: tapering down (9.5%), uniform wide (19.0%), tapering up (28.6%) and wide in the middle (42.9%). We found that the semilunar line, tapering up, was significantly more often observed in women (83.3%), uniformly wide – in brachymorphic body type (75.0%), wide in the middle – in mesomorphic body type (66.07%), and the semilunar line, tapering down, was found only in men. Variants of the shape of the semilunar line, tapering up or down, were absent in persons of the brachymorphic body type. Semilunar line, tapering up, was found (without significant differences) in persons of the mesomorphic body type in 41.6%, in the persons of the brachymorphic body type – in 58.4%, and semilunar line, tapering down, was noted in persons with a dolichomorphic body type in 75.0%.Conclusion. New data may allow to predict the location, type of spigelian hernia, and also improve the diagnosis and treatment of spigelian hernia.

New Method for Veterinary and Sanitary Control of Defrosted Meat and Fish

The freezing method allows you to extend the shelf life for a long period, however, when defrosting products there is a loss of nutrients. Currently, the assessment of the thermal state of meat and fish remains a topical issue, which today is carried out using organoleptic methods, as well as a histological method for the structure of muscle tissue. These methods lose their relevance in conditions of real circulation of meat and fish products due to subjectivity, as well as complexity. To ensure the release of benign chilled products into the market and to prevent the substitution of defrosted chilled meat, a fast, effective and easily reproducible method is required. We carried out studies to assess the thermal state of 69 experimental samples of meat of animals, poultry and fish according to organoleptic indicators and also proposed a new method for the manufacture of native drugs that allows us to evaluate the structure of muscle tissue. Native meat preparations were made from thin sections of muscle tissue, crushed between the glasses of the compressorium, followed by hematoxylon-eosin staining and microscopy. it was found that the number of fibrous sections of muscle tissue in defrosted products is 11.3 times more than in samples chilled; muscle fiber breaks - 47 times. A striking identification sign that allows to differentiate the thermal state of meat and fish raw materials is the presence of thickenings at the ends of muscle fibers, which are absent in preparations of chilled material. The obtained values are statistically significant. As a result of the studies, identification criteria were established, such as a violation of the structure of muscle tissue, which is manifested by the presence of fibrous areas, tearing of muscle fibers, as well as the presence of thickenings at the ends of muscle fibers. The obtained research results allow us to use the proposed method of manufacturing and microscopy of meat and fish preparations and establish the previous freezing of products.

Myogenic activity and serotonergic inhibition in the chromatophore network of the squid Dosidicus gigas (family Ommastrephidae) and Doryteuthis opalescens (family Loliginidae).

Seemingly chaotic waves of spontaneous chromatophore activity occur in the ommastrephid squid Dosidicus gigas in the living state and immediately after surgical disruption of all known inputs from the central nervous system. Similar activity is apparent in the loliginid Doryteuthis opalescens, but only after chronic denervation of chromatophores for 5-7 days. Electrically stimulated, neurally driven activity in intact individuals of both species is blocked by tetrodotoxin (TTX), but TTX has no effect on spontaneous wave activity in either D. gigas or denervated D. opalescens Spontaneous TTX-resistant activity of this sort is therefore likely myogenic, and such activity is eliminated in both preparations by serotonin (5-HT), a known inhibitor of chromatophore activity. Immunohistochemical techniques reveal that individual axons containing L-glutamate or 5-HT (and possibly both in a minority of processes) are associated with radial muscle fibers of chromatophores in intact individuals of both species, although the area of contact between both types of axons and muscle fibers is much smaller in D. gigas Glutamatergic and serotonergic axons degenerate completely following denervation in D. opalescens Spontaneous waves of chromatophore activity in both species are thus associated with reduced (or no) serotonergic input in comparison to the situation in intact D. opalescens Such differences in the level of serotonergic inhibition are consistent with natural chromogenic behaviors in these species. Our findings also suggest that such activity might propagate via the branching distal ends of radial muscle fibers.

Thixotropy and rheopexy of muscle fibers probed using sinusoidal oscillations.

Length changes of muscle fibers have previously been shown to result in a temporary reduction in fiber stiffness that is referred to as thixotropy. Understanding the mechanism of this thixotropy is important to our understanding of muscle function since there are many instances in which muscle is subjected to repeated patterns of lengthening and shortening. By applying sinusoidal length changes to one end of single permeabilized muscle fibers and measuring the force response at the opposite end, we studied the history-dependent stiffness of both relaxed and activated muscle fibers. For length change oscillations greater than 1 Hz, we observed thixotropic behavior of activated fibers. Treatment of these fibers with EDTA and blebbistatin, which inhibits myosin-actin interactions, quashed this effect, suggesting that the mechanism of muscle fiber thixotropy is cross-bridge dependent. We modeled a half-sarcomere experiencing sinusoidal length changes, and our simulations suggest that thixotropy could arise from force-dependent cross-bridge kinetics. Surprisingly, we also observed that, for length change oscillations less than 1 Hz, the muscle fiber exhibited rheopexy. In other words, the stiffness of the fiber increased in response to the length changes. Blebbistatin and EDTA did not disrupt the rheopectic behavior, suggesting that a non-cross-bridge mechanism contributes to this phenomenon.

Long-duration muscle dedifferentiation during limb regeneration in axolotls.

Although still debated, limb regeneration in salamanders is thought to depend on the dedifferentiation of remnant tissue occurring early after amputation and generating the progenitor cells that initiate regeneration. This dedifferentiation has been demonstrated previously by showing the fragmentation of muscle fibers into mononucleated cells and by revealing the contribution of mature muscle fibers to the regenerates by using lineage-tracing studies. Here, we provide additional evidence of dedifferentiation by showing that Pax7 (paired-box protein-7) transcripts are expressed at the ends of remnant muscle fibers in axolotls by using in situ hybridization and by demonstrating the presence of Pax7+ muscle-fiber nuclei in the early bud and mid-bud stages by means of immunohistochemical staining. During the course of regeneration, the remnant muscles did not progress; instead, muscle progenitors migrated out from the remnants and proliferated and differentiated in the new tissues at an early stage of differentiation. The regenerating muscles and remnant muscles were largely disconnected, and this left a gap between them until extremely late in the late stage of differentiation, at which point the new and old muscles connected together. Notably, Pax7 transcripts were detected in the regions of muscles that faced these gaps; thus, Pax7 expression might indicate dedifferentiation in the remnant-muscle ends and partial differentiation in the regenerating muscles. The roles of this long-duration dedifferentiation in the remnants remain unknown. However, the results presented here could support the hypothesis that long-duration muscle dedifferentiation facilitates the connection and fusion between the new and old muscles that are both in an immature state; this is because immature Pax7+ myoblasts readily fuse during developmental myogenesis.

New hypothesis for mechanism of sliding filament theory of skeletal muscle contraction

Background: Current understanding of skeletal muscle contraction is based on the sliding filament theory proposed independently by A.F. Huxley and H.E. Huxley (1954). The sliding filament theory very well explained shortening of skeletal muscle during contraction with reference to a single sarcomere with movement of Z discs toward center of that particular sarcomere. However, when we consider shortening of multiple sarcomeres in a myofibril arranged in series, the sliding filament theory fails to justify the movement of Z discs as each Z disc is being pulled in opposite direction by myosin heads of adjacent sarcomeres. Aims & Objectives: To find out alternate possible mechanism of Z disc movement on either side of A band if movement toward center by both sides is not possible. Materials and Methods: We have prepared a simulating model of sliding filament theory and observed mechanism of contraction on it. Results: The sliding filament theory fails to explain the movement of Z discs when all sarcomeres in a myofibril are contracting simultaneously. Therefore, we proposed a new hypothesis that Z discs and actin filaments in the half part of each sarcomere, which is toward origin end of muscle fiber, are not pulled toward center of myosin filament instead myosin filament slide toward origin end, and in the other half of each sarcomere (which is toward insertion end) actin filaments slide over myosin filament toward origin end of the muscle. Conclusion: This study adds a new insight into the sliding filament theory of muscle contraction and needs further confirmation on electron microscopic studies.

The role of transmembrane proteins on force transmission in skeletal muscle

Lateral transmission of force from myofibers laterally to the surrounding extracellular matrix (ECM) via the transmembrane proteins between them is impaired in old muscles. Changes in geometrical and mechanical properties of ECM of skeletal muscle do not fully explain the impaired lateral transmission with aging. The objective of this study was to determine the role of transmembrane proteins on force transmission in skeletal muscle. In this study, a 2D finite element model of single muscle fiber composed of myofiber, ECM, and the transmembrane proteins between them was developed to determine how changes in spatial density and mechanical properties of transmembrane proteins affect the force transmission in skeletal muscle. We found that force transmission and stress distribution are not affected by mechanical stiffness of the transmembrane proteins due to its non-linear stress–strain relationship. Results also showed that the muscle fiber with insufficient transmembrane proteins near the end of muscle fiber transmitted less force than that with more proteins does. Higher stress was observed in myofiber, ECM, and proteins in the muscle fiber with fewer proteins.

RacGAP50C directs perinuclear γ-tubulin localization to organize the uniform microtubule array required forDrosophilamyotube extension

The microtubule (MT) cytoskeleton is reorganized during myogenesis as individual myoblasts fuse into multinucleated myotubes. Although this reorganization has long been observed in cell culture, these findings have not been validated during development, and proteins that regulate this process are largely unknown. We have identified a novel postmitotic function for the cytokinesis proteins RacGAP50C (Tumbleweed) and Pavarotti as essential regulators of MT organization during Drosophila myogenesis. We show that the localization of the MT nucleator gamma-tubulin changes from diffuse cytoplasmic staining in mononucleated myoblasts to discrete cytoplasmic puncta at the nuclear periphery in multinucleated myoblasts, and that this change in localization depends on RacGAP50C. RacGAP50C and gamma-tubulin colocalize at perinuclear sites in myotubes, and in RacGAP50C mutants gamma-tubulin remains dispersed throughout the cytoplasm. Furthermore, we show that the mislocalization of RacGAP50C in pavarotti mutants is sufficient to redistribute gamma-tubulin to the muscle fiber ends. Finally, myotubes in RacGAP50C mutants have MTs with non-uniform polarity, resulting in multiple guidance errors. Taken together, these findings provide strong evidence that the reorganization of the MT network that has been observed in vitro plays an important role in myotube extension and muscle patterning in vivo, and also identify two molecules crucial for this process.

Deletion of Integrin-Linked Kinase from Skeletal Muscles of Mice Resembles Muscular Dystrophy Due to α7β1-Integrin Deficiency

Integrin-linked kinase (Ilk) is a serine/threonine kinase and an adaptor protein that links integrins to the actin cytoskeleton and to a number of signaling pathways involved in integrin action. We hypothesized that Ilk may act as an important effector of integrins in skeletal muscle, where these receptors provide a critical link between the sarcolemma and the extracellular matrix. Using the cre/lox system, we deleted Ilk from skeletal muscles of mice. The resulting mutants developed a progressive muscular dystrophy with multiple degenerating and regenerating muscle fibers, increased central nuclei, and endomysial fibrosis. These defects were widespread but were most severe near myofascial junctions where Ilk mutants showed displacement of focal adhesion-related proteins, including vinculin, paxillin, focal adhesion kinase, dystrophin, and the alpha 7 beta 1D-integrin subunits. Distal ends of mutant muscle fibers appeared irregular, and there was restructuring of the actin cytoskeleton. These findings resemble those seen in humans and mice lacking the alpha 7-integrin subunit and suggest that Ilk may act as a cytoplasmic effector of alpha 7 beta1-integrin in the pathogenesis of these deficiencies.

Extramuscular myofascial force transmission alters substantially the acute effects of surgical aponeurotomy: assessment by finite element modeling

Effects of extramuscular myofascial force transmission on the acute effects of aponeurotomy were studied using finite element modeling and implications of such effects on surgery were discussed. Aponeurotomized EDL muscle of the rat was modeled in two conditions: (1) fully isolated (2) with intact extramuscular connections. The specific goal was to assess the alterations in muscle length-force characteristics in relation to sarcomere length distributions and to investigate how the mechanical mechanism of the intervention is affected if the muscle is not isolated. Major effects of extramuscular myofascial force transmission were shown on muscle length-force characteristics. In contrast to the identical proximal and distal forces of the aponeurotomized isolated muscle, substantial proximo-distal force differences were shown for aponeurotomized muscle with extramuscular connections (for all muscle lengths F (dist) > F (prox) after distal muscle lengthening). Proximal optimal length did not change whereas distal optimal length was lower (by 0.5 mm). The optimal forces of the aponeurotomized muscle with extramuscular connections exerted at both proximal and distal tendons were lower than that of isolated muscle (by 15 and 7%, respectively). The length of the gap separating the two cut ends of the intervened aponeurosis decreases substantially due to extramuscular myofascial force transmission. The amplitude of the difference in gap length was muscle length dependent (maximally 11.6% of the gap length of the extramuscularly connected muscle). Extramuscular myofascial force transmission has substantial effects on distributions of lengths of sarcomeres within the muscle fiber populations distal and proximal to the location of intervention: (a) Within the distal population, the substantial sarcomere shortening at the proximal ends of muscle fibers due to the intervention remained unaffected however, extramuscular myofascial force transmission caused a more pronounced serial distribution towards the distal ends of muscle fibers. (b) In contrast, extramuscular myofascial force transmission limits the serial distribution of sarcomere lengths shown for the aponeurotomized isolated muscle in the proximal population. Fiber stress distributions showed that extramuscular myofascial force transmission causes most sarcomeres within the aponeurotomized muscle to attain lengths favorable for higher force exertion. It is concluded that acute effects of aponeurotomy on muscular mechanics are affected greatly by extramuscular myofascial force transmission. Such effects have important implications for the outcome of surgery performed to improve impeded function since muscle in vivo is not isolated both anatomically and mechanically.

Finite element modeling of aponeurotomy: altered intramuscular myofascial force transmission yields complex sarcomere length distributions determining acute effects

Finite element modeling of aponeurotomized rat extensor digitorium longus muscle was performed to investigate the acute effects of proximal aponeurotomy. The specific goal was to assess the changes in lengths of sarcomeres within aponeurotomized muscle and to explain how the intervention leads to alterations in muscle length-force characteristics. Major changes in muscle length-active force characteristics were shown for the aponeurotomized muscle modeled with (1) only a discontinuity in the proximal aponeurosis and (2) with additional discontinuities of the muscles' extracellular matrix (i.e., when both myotendinous and myofascial force transmission mechanisms are interfered with). After muscle lengthening, two cut ends of the aponeurosis were separated by a gap. After intervention (1), only active slack length increased (by approximately 0.9 mm) and limited reductions in muscle active force were found (e.g., muscle optimum force decreased by only 1%) After intervention (2) active slack increased further (by 1.2 mm) and optimum length as well (by 2.0 mm) shifted and the range between these lengths increased. In addition, muscle active force was reduced substantially (e.g., muscle optimum force decreased by 21%). The modeled tearing of the intramuscular connective tissue divides the muscle into a proximal and a distal population of muscle fibers. The altered force transmission was shown to lead to major sarcomere length distributions [not encountered in the intact muscle and after intervention (1)], with contrasting effects for the two muscle fiber populations: (a) Within the distal population (i.e. fibers with no myotendinous connection to the muscles' origin), sarcomeres were much shorter than within the proximal population (fibers with intact myotendinous junction at both ends). (b) Within the distal population, from proximal ends of muscle fibers to distal ends, the serial distribution of sarcomere lengths ranged from the lowest length to high lengths. In contrast within the proximal population, the direction of the distribution was reversed. Such differences in distribution of sarcomere lengths between the proximal and distal fiber populations explain the shifts in muscle active slack and optimal lengths. Muscle force reduction after intervention (2) is explained primarily by the short sarcomeres within the distal population. However, fiber stress distributions showed contribution of the majority of the sarcomeres to muscle force: myofascial force transmission prevents the sarcomeres from shortening to nonphysiological lengths. It is concluded that interfering with the intramuscular myofascial force transmission due to rupturing of the intramuscular connective tissue leads to a complex distribution of sarcomere lengths within the aponeurotomized muscle and this determines the acute effects of the intervention on muscle length-force characteristics rather than the intervention with the myotendinous force transmission after which the intervention was named. These results suggest that during surgery, but also postoperatively, major attention should be focused on the length and activity of aponeurotomized muscle, as changes in connective tissue tear depth will affect the acute effects of the intervention.

Greater frequency and concentration of satellite cells at the ends of developing skeletal muscle fibers

Satellite cells (SCs) are mononuclear myogenic stem cells located immediately beneath the basal laminae of skeletal muscle fibers. Once activated, SCs fuse to the fibers as new myonuclei. Since myonuclei are more concentrated in tapered fiber ends, this study tests the hypothesis that there is a greater frequency (number of SC nuclei over all nuclei within the basal laminae) and a higher concentration of SCs at the ends of skeletal muscle fibers. Chicken pectoralis was used as the experimental model because of its overlapping fibers and the presence of neonatal myosin in the fiber ends. Samples were excised from birds ranging in age from nine to 115 days. Antibodies against Pax7 were used to identify SC nuclei. Applying immunocytochemical techniques and computer image analyses, the frequency of SCs was quantified for different types of fiber profiles (neonatal, transforming and adult) and with regards to fiber size. While the frequency of SCs decreased significantly (P<0.01) with age from about 20% to less than 5%, along the fiber lengths the ends retained the highest frequency ranging between 20-13%. Also, the area of plasmalemma per SC was calculated for each type of fiber profile. There was significantly (P<0.05) less plasmalemma per SC at the ends of the fibers. This study demonstrates that throughout development there is a higher frequency and concentration of SCs at the tapered ends of skeletal muscle fibers. Funds for this study were provided by the Natural Sciences and Engineering Research Council of Canada.

Analysis of Calculated Electrical Activation of Denervated Muscle Fibers in the Human Thigh

Finite difference models of the human thigh are used to analyze the excitation process in the fibers of denervated skeletal muscles in conjunction with functional electrical stimulation (FES) via surface electrodes. The Matlab tool "FES-Analyze" was developed to simulate and analyze the super-threshold regions in a human thigh. Action potential is simulated with a muscle fiber model of the Hodgkin Huxley type and with a generalized form of the activating function. With FES-Analyze it is possible to compare the stimulation at the end of the muscle fiber and the stimulation at the central part of the muscle fiber, both in cross- and longitudinal-section, as well as to observe the effect of different impulse intensities and lengths during FES. Simulating with "Standard Values" of the pulse duration (20 ms) and the amplitude (80 V) one discovers that the main part of the activation takes place at the end of the muscle fiber. To obtain activation at the central part of the muscle fiber, higher amplitudes and longer pulse durations are needed.

Zebrafish as a model for caveolin-associated muscle disease; caveolin-3 is required for myofibril organization and muscle cell patterning

Caveolae are an abundant feature of many animal cells. However, the exact function of caveolae remains unclear. We have used the zebrafish, Danio rerio, as a system to understand caveolae function focusing on the muscle-specific caveolar protein, caveolin-3 (Cav3). We have identified caveolin-1 (alpha and beta), caveolin-2 and Cav3 in the zebrafish. Zebrafish Cav3 has 72% identity to human CAV3, and the amino acids altered in human muscle diseases are conserved in the zebrafish protein. During embryonic development, cav3 expression is apparent by early segmentation stages in the first differentiating muscle precursors, the adaxial cells and slightly later in the notochord. cav3 expression appears in the somites during mid-segmentation stages and then later in the pectoral fins and facial muscles. Cav3 and caveolae are located along the entire sarcolemma of late stage embryonic muscle fibers, whereas beta-dystroglycan is restricted to the muscle fiber ends. Down-regulation of Cav3 expression causes gross muscle abnormalities and uncoordinated movement. Ultrastructural analysis of isolated muscle fibers reveals defects in myoblast fusion and disorganized myofibril and membrane systems. Expression of the zebrafish equivalent to a human muscular dystrophy mutant, CAV3P104L, causes severe disruption of muscle differentiation. In addition, knockdown of Cav3 resulted in a dramatic up-regulation of eng1a expression resulting in an increase in the number of muscle pioneer-like cells adjacent to the notochord. These studies provide new insights into the role of Cav3 in muscle development and demonstrate its requirement for correct intracellular organization and myoblast fusion.

Scanning Electron Microscopic Study of the Muscle Fiber Ends at the Myotendinous Junction in the Posterior Cricoarytenoid and Cricothyroid Muscles in Rats

Scanning Electron Microscopic Study of the Muscle Fiber Ends at the Myotendinous Junction in the Posterior Cricoarytenoid and Cricothyroid Muscles in Rats

Scanning Electron Microscopic Study of the Muscle Fiber Ends at the Myotendinous Junction in the Posterior Cricoarytenoid and Cricothyroid Muscles in Rats

The fine structural organization of muscle fiber ends at the myotendinous junction in the posterior cricoarytenoid (PCA) and cricothyroid (CT) muscles in adult rats was studied by scanning electron microscopy, after removal of tendon connective tissues using the HCl-hydrolysis method. The muscle fiber ends in the PCA muscle had a relatively simple conical appearance and contained a great number of longitudinal slits on the surface. In the CT muscle, the muscle fiber ends were classified into two types. One type had a conical appearance similar to the PCA muscle with many slits on the surface, while the other type was characterized by a complex arrangement of cylindrical cytoplasmic processes and deep clefts, as seen in common skeletal muscles. These findings suggest that the PCA muscle is evolutionarily primitive, and that the CT muscles represent a transitional form between primitive and evolved muscles.

Persistent expression of developmental myosin heavy chain isoforms in the tapered ends of adult pigeon pectoralis muscle fibres.

We have shown previously that in addition to the adult myosin heavy chain (MyHC) isoform present throughout the length of each fast-twitch glycolytic muscle fibre within the pectoralis of the mature chicken, the neonatal isoform is retained in the tapered ends of these fibres. This work, however, has been the only published report of this phenomenon. Here, we tested the hypothesis that similar to the chicken, the ends of mature pigeon pectoralis muscle fibres contain developmental MyHC isoform(s). A histological stain was used to visualize endomysium to assist in the analysis of transverse sections of pectoralis muscle from four mature pigeons. Immunocytochemical techniques were used to localize MyHC isoform(s) characteristic of pigeon pectoralis development. We show that within mature pigeon pectoralis, the ends of both fast-twitch glycolytic and fast-twitch oxidative-glycolytic fibre types express MyHC isoform(s) characteristic of their earlier development. Thus, we extend our findings on chicken to another species and an additional muscle fibre type. Retention of developmental MyHC isoform(s) within the tapered ends of mature muscle fibres may be more widespread than is currently appreciated.

Muscular Force Transmission: A Unified, Dual or Multiple System? A Review and Some Explorative Experimental Results

Structures contributing to force transmission in muscle are reviewed combining some historical and relatively recently published experimental data. Also, effects of aponeurotomy and tenotomy are reviewed shortly as well as some new experimental results regarding these interventions that reinforce the concept of myofascial force transmission. The review is also illustrated by some new images of single muscle fibres from Xenopus Laevis indicative of such transmission and some data about locations of insertion of human gluteus maximus muscle. From this review and the new material, emerges a line of thought indicating that mechanical connections between muscle fibres and intramuscular connective tissue play an important role in force transmission. New experimental observations are presented for non-spanning muscle (i.c., rat biceps femoris muscle), regarding the great variety of types of intramuscular connections that exist i n addition to myo-tendinous junctions at the perimuscular ends of muscle fibres. Such connections are classified as (1) tapered end connections, (2) Myo-myonal junctions, (3) myo-epimysial junctions and (3) Myo-endomysial junctions. This line of thought is followed up by consideration of a possible role of connections of intra- and extramuscular connective tissue in force transmission out of the muscle. Experimental results of an explorative nature, regarding the interactions of extensor digitorum longus (EDL), tibialis anterior (TA) and hallucis longus (HAL) muscles within a relatively intact dorsal flexor compartment of the rat hind leg, indicate that: (1) length force properties of EDL are influenced by TA activity in a length dependent fashion. Depending on TA length, force exerted by EDL, kept at constant origin insertion distance, is variable and the effect is influenced by EDL length itself as well; (2) Force is transmitted from muscle to extramuscular connective tissue and vice versa. As a consequence force exerted at proximal and distal tendons of a muscle are not always equal. The difference being transmitted by extramuscular connective tissue and may appear at the tendons of other muscles or may be transmitted via connective tissue directly to bone. It is concluded that the system of force transmission from skeletal muscle should be considered as a multiple system.

Morphodifferentiation of skeletal muscle fiber ends at the myotendinous junction in the postnatal Chinese hamster: a scanning electron microscopic study.

The postnatal morphodifferentiation of muscle fiber ends at the myotendinous junction was examined by scanning electron microscopy in gastrocnemius muscles of the Chinese hamster. Muscle fiber ends during the first week were simply conical or slightly complex, having a few pit-like invaginations and longitudinal and narrow clefts. By the second week, fiber ends gradually became complicated with an increased number of clefts and finger-like processes. Pits and short clefts linearly arranged towards the fiber tip were visible until the second week. This possibly indicates the linear elongation of the clefts by the fusion of adjacent pits to one another and to preexisting clefts. By the fourth week, the fiber ends had almost fully matured and displayed numerous cytoplasmic processes and lateral grooves.