The orientation of muscle fibres in the tail musculature ofRana temporaria tadpoles (Amphibia,Anura)

Summary Shape of the myosepts and arrangement of the muscle fibres were recorded in the lateral musculature of the tail ofRana temporaria embryos and larvae. Well developed myomeres are present as early as st. 18–19. The main characteristics—ie. those related to functional properties—of myoseptal shape as well as of muscle fibre arrangement, remain unchanged throughout further development until degeneration of the tail occurs during metamorphosis. The rather simple myoseptal shape observed inRana—as compared to the multiple cone-form observed in most fishes—shows a close agreement to hypothetical myosept models described in papers by Jarman (1961), van der Stelt (1968) and Willemse (1966). The muscle fibres in the m. lateralis ofRana are arranged in trajectorial patterns that show a close similarity to the trajectorial patterns observed in typical teleosts. Both arrangements agree with trajectorial models based on the mathematical analyses of Alexander (1968).Neurulas anaesthetized with 1:10000 MS-222 and exposed up two weeks to this anaesthetic developed the same shape of the myosepts and arrangement of muscle fibres as in controls. Thus even the details of the function-related features of the myomere structure develop without functioning. In this field possible feedback meachisms are either not affected by anaesthesia or do not exist at all.     

Morphology and mechanics of myosepta in a swimming salamander (Siren lacertina)

In contrast to the complex, three-dimensional shape of myomeres in teleost fishes, the lateral hypaxial muscles of salamanders are nearly planar and their myosepta run in a roughly straight line from mid-lateral to mid-ventral. We used this relatively simple system as the basis for a mathematical model of segmented musculature. Model results highlight the importance of the mechanics of myosepta in determining the shortening characteristics of a muscle segment. We used sonomicrometry to measure the longitudinal deformation of myomeres and the dorsoventral deformation of myosepta in a swimming salamander (Siren lacertina). Sonomicrometry results show that the myosepta allow some dorsoventral lengthening, indicating an amplification of myomere shortening that is greater than that produced by muscle fiber angle alone (10% muscle fiber shortening produces 28.7% myomere shortening). Polarized light and DIC microscopy of isolated hypaxial myosepta revealed that the collagen fiber orientation in hypaxial myomeres is primarily mediolateral. The mediolateral collagen fiber orientation, combined with our finding that the hypaxial myosepta lengthen dorsoventrally during swimming, suggests that one possible function of hypaxial myosepta in S. lacertina is to increase the strain amplification of the muscle fibers by reducing the mediolateral bulging of the myomeres and redirecting the bulging toward the dorsoventral direction.     

Histochemical definition of muscle fibre types in the trunk musculature of a teleost fish (cod,Gadus morhua,L.)

Summary Cryostat sections incubated for myofibrillar ATPase, SDH, LDH, and -GPDH as well as p-phenylene-diamine stained semithin sections were used to define muscle fibre types in the trunk musculature of the cod (Gadus morhua, L.).Three zones (superficial, intermediate, deep) containing different muscle fibre types are present within both epaxial and hypaxial parts of each myomere subjacent to the lateral line.Atypical relations concerning myofibrillar ATPase activity probably reflects instability of myosin during storage of frozen tissue. The histochemical reaction does not distinguish between myofibrillar and mitochondrial ATPase in cod muscle.Based on ATPase and SDH activities, seven different histochemical profiles of muscle fibres can be identified in trunk musculature of this teleost fish. Attempts to homologize these fibre types with those in cyclostomes or those in higher animals proved futile. The higher number of histochemically defined muscle fibre types in cod might be explained by developmental processes and an admixture of immature fibres throughout life.     

Muscle architecture and fibre characteristics of rat gastrocnemius and semimembranosus muscles during isometric contractions

Rat gastrocnemius medialis (GM) and semimembranosus (SM) muscles have a very different morphology. GM is a very pennate muscle, combining relatively short muscle fibre length with sizable fibre angles and long muscle and aponeurosis lengths. SM is a more parallel-fibred muscle, combining a relatively long fibre length with a small fibre angle and short aponeurosis length. The mechanisms of fibre shortening as well as angle increase are operational in GM as well as SM. However, as a consequence of isometric contraction, changes of fibre length and angle are greater for GM than for SM at any relative muscle length. These differences are particularly notable at short muscle lengths: at 80% of optimum muscle length, fibre length changes of approximately 30% are coupled to fibre angle changes of 15 degrees in GM, while for SM these changes are 4% and 0.6 degrees, respectively. A considerable difference was found for normalized active slack muscle length (GM approximately 80 and SM approximately 45%). This is explained by differences of degree of pennation as well as factors related to differences found for estimated fibre length-force characteristics. Estimated normalized active fibre slack length was considerably smaller for SM than for GM (approximately 40 and 60%, respectively). The most likely explanation of these findings are differences of distribution of optimum fibre lengths, possibly in combination with differences of myofilament lengths and/or fibre length distributions.     

Myosin filament sliding through the Z-disc relates striated muscle fibre structure to function

Striated muscle contraction requires intricate interactions of microstructures. The classic textbook assumption that myosin filaments are compressed at the meshed Z-disc during striated muscle fibre contraction conflicts with experimental evidence. For example, myosin filaments are too stiff to be compressed sufficiently by the muscular force, and, unlike compressed springs, the muscle fibres do not restore their resting length after contractions to short lengths. Further, the dependence of a fibre''s maximum contraction velocity on sarcomere length is unexplained to date. In this paper, we present a structurally consistent model of sarcomere contraction that reconciles these findings with the well-accepted sliding filament and crossbridge theories. The few required model parameters are taken from the literature or obtained from reasoning based on structural arguments. In our model, the transition from hexagonal to tetragonal actin filament arrangement near the Z-disc together with a thoughtful titin arrangement enables myosin filament sliding through the Z-disc. This sliding leads to swivelled crossbridges in the adjacent half-sarcomere that dampen contraction. With no fitting of parameters required, the model predicts straightforwardly the fibre''s entire force–length behaviour and the dependence of the maximum contraction velocity on sarcomere length. Our model enables a structurally and functionally consistent view of the contractile machinery of the striated fibre with possible implications for muscle diseases and evolution.     

Effects of short length immobilization of medial gastrocnemius muscle of growing young adult rats.

Effects of four and six weeks of immobilization at short length of gastrocnemius muscle on its architecture at optimum muscle length and length-force characteristics were studied. In general, immobilization effects were similar after 4 and 6 weeks. Smaller physiological cross-sectional area and lower muscle force were found as a consequence of immobilization. Muscle and aponeurosis were shorter. This was shown to be quantitatively related to atrophy i.e. smaller fibre diameter. Despite this atrophy no effects of immobilization on fibre and aponeurosis angles could be shown. Adaptation of the number of sarcomeres in series was found exclusively in distal fibres after 4 weeks of immobilization. No significant effects were found for proximal fibres of muscles at this time nor for any fibres after 6 weeks of immobilization. The effects of immobilization on muscle architecture did not affect the length range of active force exertion. It is concluded that muscle length adaptation as a consequence of short length immobilization is not related to adaptation of number of sarcomeres in series but to the occurrence of atrophy. It is also concluded that atrophy of pennate muscles does not have to be accompanied by a lower fibre and aponeurosis angle. Comparison of immobilized and control group rats indicates that the effects of immobilization can be characterized as a combination of retarded development of several variables and the influence of atrophy and its consequences.     

Three muscle fibre types in the axial muscle of axolotl (Ambystoma mexicanum Shaw) A quantitative lightand electron microscopic study

Morphometric analysis by light microscopy of p-phenylene-diamine stained semithin sections of axolotl tail muscle revealed differences in the cross-sectional area of the fibres and in the number of mitochondria and of lipid inclusions per fibre, and indicated the presence of three distinct types of fibres. The tripartition was found to be statistically highly significant. Representative fibres from each group established by light microscopic morphometry were subjected to an ultrastructural morphometric analysis. The volume content of mitochondria amounted to 9.8% of the fibre volume for red, 4.0% for intermediate and 0.8% for white fibres. The myofibrils composed 60%, 70% and 83% in the same fibres. The volume of the sarcotubular system (t-tubuli and sarcoplasmic reticulum) was 2.5% in red, 4.5% in intermediate and 11.7% in white fibres. The three fibre types also demonstrated differences in myofibrillar cross-striation pattern and number of triads. The reliability of the light microscopic morphometry was tested by correlation with EM montages of the representative fibres.     

The response of fast and slow nuclear bag fibres and nuclear chain fibres in isolated cat muscle spindles to fusimotor stimulation, and the effect of intrafusal contraction on the sensory endings.

1. The mechanical behaviour of intrafusal muscle fibres during fusimotor stimulation and passive stretch was observed directly in muscle spindles isolated from the cat tenuissimus muscle. 2. Mammalian intrafusal muscle fibres are of three functional types. Most spindles contain one slow nuclear bag fibre, one fast nuclear bag fibre, and four or five nuclear chain fibres. 3. Contraction in slow nuclear bag fibres is characterized by a long latency and very slow initial velocity, whereas the latency for the other intrafusal fibres is short and the inital velocity rapid. The mean time for maximum contraction (at 75 Hz to 100 Hz) and relaxation is significantly longer for slow nuclear bag fibres (0-8s) than for other intrafusal fibres (0-5 s). The contraction time of fast nuclear bag fibres is sometimes longer than that of nuclear chain fibres but the mean values are not significantly different; a difference in the time to attain 90% contraction is more obvious. 4. At low stimulation frequencies (10 Hz) contraction in slow nuclear bag fibres and in most fast nuclear bag fibres is smooth whereas nuclear chain fibres exhibit marked oscillations. Single stimuli elicit small local twitches in nuclear chain fibres and occasionally in fast nuclear bag fibres but produce no visible effect in slow nuclear bag fibres. 5. Maximum contraction of slow and fast nuclear bag fibres at body temperature is attained at a stimulation frequency of 75 Hz to 100 Hz, whereas a frequency of 150 Hz or more is required for maximum contraction of nuclear chain fibres. At 50 Hz at body temperature contraction in nuclear bag fibres is at least half the maximum, whereas in many spindles nuclear chain fibres show only a very small contraction at this frequency. 6. Contraction in slow nuclear bag fibres occurs at one or two discrete foci, most of which lie in the intracapsular region beyond the end of the fluid space. Weak contraction extends the primary sensory spiral by a small amount (2%-8%) at a low velocity (5%-10%s-1). When the fibre is passively stretched the spiral opens and then creeps back to about 75% of the extension at the end of the stretch due to yielding in the poles of fibre; creep is complete in 0-5s to 2-5s. 7. Contraction in fast nuclear bag fibres also occurs at one or two discrete foci, most of which lie in the intracapsular region beyond the end of the fluid space. Shortening of sarcomeres at the foci and extension of the sensory spiral are, however, up to eight times greater (up to 25%) than in slow nuclear bag fibres, and the velocity of stretch of the spiral is three to eight times greater (25%-40%s-1). Fast nuclear bag fibres exhibit little or no creep following passive stretch. 8. Contraction in the nuclear chain fibre bundle is localized to the intracapsular region, centered on a point in the intracapsular region between 0-9 mm and 1-6 mm from the spindle equator. Maximal contraction stretches primary and secondary sensory endings by 15% to 20%, at 30% to 40% s-1...     

X-ray microanalysis of myotomal muscle in the sunbleak, Leucaspius delineatus (Heckel) and goldfish, Carassius auratus gibelio

The functioning of a group of muscle fibres as a tissue that performs a well characterized type of contraction (slow or fast) depends on their biochemical and structural organization that is already well established. The biochemical and structural diversities between three types of fish muscle fibres found also a reflection in the content of light elements. The present work demonstrates significant differences in the content of diffusible elements (Cl, K, Na, and Mg) and bound elements (P and S) between the muscle fibres types. In general all muscle fibre types of goldfish (Carassius auratus gibelio) that belongs to stationary slow-swimming fish has lower K/Na ratios than those in all three fibre types of fast swimming sunbleak (Leucaspius delineatus).     

Human skeletal muscle architecture studied in vivo by non-invasive imaging techniques: functional significance and applications

The internal architecture plays an essential role in determining the functional features of skeletal muscle. Both length–force and force–velocity relationships depend on the spatial arrangement of muscle fibres in skeletal muscle. The degree of muscle pennation determines both the amount of contractile tissue packed along the tendons and fibre length, and is reflected by the force-generating capacity and shortening velocity of the muscle and by the elastic properties of the muscle–tendon complex. Until recently, knowledge on human muscle architecture was based on measurements performed on cadavers, whose muscle fibres were often shrunk by the preserving medium and by age. With the introduction of non-invasive imaging techniques, it has become possible to study muscle architecture in vivo at rest and the changes thereof upon contraction. This paper discusses the applications of these techniques, namely ultrasonography and nuclear magnetic resonance imaging, and their relevance in physiology and biomechanics.     

Strain-induced reorientation of an intramuscular connective tissue network: implications for passive muscle elasticity

The most abundant intramuscular connective tissue component, the perimysium, of bovine M. sternomandibularis muscle was shown to be a crossed-ply arrangement of crimped collagen fibres which reorientate and decrimp on changing muscle fibre sarcomere length. Reorientation of perimysial strands was observed by light microscopy and identification of these strands as collagen fibres was confirmed by high-angle X-ray diffraction. Mean collagen fibre direction with respect to the muscle fibres ranged from approximately 80 degrees at sarcomere length = 1.1 micron to approximately 20 degrees at 3.9 microns. This behaviour was well described by a model of a crimped planar network surrounding a muscle fibre bundle of constant volume but varying length. Modelling of the mechanical properties of the perimysium at different sarcomere lengths produced a load-sarcomere length curve which was in good agreement with the passive elastic properties of the muscle, especially at long sarcomere lengths. It is concluded that the role of the perimysial collagen network is to prevent over-stretching of the muscle fibre bundles.     

Behaviour of the crossbridges in stretched or compressed muscle fibres

The maximum chord of the myosin heads is comparable to the closest surface-to-surface spacing between the myofilaments in a muscle at the slack length. Therefore, when the sarcomere length increases or when the fibre is compressed, the surface-to-surface myofilament spacing becomes lower than the head long axis. We conclude that, in stretched or compressed fibres, some crossbridges cannot attach, owing to steric hindrance. When the amount of compression is limited, this hindrance may be overcome by a tilting of the heads in the plane perpendicular to the filament axes; in this case, there is no consequence as concerns the crossbridge properties. In highly compressed fibres, the crossbridges become progressively hindered and all the crossbridges are hindered for an axis-to-axis spacing representing about 60% of the spacing observed under zero external osmotic pressure. In this case, both the isometric tension and the ATPase activity of the fibre are zero. In fibres stretched up to 3.77 microns (sarcomere length corresponding to the disappearance of the overlap between the thick and the thin filaments), the ratio of hindered crossbridges over the functional crossbridges may be estimated at about 55%. In stretched fibres, a noticeable proportion of crossbridges are sterically hindered and the crossbridges performance (e.g. constants of attachment and detachment) depends on filament spacing, i.e. on sarcomere length. Therefore, we think it is probably impossible to consider the crossbridges as independent force converters, since this idea requires that the crossbridge properties are independent of sarcomere length. In this connection, all the experiments performed on osmotically compressed fibres are of major importance for the understanding of the true mechanisms of muscle contraction.     

Electrophysiological experiments on the mechanism and accuracy of neuromuscular specificity in the axolotl.

The supracoracoideus muscle of the axolotl shoulder girdle is innervated by two nerves, the supracoracoideus nerve (SC) supplying most of the muscle and the posterior supracoracoideus (PSC) supplying the posterior corner. All the muscle fibres are multiply innervated and at the border between the two innervations many muscle fibres, when penetrated by a microelectrode, show junction potentials from both nerves. In such cases one junction potential is often very small, below the threshold for exciting muscle contraction, the other large and effective at exciting the muscle. If the SC nerve is cut, the territory of the PSC nerve expands over several weeks. Upon regrowth of the cut nerve it reinnervates its old muscle fibres and removes the previous foreign innervation, the borderline between the two nerve territories being established exactly as before. This depends upon two processes, sprouting of nerves and a competitive repression of transmission from nerves ending on foreign muscle fibres.     

The ultrastructure and innervation of muscles controlling chromatophore expansion in the squid,Loligo vulgaris

Squid chromatophores are organs of colour change, consisting of a pigment sac opened by contraction of 10–24 radial muscle fibres. The ultrastructure and innervation of these muscle fibres were examined by electron microscopy and diagramatic reconstructions made on the basis of serial ultra-thin sections. At the proximal end of the fibre, nearest the pigment sac a cortical myofilament zone surrounds 2 cores containing mitochrondria; further along the fibre these merge to form one central core. The myofilament zone forms a groove containing a nerve bundle consisting of 2 to 4 axons per muscle fibre. The axons are surrounded by glial cell processes, and either originate from a neighbouring fibre, or join the fibre at some point along its length. Axons twist around each other, forming a series of synapses with the muscle fibre. As many as 6–37 synapses exist along the length of each muscle fibre; the mean synapse interval is 9.05 m, but the largest may be 123 m. At the distal end of the muscles, the nerve is located towards the middle of the fibre, which it penetrates as the muscle splits up. Electron-lucent vesicles are present in all synaptic regions, but electron-dense vesicles are only found towards the distal end of the fibre. There is thus a possibility that more than one neurotransmitter is present in the nerves innervating chromatophores. Electron-lucent and dense-cored vesicles are not colocalised.This work was carried out during the tenure of a BBSRC CASE studentship     

Characterization of the capillary network in skeletal muscles from 3D data

In this review we present immunohistochemical methods for visualization of capillaries and muscle fibres in thick muscle sections. Special attention is paid to the procedures that preserve good morphology. Applying confocal microscopy and virtual 3D stereological grids, or tracing of capillaries in virtual reality, length of capillaries within a muscle volume or length of capillaries adjacent to a muscle fibre per fibre length, fibre surface area or fibre volume can be evaluated by an unbiased approach. Moreover, 3D models of capillaries and muscle fibres can be produced. Comparison of the developed methods with counting capillary profiles from 2D sections is discussed and the reader is warned that counting capillary profiles from 2D sections can underestimate the capillary length by as much as 75 percent. Application of the described 3D methodology is illustrated by the anatomical remodelling of capillarity during acute denervation and early reinnervation in the rat soleus and extensor digitorum longus muscles.     

The force-length relationship of a muscle-tendon complex: experimental results and model calculations

Models are useful when studying how architectural and physiological properties of muscle-tendon complexes are related to function, because they allow for the simulation of the behaviour of such complexes during natural movements. In the construction of these models, evaluation of their accuracy is an important step. In the present study, a model was constructed to calculate the isometric force-length relationship of the rat extensor digitorum longus muscle-tendon complex. The model is based on the assumption that a muscle-tendon complex is a collection of independent units, each consisting of a muscle fibre in series with a tendon fibre. By intention, values for model parameters were derived indirectly, using only the measured maximal isometric tetanic force, the distance between origin and insertion at which it occurred (optimum lOI) and an estimate of muscle fibre optimal length. The accuracy of the calculated force-length relationship was subsequently evaluated by comparing it to the relationship measured in isometric tetanic contractions of a real complex in the rat. When the length of distal muscle fibres, measured during isometric contraction at optimal lOI of the whole complex, was used as an estimate for muscle fibre optimal length of all muscle fibre-tendon fibre units in the model, the calculated relationship was too narrow. That is, both on the ascending limb and on the descending limb the calculated tetanic force was lower than the measured tetanic force.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adaptation of muscle fibre types and capillary network to acute denervation and shortlasting reinnervation

We postulated that, in rat extensor digitorum longus muscle (EDL), the length of capillaries per fibre surface area (Lcap/Sfib) and per fibre volume (Lcap/Vfib) could reflect fibre-type transformations accompanied by changes in oxidative metabolic profile and selective fibre-type atrophy. We excised rat EDL muscle 2 weeks after the sciatic nerve was cut (acute denervation; DEDL) and 4 weeks after the nerve was crushed (early reinnervation; REDL) and characterised muscle fibre-type transformation by the expression of myosin heavy-chain isoforms and by succinate dehydrogenase (SDH) and nicotinoamide adenine dinucleotide-tetrazolium reductase (NADH-TR) reactions. The numerical percentage (N/N) and area percentage (A/A) of pure and hybrid fibres and their diameter were determined, as was the A/A of SDH- and NADH-TR-positive fibres. The length of capillaries per fibre length (Lcap/Lfib), Lcap/Sfib and Lcap/Vfib were estimated in REDL and Lcap/Vfib in DEDL. In DEDL, the type 2x and 2b fibres evidently atrophied, with the N/N of type 2x fibres being lower and that of hybrid fibres higher. In REDL, the N/N of hybrid fibres was even higher, consequent to a lower N/N of type 2b fibres; however, fibre diameters approached values of the control EDL. Compared with control EDL, denervated and reinnervated muscles exhibited a higher A/A of oxidative fibres. This is probably the result of fibre-type transformation and selective fibre atrophy. We conclude that capillary length does not change during acute denervation and early reinnervation. The obtained higher values of Lcap/Sfib and Lcap/Vfib are related to changes in muscle fibre cross-sectional area. This study was supported by the Slovenian Research Agency and the Ministry of Education, Youth and Sport of the Czech Republic (KONTAKT grant no. 9-06-6 and grant no. LC06063).     

Residual force enhancement and force depression in human single muscle fibres

Residual force depression (rFD) and residual force enhancement (rFE) are intrinsic contractile properties of muscle. rFD is characterized as a decrease in steady-state isometric force following active shortening compared with a purely isometric contraction at the same muscle length and level of activation. By contrast, isometric force is increased following active lengthening compared to a reference isometric contraction at the same muscle length and level of activation; this is termed rFE. To date, there have been no investigations of rFD and rFE in human muscle fibres, therefore the purpose of this study was to determine whether rFD and rFE occur at the single muscle fibre level in humans. rFD and rFE were investigated in maximally activated single muscle fibres biopsied from the vastus lateralis of healthy adults. To induce rFD, fibres were activated and shortened from an average sarcomere length (SL) of 3.2–2.6 μm. Reference isometric contractions were performed at an average SL of 2.6 μm. To induce rFE, fibres were actively lengthened from an average SL of 2.6–3.2 μm and a reference isometric contraction was performed at an average SL of 3.2 μm. Isometric steady-state force was lower following active shortening (p < 0.05), and higher following active lengthening (p < 0.05), as compared to the reference isometric contractions. We demonstrated rFD and rFE in human single fibres which is consistent with previous animal models. The non-responder phenomenon often reported in rFE studies involving voluntary contractions at the whole human level was not observed at the single fibre level.     

Ultrastructure of muscle fibres in head and axial muscles of the perch (Perca fluviatilis L.)

Summary White, pink, red and deep red fibres, selected from a head muscle and from axial muscles of the perch, show significant differences in actin filament length, Z line thickness, Z line lattice space, myofibril girth, the percentages volume occupied by T system and terminal cisternae of the SR, and in the degree of T system SR contact per sarcomere. In both muscles the degree of T system SR contact decreases in the order: white, pink, red, deep red, which suggests a decrease of contraction velocity in the same order.The position of the T system (at the Z line or at the AI junction) is related to the actin filament length. The actin filaments in the red fibres are appreciably longer than in the white, which suggests that the sarcomeres of the red fibres have a broader length-tension curve. The Z line thickness is positively correlated with the actin filament length and, in the white and the red fibres, negatively with the degree of sarcomere shortening. Thicker Z lines are suggested to allow greater sarcomere sizes (length or girth).The percentage volume occupied by mitochondria varies independently of the extent of membrane systems.The ultrastructural characteristics of the fibre types are in agreement with the functional roles as reported in literature.