THE MAXIMUM SARCOMERE LENGTH FOR CONTRACTION OF ISOLATED MYOFIBRILS

The passive mechanical properties of muscle and their adaptations to altered patterns of use

The length and stiffness of a relaxed muscle are determined by the mechanical properties of its intramuscular connective tissue and/or intracellular structures. Viscous deformation of these components of muscle is responsible for the increase in muscle length seen immediately after stretching, but this increase is transient. Lasting changes in muscle length can only be brought about by adaptations of the structure of muscle. An understanding of the nature of the stimulus for muscle to adapt can provide therapists with a theoretical basis for therapeutic intervention aimed at producing changes in muscle length.

Stretch-induced membrane damage in muscle: comparison of wild-type and mdx mice

One component of stretch-induced muscle damage is an increase in the permeability of the cell membrane. As a result soluble myoplasmic proteins leak out of the muscle into the plasma, extracellular proteins can enter the muscle, and extracellular ions, including calcium, are driven down their electrochemical gradient into the myoplasm. In Duchenne muscular dystrophy, caused by the absence of the cytoskeletal protein dystrophin, stretch-induced membrane damage is much more severe. The most popular theory to explain the occurrence of stretch-induced membrane damage is that stretched-contractions cause transient mechanically-induced defects in the membrane (tears or rips). Dystrophin, which is part of a mechanical link between the contractile machinery and the extracellular matrix, is thought to contribute to membrane strength so that in its absence mechanically-induced defects are worse. In our view the evidence that stretch-induced muscle damage causes increased membrane permeability is overwhelming but the evidence that the increased permeability is caused by mechanically-induced defects is weak. Instead we review the substantial evidence that the membrane permeability is a secondary consequence of the mechanical events in which elevated intracellular calcium and reactive oxygen species are important intermediaries.

Micromechanical modeling of the epimysium of the skeletal muscles

A micromechanical model has been developed to investigate the mechanical properties of the epimysium. In the present model, the collagen fibers in the epimysium are embedded randomly in the ground substance. Two parallel wavy collagen fibers and the surrounding ground substance are used as the repeat unit (unit cell), and the epimysium is considered as an aggregate of unit cells. Each unit cell is distributed in the epimysium with some different angle to the muscle fiber direction. The model allows the progressive straightening of the collagen fiber as well as the effects of fiber reorientation. The predictions of the model compare favorably against experiment. The effects of the collagen fiber volume fraction, collagen fiber waviness at the rest length and the mechanical properties of the collagen fibers and the ground substance are analyzed. This model allows the analysis of mechanical behavior of most soft tissues if appropriate experimental data are available.

Lateral force transmission across costameres in skeletal muscle

In skeletal muscle, contractile force can be transmitted laterally between the z-disks and M-lines of neighboring myofibrils, across the sarcolemma, and through the extracellular matrix to the tendon. Here we examine the data that support this model and the sarcolemmal properties and structures, termed "costameres," that are consistent with it.

Supercontracting muscle: producing tension over extreme muscle lengths

Muscle mechanics dictates a trade-off between the ability of a muscle to generate isometric force and its length. This intrinsic trade-off is the result of the need for overlap between thick and thin filaments upon extension of the sarcomere and of the limitations imposed by the physical interference between the thin filaments and the thick filaments with the Z-disk upon contraction. However, previously published data indicate that chameleons are able to produce a nearly constant tongue retraction force over a wide range of tongue extension lengths, made possible by the presence of supercontracting muscle in the tongue retractors. Investigation of the length/tension properties and ultrastructure of the tongue retractor in a closely related agamid lizard (Pogona vitticeps) indicates that the ability to generate tension at extreme elongation is probably a derived feature for chameleons. Whereas chameleons are unique among vertebrates in possessing supercontracting muscle, this seems to be a common phenomenon in invertebrates. However, the presence of supercontracting muscle in chameleons and in several invertebrate groups seems to be coupled to the need to generate tension over large changes in muscle length and might be a more general solution for this problem.

The filament lattice of striated muscle

The filament lattice of striated muscle is an overlapping hexagonal array of thick and thin filaments within which muscle contraction takes place. Its structure can be studied by electron microscopy or X-ray diffraction. With the latter technique, structural changes can be monitored during contraction and other physiological conditions. The lattice of intact muscle fibers can change size through osmotic swelling or shrinking or by changing the sarcomere length of the muscle. Similarly, muscle fibers that have been chemically or mechanically skinned can be compressed with bathing solutions containing very large inert polymeric molecules. The effects of lattice change on muscle contraction in vertebrate skeletal and cardiac muscle and in invertebrate striated muscle are reviewed. The force developed, the speed of shortening, and stiffness are compared with structural changes occurring within the lattice. Radial forces between the filaments in the lattice, which can include electrostatic, Van der Waals, entropic, structural, and cross bridge, are assessed for their contributions to lattice stability and to the contraction process.

Inhibition of mitochondrial calcium uptake slows down relaxation in mitochondria-rich skeletal muscles

Isolated fibres from various muscles were skinned mechanically in oil. From a Ca2+-loaded micropipette, local applications of Ca2+ were made. These produced a limited contraction which relaxed spontaneously. The time-course of sarcomere shortening and re-lengthening was recorded by microcinephotography. Application of Ruthenium Red, a potent and specific inhibitor of Ca2+ uptake by mitochondria, did not affect the contraction-relaxation cycles of typical glycolytic white fibres (frog sartorius, pigeon breast). By contrast, Ruthenium Red greatly slowed down the relaxation rate in mitochondria-rich fibres (rat soleus and rabbit masseter). In these fibres, Ca2+ uptake by mitochondria seems to play an active role in promoting relaxation.

Effect of the organic Ca2+ channel blocker D-600 on sarcoplasmic reticulum Ca2+ uptake in skeletal muscle

Experiments were undertaken to study the possibility that the calcium channel blocker D-600 (gallopamil), which penetrates into muscle cells (20), facilitates excitation-contraction coupling in skeletal muscle (7) by a direct effect on the sarcoplasmic reticulum (SR). The effects of D-600 were studied on single phasic muscle fibers, either intact or split open. D-600 potentiated twitches in intact fibers at concentrations lower than those reported in whole muscles. In split fibers, the force produced by caffeine-induced Ca2+ release from the SR was reversibly inhibited by 5 microM D-600, when added to the Ca2+ loading solution. This inhibitory effect was inversely related to temperature, and it was dose dependent. When D-600 was added after Ca2+ loading and before caffeine exposure, or during the caffeine exposure itself, it did not inhibit Ca2+ release, but rather increased the development of force. We conclude that, apart from the blocking effect that D-600 may have on the voltage sensor, the drug penetrates into the myoplasm and affects excitation-contraction coupling by inhibiting the SR Ca2+ pump. This may be the consequence of a conformational change in the transmembrane Ca2+ binding domain of the ATPase.

Free energy and enthalpy of ATP hydrolysis in the sarcoplasm

A rigorous calculation of the free energy available in vivo from ATP hydrolysis requires the following information which is not all available, namely: (i) intra­cellular pH, (ii) activities of all the species of reactants and products in sarcoplasm, (iii) thermodynamic data for all the reactions involved, including values for ionic strength and temperature dependence, and (iv) the extent of deviation from equilibrium conditions, i. e. during contraction. We shall discuss each of these factors in turn and state the assumptions made that allow the approximate calculation of the free energy made available by the following net reaction in the sarcoplasm: ATP +H 2 O → ADP + Pi + H + . (1) Although it can only be an approximation this calculation is useful since it will take into account recent thermodynamic measurements in vitro .

LOCALIZATION OF CALCIUM-ACCUMULATING STRUCTURES IN STRIATED MUSCLE FIBERS

When frog muscle fibers from which the sarcolemma had been dissected away were perfused with a calcium solution and then treated with oxalate, electron-opaque material, probably calcium oxalate, accumulated in the terminal sacs of the sarcoplasmic reticulum. These regions of calcium accumulation were identified with the intracellular calcium sink that controls the relaxation phase of the contraction-relaxation cycle; their proximity to tubules implicated in intracellular stimulus conduction suggests that they might also be regions from which calcium is released to trigger contraction.

The morphology and mechanical properties of endomysium in series-fibred muscles: variations with muscle length

In the series-fibred muscle architecture commonly found in large muscles of mammals and birds, the intrafasciculary-terminating muscle fibres have no direct tendinous attachments. Contractile force produced in these fibres must be transmitted between adjacent muscle fibres via the endomysial connective tissue which separates them. The endomysium is thus an essential mechanical component in such muscles. Studies of motor end-plate banding patterns and the frequent occurrence of tapering ends of fibres within the fascicles of the bovine sternomandibularis muscle show it to be a series-fibred muscle. Sodium hydroxide digestion of fixed samples of this muscle to remove the myofibrillar apparatus revealed the endomysium to be a disordered planar network of mainly curvilinear collagen fibrils. The orientation distribution of the collagen fibrils in the endomysial network was measured by image analysis of scanning electron micrographs. Analysis of endomysial preparations from muscle fixed at sarcomere lengths between 1-4 microns showed that the orientation distribution of collagen fibrils is quantitatively related to muscle length. At rest sarcomere length the collagen fibril network is not completely random, but has a slight circumferential bias. The orientation distribution shows a progressive shift towards the circumferential direction at short sarcomere lengths and towards the longitudinal direction at long sarcomere lengths. The relationship between the number-weighted mean collagen orientation and sarcomere length was compared to two geometric models of network behaviour, the isoareal and constant shape models. Both fitted the data reasonably, although the constant shape model described the rate of change of mean orientation more closely. From fibrous composites theory, the reinforcement efficiency factor, eta, was calculated from the measured collagen fibril orientation distributions. These calculations predict a non-linearly increasing longitudinal tensile modulus for the endomysium with increasing sarcomere length, in agreement with its known non-linear properties, but confirm that the tensile properties of the endomysium are unsuitable for transmission of tensile force from muscle fibres contracting near rest length. This reinforces a previous interpretation that contractile force is transmitted between neighbouring muscle fibres by trans-laminar shear through the endomysium rather than by in-plane tension.

Ultrastructural comparison of slack and stretched myotendinous junctions, based on a three-dimensional model of the connecting domain

The vertebrate myotendinous junction contains junctional microfibrils, located in the lamina lucida of the basement membrane. The junctional microfibrils are thought to transmit muscular force across the junctional lamina lucida, also called the connecting domain. If true, deformation of the terminal muscle cell processes and connecting domain during force transmission would be detected as a change in spacing and/or orientation of the junctional microfibrils. This study compared connecting domain morphology in frog semitendinosus muscles fixed in two extremes of resting tension, to elucidate the mechanical properties of the myotendinous junction. An initial study of connecting domain ultrastructure revealed that junctional microfibrils are punctate or spinelike in shape, and that they are distributed in a linear, helically-oriented array on the muscle cell surface. The rows in the surface lattice are 10-15 nm in thickness, have a centre-to-centre distance between rows of approximately 24 nm, and are oriented at approximately 41 degrees with respect of the long axis of the muscle fibre. Comparison of slack and highly stretched myotendinous junctions shows no significant changes in spacing or orientation of either individual junctional microfibrils or rows in the helical surface lattice. Thus, both the connecting domain and terminal cell processes at the myotendinous junction are essentially inextensible under the loading conditions used in this study.

Divalent cation-dependent adhesion at the myotendinous junction: ultrastructure and mechanics of failure

Junctional microfibrils, which span the lamina lucida of the vertebrate myotendinous junction, are thought to function in force transmission at the junction. This hypothesis has been tested by disrupting junctional microfibrils through elimination of extracellular divalent cations, and determining the effects of this treatment on the ultrastructure and mechanics of whole frog skeletal muscles passively stretched to failure. Muscles incubated in divalent cation-free solution failed exclusively in the lamina lucida of the myotendinous junction, while control muscles all failed within the muscle fibres, several millimetres away from the junction. Failure sites from divalent cation-free muscles incubated with antibodies against collagen type IV, laminin, and tenascin showed no labelling of the avulsed ends of the muscle fibres, indicating that remnants of junctional microfibrils observed on the cell surface are not composed of any of these extracellular proteins. All three proteins were present on the tendon side of the failure site, confirming that the lamina densa remains attached to the tendon. Breaking stress for control muscles was 3.47 x 10(5) N m-2, and for divalent cation-free muscles, 1.84 x 10(5) N m-2, or approximately half the control value. Breaking strain averaged 1.17 for divalent cation-free muscles and 1.39 for controls, although the difference was not significant. We conclude that junctional microfibrils are components of a divalent cation-dependent adhesion mechanism at the myotendinous junction. In addition, ultrastructural analysis of divalent cation-free fibres stretched just short of failure suggests that a second, divalent cation-independent mechanism persists along the non-junctional cell surface, and can transmit substantial passive tension from myofibrils laterally to the extracellular matrix, bypassing the failed myotendinous junction.

Scattering of ultrasound from skeletal muscle tissue

The purpose of this work was to explore the mechanisms which are responsible for the scattering of ultrasound from skeletal muscle tissue. It was undertaken in response to an interesting phenomenon observed in the authors' laboratory whereby scattering power from avian skeletal muscle changed in concordance with passive stretch. Ultrasonic scattering from skeletal muscle samples was measured as they were stretched passively in increments of 10% of their original length up to 40%. The samples were illuminated with an ultrasound beam from a transducer which was oriented orthogonally to and at 20 degrees from the normal to the long axis of the muscle sample. It was found that the integrated backscatter increased significantly over the strain range for the orthogonal orientation, but it changed very little after the initial stretch when the orientation was 20 degrees . It is postulated that this phenomenon may be caused by reorientation of the endomysial collagen fibers surrounding each muscle fiber.

Functional morphology of the endomysium in series fibered muscles

Many skeletal muscles, including the feline biceps femoris, are composed of short, tapered myofibers arranged in an overlapping longitudinal series. The endomysium of such muscles transfers tension between overlapping myofibers, and is thus an elastic element in series with them. The endomysium of the cat biceps femoris contains curvilinear collagen fibrils in an approximately isotropic (random) array. The collagen fibrils undergo only a modest reorientation as the myofibers shorten or lengthen within the physiological range. A geometrical model predicts no change in the thickness of the endomysium on changing muscle fiber length and quantifies the expected collagen fibril reorientation in the endomysium as a function of muscle extension. It is also demonstrated that a high proportion of the collagen fibrils will be curvilinear at all sarcomere lengths. The organization of endomysial collagen is appropriate for the transfer of loads between myofibers by means of shear.

Effect of thin filament length on the force-sarcomere length relation of skeletal muscle

We studied a slow- and a fast-twitch muscle fiber type of the perch that have different thin filament lengths. The force-sarcomere length relations were measured, and it was tested whether their descending limbs were predicted by the cross-bridge theory. To determine the predicted relations, filament lengths were measured by electron microscopy. Measurements were corrected for shrinkage with the use of I-band and H-zone periodicities. Thick filament lengths of the two fiber types were found to be similar (1.63 +/- 0.06 and 1.64 +/- 0.10 microns for slow- and fast-twitch fibers, respectively), whereas the thin filament lengths were clearly different: 1.24 +/- 0.10 microns (n = 86) for the slow-twitch type and 0.94 +/- 0.04 microns (n = 94) for the fast type. The descending limbs of the two fiber types are therefore predicted to be shifted along the sarcomere length axis by approximately 0.6 microns. Sarcomere length was measured on-line by laser diffraction in a single region in the center of the fibers. The passive force-sarcomere strain relation increased much more steeply in the slow-twitch fibers. The descending limb of the active force-sarcomere length relation of fast twitch fibers was linear (r = 0.92), but was found at sarcomere lengths approximately 0.1 micron greater than predicted. The descending limb of the slow-twitch fibers was also linear (r = 0.87) but was now found at sarcomere lengths approximately 0.05 microns less than predicted. The difference in position of the descending limbs of the two fiber types amounted to 0.5 microns, approximately 0.1 micron less than predicted. The difference between measured and predicted descending limbs was statistically insignificant.

Effect of active pre-shortening on isometric and isotonic performance of single frog muscle fibres

1. We studied the effects of shortening history on isometric force and isotonic velocity in single intact frog fibres. Fibres were isometrically tetanized. When force reached a plateau, shortening was imposed, after which the fibre was held isometric again. Isometric force after shortening could then be compared with controls in which no shortening had taken place. 2. Sarcomere length was measured simultaneously with two independent methods: a laser-diffraction method and a segment-length method that detects the distance between two markers attached to the surface of the fibre, about 800 microns apart. 3. The fibre was mounted between two servomotors. One was used to impose the load clamp while the other cancelled the translation that occurred during this load clamp. Thus, translation of the segment under investigation could be minimized. 4. Initial experiments were performed at the fibre level. We found that active preshortening reduced isometric force considerably, thereby confirming earlier work of others. Force reductions as large as 70% were observed. 5. Under conditions in which there were large effects of shortening at the fibre level, we measured sarcomere length changes in the central region of the fibre. These sarcomeres shortened much less than the fibre's average. In fact, when the load was high, these sarcomeres lengthened while the fibre as a whole shortened. Thus, while the fibre-length signal implied that sarcomeres might have shortened to some intermediate length, in reality some sarcomeres were much longer, others much shorter. 6. Experiments performed at the sarcomere level revealed that isometric force was unaffected by previous sarcomere shortening provided the shortening occurred against either a low load or over a short distance. However, if the work done during shortening was high, force after previous shortening was less than if sarcomeres had remained at the final length throughout contraction. The correlation between the force deficit and the work done during shortening was statistically significant (P = 0.0001). 7. Interrupting the tetanus for 0.5-3.0 s did not reverse the effects of shortening on isometric force; at least 5-10 min of rest were required before force recovered completely. 8. Sarcomeres accelerated during the period of shortening under constant load, indicating that the sarcomeres became progressively stronger. However, the acceleration was less than that predicted from the force-velocity relation applicable at each of the sarcomere lengths transversed during shortening. 9. Velocity of shortening appeared to be much more sensitive to previous shortening than isometric force. 10. Results obtained with the diffraction method were the same as those obtained with the segment method.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of collagen crosslinking in the increased stiffness of avian dystrophic muscle

The resting tension and stiffness in the range of sarcomere lengths 2.4-3.6 microns were studied in highly inbred normal and dystrophic chicken pectoral muscle bundles, and the results were compared with the collagen content and the extent of crosslinkage of the collagen. All parameters increased in the order normal homozygote (003/003) less than heterozygote (003/433) less than dystrophic homozygote (433/433) chickens, with the data from the heterozygotes being halfway between the two homozygotes, thus exhibiting a semi-dominant inheritance pattern. In separate experiments, lathyrism was induced by treating normal (412/412) and dystrophic (413/413) chickens with alpha-acetoaminonitrile, an inhibitor of lysyl oxydase, the enzyme responsible for the initiation of collagen crosslinkage formation. These experiments showed that the tension and stiffness in response to passive stretch did not change with lathyrism in normal muscles, whereas the tension and stiffness decreased significantly with lathyrism in dystrophic muscles. The collagen content did not change with lathryrism in both normal and dystrophic muscles. These results indicate that the increased content of collagen crosslinkages is the basis for the increased resting tension and stiffness in the dystrophic muscles of the chicken, and that the effects can be reversed by treatment with an inhibitor of collagen crosslinkage formation.

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.

Positioning of actin filaments and tension generation in skinned muscle fibres released after stretch beyond overlap of the actin and myosin filaments

Skinned fibres from frog semitendinosus muscle were stretched in relaxing solution from a sarcomere length of 2.5 microns to greater sarcomere lengths, and then shortened back to the original length. Fibres could be stretched up to sarcomere lengths of 3.3 microns, and reshortened fully. If the original stretch was to a sarcomere length greater than 3.3 microns, the extent of recovery was dependent on the magnitude of the stretch and the number of times the stretch/shorten cycle was repeated. When the original stretch was to sarcomere lengths beyond overlap of the thick and thin filaments, the thin filaments did not re-enter the thick filament array but buckled at the A-I junction. If these fibres were subsequently activated and contracted, the thin filaments re-entered the thick filament array, taking up approximately their former positions, and allowing reduced development of isometric tension.

A physiological role for titin and nebulin in skeletal muscle

Production of active force in skeletal muscle results from the interaction of myosin-containing thick filaments with actin-containing thin filaments. These muscles are also passively elastic, producing forces that resist stretch independently of ATP splitting or of interaction between the filaments. The mechanism of this passive elasticity is unknown; suggestions include gap filaments in the region between thick and thin filaments in muscles stretched beyond filament overlap, or intermediate filaments which connect successive Z-disks. Recently, the two exceptionally large proteins titin (also called connectin) and nebulin (originally called band 3) have been implicated in passive elasticity (for review see refs 7, 8). Here, we show that after these proteins are degraded by low doses of ionizing radiation, the ability of single skinned muscle cells to generate both passive tension in response to stretch and active tension in response to calcium is greatly reduced. These effects are accompanied by axial misalignment of thick filaments. Titin and/or nebulin apparently provide axial continuity for the production of resting tension on stretch and also tend to keep the thick filaments centred within the sarcomere during force generation.

Lattice shrinkage with increasing resting tension in stretched, single skinned fibers of frog muscle

The 1,0 lattice spacing d1,0 in chemically and mechanically skinned single fibers of frog muscle was measured at various sarcomere lengths, L, in the range from L = 2.1 to 6.0 microns by an x-ray diffraction method. In chemically skinned fibers, d1,0 decreased with a similar slope to that of mechanically skinned fibers up to L congruent to 3 microns, but beyond this point d1,0 steeply decreased with further stretching. This steep decrease in d1,0 could be ascribed mainly to an increase in the compressing force associated with the longitudinal extension of a remnant of the sarcolemma. In mechanically skinned fibers, the gradual decrease in d1,0 continued beyond filament overlap (L greater than or equal to 3.5 microns) and was highly proportional to a resting tension. This decrease in d1,0 at L greater than or equal to 3.5 microns could be ascribed to an increase in the force exerted by lateral elastic components, which is proportional to the longitudinal resting tension. A conceptual model is proposed of a network structure of elastic components in a sarcomere.

Myofibrils bear most of the resting tension in frog skeletal muscle

The tension that develops when relaxed muscles are stretched is the resting (or passive) tension. It has recently been shown that the resting tension of intact skeletal muscle fibers is equivalent to that of mechanically skinned skeletal muscle fibers. Laser diffraction measurements of sarcomere length have now been used to show that the exponential relation between resting tension and sarcomere length for whole frog semitendinosus muscle is similar to that of single fibers. Slack sarcomere lengths and the rates of stress relaxation in these muscles were similar to those in skinned fibers, and sarcomere length remained unchanged during stress relaxation, as in skinned fibers. Thus, in intact semitendinosus muscle of the frog up to a sarcomere length of about 3.8 micrometers, resting tension arises, not in the connective tissue as is commonly thought, but in the elastic resistance of the myofibrils.

Thixotropic behaviour of human finger flexor muscles with accompanying changes in spindle and reflex responses to stretch

Prompted by previous reports on muscle thixotropy, we have investigated changes in inherent and reflex stiffness of the finger flexor muscles of human subjects at rest, following transient conditioning manoeuvres involving contractions and/or length changes of the finger flexors. The stiffness measurements were combined with electromyographic recordings from forearm and hand muscles and with microneurographic recordings of afferent stretch responses in finger flexor nerve fascicles. Finger flexor stiffness was evaluated by measuring (a) the flexion angle of the metacarpo-phalangeal joints at which the system during rest balanced the force of gravity and (b) the speed and amplitude of angular finger extensions induced by recurrent extension torque pulses of constant strength delivered by a torque motor. In the latter case, extension drifts in the resting position of the fingers were prevented by a weak flexion bias torque holding the fingers in a pre-determined, semiflexed position against a stop-bar. Stiffness changes following passive large amplitude finger flexions and extensions were studied in subjects with nerve blocks or nerve lesions preventing neurally mediated contractions in the forearm and hand muscles. Inherent stiffness was enhanced following transient finger flexions and reduced following transient finger extensions. The after-effects gradually declined during observation periods of several minutes. Similar results were obtained in subjects with intact innervation who succeeded during the pre- and post-conditioning periods in keeping the arm and hand muscles relaxed (i.e. showed no electromyographic activity). In these subjects it was also found that the after-effects were similar for active and passive finger movements and that isometric voluntary finger flexor contractions loosened the system in a way similar to finger extensions. In some subjects electromyographic reflex discharges appeared in the finger flexors in response to the extension test pulses. When elicited by small ramp stretch stimuli of constant amplitude, the stretch reflex responses were found to vary in strength in parallel with the changes in inherent stiffness following the various conditioning manoeuvres. The strength of the multi-unit afferent stretch discharges in the muscle nerve, used as index of muscle spindle stretch sensitivity, varied in parallel with the changes in inherent stiffness. Post-manoeuvre changes in muscle spindle stretch sensitivity were seen also when the spindles were de-efferented by a nerve block proximal to the recording site. The results can be explained in terms of thixotropic behaviour of extra- and intrafusal muscle fibres.(ABSTRACT TRUNCATED AT 400 WORDS)

The descending limb of the sarcomere length-force relation in single muscle fibres of the frog

Single muscle fibres, isolated from the tibialis anterior muscle of the frog, were used to study intersarcomere dynamics during muscle-isometric (fixed-end) tetani at long sarcomere lengths. Sarcomere length was measured by an online laser diffraction technique. On the descending limb of the length-force relation, the slow rise of force (creep) was always associated with changes in sarcomere length. Sarcomeres at the ends of the fibres shortened, while those of the central 90% of the fibre length were stretched. Fibres were found to have a range of passive length-force curves, those with high resting forces developed little creep force, while low resting force fibres developed substantial creep, resulting in a fixed-end sarcomere length-force relation which deviated greatly from that expected from crossbridge theory. These differences in creep force can be qualitatively accounted for by differences in sarcomere dynamics. The simultaneous measurement of force and sarcomere length during force development allows the construction of a 'sarcomere-isometric' length-force curve from minima in the sarcomere length record. Force declined linearly from a plateau at 2.2 microns to zero at a sarcomere length close to 3.65 microns. The online, diffraction-derived sarcomere length was used in a feedback loop to clamp sarcomere length in short (100-200 microns) segments of fibres. A length-force curve constructed from sarcomere length-clamped tetani shows a linear decline in force from a plateau at 2.2 microns to zero at a sarcomere length of 3.65 microns.

Stiffness and contractile properties of avian normal and dystrophic muscle bundles as measured by sinusoidal length perturbations

Both tension and stiffness as a function of muscle length were measured under relaxing conditions on isolated small bundles of chemically skinned myofibers from normal and dystrophic chicken pectoral muscles. It was shown that the dystrophic muscle was stiffer than normal muscle and developed more tension for the same amount of stretch. A fraction of stiffness was not removed by extraction with either 0.6 M KI or with 5 M guanidine HCl mixed with 1% mercaptoethanol. The stiffness of dystrophic muscle was also unaffected by treatment with bacterial collagenase under conditions that destroyed the stiffness of tendon. Nyquist plots of normal and dystrophic muscles during calcium-activated isometric contraction were very similar and were characteristic of fast-twitch muscle, as evidenced by three clear exponential processes. The normal appearance of the Nyquist plot of dystrophic muscle demonstrates that cross-bridge function is not altered, and the characteristic slowing of contraction and relaxation is not a consequence of a fast-to-slow transformation of muscle types. The increased stiffness of dystrophic muscle may be a very fundamental change in the biomechanics of dystrophy. We postulate that the stiffness is mediated by an altered form of collagen, which is collagenase-resistant by virtue of excessive crosslinking.

Localization of the parallel elastic components in frog skinned muscle fibers studied by the dissociation of the A- and I-bands

Localization of the parallel elastic components (PECs) in skinned muscle fibers was investigated by analyzing the change of the resting tension, which accompanies the dissociation of the A- and I-bands. The A-band was dissociated from both ends by increasing the concentration of KCl under relaxing conditions (0.09-0.54 M KCl, 4.0 mM MgATP, 1.0 mM Mg2+, 4.0 mM EGTA, pH 6.0-9.0, 20 degrees C). At sarcomere lengths greater than or equal to 3.5 microns, the length of the A-band was estimated by comparing the intensity of the first-order optical diffraction line with the results of model calculations. These results were supported by differential-interference microscopy and sodium dodecyl sulfate gel electrophoresis. It was shown that the resting tension decreased nearly in proportion to the residual length of the A-band. At sarcomere lengths less than or equal to 4.0 microns, the resting tension after the dissociation of the A-band was lowered to less than 10% of the initial value. On the other hand, at sarcomere lengths greater than or equal to 5.0 microns the resting tension after the dissociation of the A-band still showed approximately 35% of the initial value and did not change even after the I-band was dissociated by a solution containing KI. From these results, we propose that most of the PECs contributing to resting tension bind almost uniformly to the A-band and there are also PECs connecting Z-lines.

Resonance at the wrist demonstrated by the use of a torque motor: an instrumental analysis of muscle tone in man

The resonance of the relaxed wrist for flexion-extension movements in the horizontal plane has been investigated by using rhythmic torques generated by a printed motor. In the normal subject the resonant frequency of the wrist is ca. 2 Hz unless the torque is reduced below a certain critical value when the system is no longer linear and the resonant frequency rises. This critical torque level, and the damping are both less in women than men. The resonant frequency is uninfluenced by surgical anaesthesia. With added bias the increase of resonant frequency at low torques still occurs although the hand is now oscillating about a displaced mean position. It follows that the stiffening implied by this elevation of resonant frequency for small movements is neither the result of pre-stressing of the muscles nor of reflex activity. With velocity feed-back of appropriate polarity the system will oscillate spontaneously at its resonant frequency. If the peak driving torque is progressively reduced the resonant frequency increases abruptly, indicating that the system has stiffened. Perturbations delivered to the wrist may reduce its stiffness. The postural system is thixotropic with a 'memory time' of 1-2 s. The resonant frequency is elevated in voluntary stiffening.

Cinematographic studies on the A-band length changes during Ca-activated contraction in horseshoe crab muscle myofibrils

Cinematographic recordings of sarcomere shortening were performed on glycerinated horseshoe crab muscle myofibrils during Ca-activated contraction. When the preparations at slack length (sarcomere length, 7-9 microns) were locally activated with iontophoretically applied Ca ions, the A-band length did not change appreciably while the activated sarcomeres shortened linearly with a velocity similar to the maximum shortening velocity measured on intact muscle fibers. If, on the other hand, previously stretched preparations (sarcomere length, 11-14 microns) were locally activated, the A-band length first increased by 40-50% and then shortened to the initial length, while the activated sarcomeres continued to shorten. These results indicate that the thick filament shortening may not be associated with the physiological sarcomere shortening; the transient A-band lengthening with long initial sarcomere lengths may result from the transient misalignment of the thick filaments followed by their realignment, implying that the force exerted by the cross-bridge is not constant but may vary according to its past history.

Lateral transmission of tension in frog myofibers: a myofibrillar network and transverse cytoskeletal connections are possible transmitters

The extensibility of the sarcolemma of single myofibers can be reduced locally by leaving a segment covered by a sleeve of surrounding tissue composed of cut myofibers, blood vessels, and connective tissue, hereafter referred to as "the splint." Splinted fibers from frog semitendinosus muscle were used to study mechanical connections (transverse coupling) between myofibrillar components and sarcolemma. The transverse coupling is strong enough to insure a tight correlation between myofibril length and overlying sarcolemma length in both resting and activated fibers and to transmit nearly maximum isometric tension to the splint. Lateral transmission of active tension was demonstrated with a preparation which had the distal two-thirds of an intact fiber covered by a splint and the proximal third dissected clean. When the outer end of the splint was pinned down and only the distal tendon was held, tension generated in the splinted fiber was transmitted to, and recorded from, the splint. Parameters of isometric tension transmitted laterally were not significantly different from those of tension transmitted longitudinally. Myofibrils branch profusely and form a network that may act as a unitary force generator and transmitter. In splinted fibers its output is possibly picked up circumferentially and transmitted across the sarcolemma by a microfilament network. A cap of relatively inextensible sarcolemma "splints" myofiber ends. Resting tension is transmitted to and from the myofibrils by transverse coupling beyond the cap and the region of short sarcomere spacing it covers. Transverse cytoskeletal connections at Z and M regions are described. Immobilization of the sarcolemma allows study of myofibril-sarcolemma linkage in intact fibers. Both active and resting tension were transmitted laterally.

Activation of fast skeletal muscle: contributions of studies on skinned fibers

The membrane potential of vertebrate twitch fibers closely controls Ca fluxes between intracellular compartments, which in turn control contraction. Recent work on intracellular Ca movement is reviewed in the general context of current efforts to synthesize physiological, biochemical, and structural observations on the contractile mechanism and its regulation, emphasizing the increasing role of functionally skinned fibers in this synthesis. Skinned fiber preparations, with removed or disrupted sarcolemma, bridge the gap between properties of isolated subsystems and their constrained operation in the intact fiber. Recent studies indicate that the surface action potential propagates along the transverse tubules, but not the sarcoplasmic reticulum (SR), which appears to be a distinct intracellular compartment. Voltage-dependent charge movements in the transverse tubules probably control Ca flux across the SR membranes. Current questions concern the mechanism of the signal that bridges the junctional gap between the two membrane systems, the mechanism and properties of the activated Ca efflux to the myofilament space, and the operation of the Ca pump of the SR during activation. New methods applied to intact fibers, cut fibers, skinned fibers, and subcellular systems are yielding the kind of information needed for a complete description of these central steps in excitation-contraction coupling and of Ca regulation of the myofilaments.

Elastic and viscous properties of resting frog skeletal muscle

The mechanical properties of the resting, whole semitendinosus muscle of the frog have been characterized as functions of both muscle length and temperature. Measurements were made of pseudorandom white noise (PRWN) displacements (less than 10 A/half-sarcomere) applied to the muscle and the force responses to these movements. Signal correlation techniques were then used to obtain the dynamic modulus function for the muscle in the frequency range 2.44-320 Hz. This function was represented by a series combination of a Voigt element and a time delay element for tension propagation along the muscle. A dynamic elastic modulus (E), coefficient of damping (B), and tension transmission velocity (V) were measured for resting muscle on the basis of this model. For each of these parameters, a marked variation with sarcomere length (s) was found. The mean values for E and B at LO (s=2.25 mum) were 1.84+/-0.24 X 10(5) N/m2 and 2.33+/-0.25 X 10(2) Ns/m2, respectively. Further, B demonstrated a negative temperature dependence, Q10=0.78 (P less than 0.05), in the range s=2.6-3.0 mum, while E was not significantly temperature dependent. The length-dependent variations of E and B are interpreted as deriving from both passive muscle elements and attached crossbridges. Velocity was calculated at a single displacing frequency for every experiment; the mean value at LO and all temperatures was v=11.7+/-0.6 m/s. Velocity was also calculated as a function of frequency within several experiments: the results indicate considerable variation of v with frequency.

Concentration and adenosinetriphosphatase activity on left ventricular actomyosin in Goldblatt rats during the compensatory stage of hypertrophy

Left ventricular myocardia of Goldblatt rats with an average increase in arterial blood pressure to about 200 mm Hg showed a progressive reduction of the Ca-activated specific acotmyosin ATPase activity 4 -12 weeks after the coarctation of one renal artery, as compared with controls of the same age. During the same period, a significant increase in the concentration of contractile proteins was noticeable, whereas the content of nonprotein substances and of water corresponded to the control values. The hydroxyproline concentration, as a measure of the collagen tissue content, increased only after 24 weeks. The time course of the specific ATPase activity was closely parallel to the decrease in the unloaded myocardial shortening velocity, as estimated at the same stage by our group. This is in accordance with the assumption of a fundamental relationship between the two values. The reduced rate of energy turnover and of the shortening velocity is regarded as an adaptive mechanism which, however, has a negative effect in advanced hypertrophy when further diminution takes place. The decrease in the specific enzymatic activity of actomysin is not necessarily linked to a large increase in myocardial mass, but is already apparent at moderate degrees of hypertrophy (34%).

Studies on the relation between latency relaxation and resting cross-bridges of frog skeletal muscle

Latency relaxation of frog skeletal muscle (LR) was investigated with respect to its relation to resting cross-bridges. A decrease in the initial stiffness of the resting muscle (stiffness of the muscle during the beginning of a length-change) was found, when repeated triangular length-changes were imposed on the muscle. This decrease in the initial stiffness depends on the velocity of the length-change. It is interpreted that the decrease in the initial stiffness reflects a detachment of the resting cross-bridges from their binding-sites. The LR, induced immediately after the offset of the length-changes, i.e. when the cross-bridges are still detached, showed an increased depth, its time course remaining unchanged. There is a strong correlation between the increase in the depth of the LR and the decrease in the initial stiffness. The LR regained its original depth (depth without a preceding length-change) about 5 s after the offset of the length-changes (20 degrees C). It is suggested that the LR of skeletal muscle is not due to a detachment of resting cross-bridges.

The relative contributions of the folds and caveolae to the surface membrane of frog skeletal muscle fibres at different sarcomere lengths

The plasmalemmal area of striated muscle fibres is greater than the apparent surface area (A = circumference x length) because of variable folds and the invaginations of the caveolae and T-tubules. Freeze-fracture replicas of the surface membrane of sartorius and semitendinosus muscles from Rana pipiens have been used to determine the numbers and distribution of folds and caveolae at different sarcomere lengths. (1) The plasmalemma folds are variable in size and shape, but are always oriented perpendicular to the long axis of the fibre. The folds vary with stretch, being more prominent at short sarcomere lengths. The caveolae are elliptical invaginations of the plasmalemma which open to the outside by a narrow "neck" of approximately 20 nm. The caveolar lumen has an average long dimension of 81.6 +/- 11.7 nm and an average short dimension of 66.9 +/- 7.9 nm. The caveolar "necks" only can be seen in freeze-fracture replicas and these are distributed in two circumferential bands on either side of the Z-line, and in longitudinal bands separated by distances of 1-5 mum. In the sartorius muscle, at a sarcomere length of 2.8 mum, there is an average number of thirty-seven caveolae per square micrometer of fibre surface. (2) During passive stretch the opening of folds provides membrane for the necessary increase in surface area up to a sarcomere length of about 3.0 mum. This length is defined as the critical sarcomere length (Sc). The number of caveolae remains constant at all sarcomere lengths less than Sc and thus their "necks" have been used as membrane markers to determine the amount of folding at different sarcomere lengths. The membrane area contained in folds and caveolae is expressed as a fraction of the apparent surface area (A). For example, in the sartorius muscle, at a sarcomere length of 2.4 mum, the membrane area, excluding the T-tubules, is: A + 0.1A (folding) + 0.7A (caveolae) = 1.8A. (3) For stretch beyond Sc membrane is provided by the opening of caveolae. At a sarcomere length of about 8 mum all the caveolae are open and the fibres rupture with further stretch. (4) The relative contributions of folds and caveolae vary with sarcomere length in a way that is consistent with assumptions of constant volume and plasmalemma area. The maintenance of constant plasmalemma area, even after excessive stretch, suggests that the plasmalemma is relatively inelastic in this situation.

Muscular contraction

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