High frequency characteristics of elasticity of skeletal muscle fibres kept in relaxed and rigor state

Summary The viscoelastic properties of crossbridges in rigor state are studied by means of application of small length changes, completed within 30 s, to isometric skinned fibre segments of the iliofibularis muscle of the frog in relaxed and rigor state and measurement of the tension response. Results are expressed as a complex Young's modulus, the real part of which denotes normalized stiffness, while the imaginary part denotes normalized viscous mechanical impedance. Young's modulus was examined over a wide frequency range varying from 5 Hz up to 50 kHz. Young's modulus can be interpreted in terms of stiffness and viscous friction of the half-sarcomere or in terms of elastic changes in tension and recovery upon a step length change.The viscoelastic properties of half-sarcomeres of muscle fibre segments in rigor state showed strong resemblance to those of activated fibres in that shortening a muscle fibre in rigor state resulted in an immediate drop in tension, after which half of the drop in tension was recovered. The following slower phases of tension recovery—a subsequent drop in tension and slow completion of tension recovery—as seen in the activated state, do not occur in rigor state. The magnitude of Young's moduli of fibres in rigor state generally decreased from a value of 3.12×10⁷ N m^-2 at 40 kHz to 1.61×10⁷ N m^-2 at about 100 Hz.Effects of increased viscosity of the incubation medium, decreased interfilament distance in the relaxed state and variation of rigor tension upon frequency dependence of complex Young's modulus have been investigated. Variation of tension of crossbridges in rigor state influenced to some extent the frequency dependence of the Young's modulus. Recovery in relaxed state is not dependent on the viscosity of the medium. Recovery in rigor is slowed down at raised viscosity of the incubation medium, but less than half the amount expected if viscosity of the medium would be the cause of internal friction of the half-sarcomere. Internal friction of the half-sarcomere in the relaxed fibre at the same interfilament distance as in rigor is different from internal friction in rigor. It will be concluded that time necessary for recovery in rigor cannot be explained by friction due to the incubation medium. Instead, recovery in rigor expressed by the frequency dependence of the Young's modulus has to be due to intrinsic properties of crossbridges. These intrinsic properties can be explained by the occurrence of state transitions of crossbridges in rigor. Similarity of Young's modulus of fibre segments in the activated state and in rigor in the frequency range above 5 kHz strongly suggests that the same state transitions occur in force generating crossbridges in the activated fibre.     

The complex Young's modulus of skeletal muscle fibre segments in the high frequency range determined from tension transients

Summary Stiffness measurements of muscle fibres are often based on application of a length change at one end of the muscle fibre and recording of the following tension change at the other end. In this study a method is developed to determine in the high frequency range (up to 40 kHz) the complex Young's modulus of skeletal muscle fibre as a function of frequency from the tension transient, following a rapid stepwise length change completed within 40 s. For this purpose both a new mechanical moving part of the displacement generating system and a force transducer with a high natural frequency (70 kHz) had to be developed. In addition to stiffness measurements of a silk fibre to test the displacement generating system and the method of analysis, stiffness of skeletal muscle fibres in relaxed and rigor state have been measured. The complex Young's moduli of relaxed muscle fibres as well as muscle fibres in rigor state are frequency dependent. In both cases the complex Young's modulus increases smoothly with increasing frequency over a range of 250 Hz up to 40 kHz. The phase angles of the responses remained almost constant at a value of 0.3 radians for a fibre in rigor and 0.6 radians for a relaxed fibre. This leads to the conclusion that for muscle fibres in rigor state the recovery in the tension response to a step length change shows a continuous distribution of relaxation times rather than a few discrete ones. Results of our stiffness measurements are compared with results obtained from current viscoelastic models used to describe stiffness of muscle fibre in this frequency range. The frequency dependence of stiffness of muscle fibres in rigor state suggests that the frequency used has to be taken into account while determining the elastic range of a crossbridge.     

Tension and stiffness of frog muscle fibres at full filament overlap

Summary Stiffness measurements in activated skeletal muscle fibres are often used as one means of estimating the number of attached crossbridges on the assumption that myofilament compliances do not contribute significantly to the fibre compliance. This assumption was tested by studying the effects of sarcomere length on fibre stiffness in the plateau region of the length-tension diagram (from 1.96 to 2.16m sarcomere length in the tibialis anterior muscle of the frog). Lengthening of the sarcomere across this region in fact, produces only an increase in the proportion of actin filament free from cross-bridges without altering the amount of effective overlap; no change in fibre stiffness is therefore expected if actin filaments are perfectly rigid. The results show that while tetanic tension remained constant within 1.5%, as the sarcomere length was increased from 1.96 to 2.16m fibre stiffness decreased by about 4%, indicating that a significant proportion of sarcomere compliance is localized in the actin filaments. A simple model based on the sliding filament theory was used in order to calculate the relative contribution of actin filaments to fibre compliance. In the model it was assumed that fibre compliance resulted from the combination of crossbridge compliance (distributed over the overlap zone) in series with thin filament and tendon compliances. The calculations show that the experimental data could be adequately predicted only assuming that about 19% of sarcomere compliance is due to actin filament compliance.     

Stiffness of carbodiimide-crosslinked glycerinated muscle fibres in rigor and relaxing solutions at high salt concentrations

Summary In this article, we have applied a crosslinking technique with a water-soluble carbodiimide to single glycerol-extracted muscle fibres from the rabbit. We have measured the stiffness of the fibres in a relaxing solution at high salt concentration. These fibres were crosslinked to varying extents in the rigor state. The relaxing solution caused uncrosslinked crossbridge heads (S1) to detach. High salt concentrations were used because the fibres were not activated by the crosslinked crossbridges under these conditions, although they were at physiological ionic strength. We found (1) a linear correlation between the extent of S1 crosslinking to thin filaments and the stiffness and (2) that the stiffness in the relaxing solution of muscle fibres with all the S1 heads crosslinked to thin filaments was the same as the rigor stiffness of the fibres before crosslinking. We conclude that the sarcomere compliance is mostly a property of the crossbridges (with more than 65% of the crossbridge compliance in the S1 portions and less than 35% in the S2 portion) and little of other sarcomere structures. In an earlier paper [Kimura & Tawada,Biophys. J. 603–10 (1984)], we demonstrated that the S2 portion of the crossbridge was stiff. It then follows that the crossbridge compliance, and thus the sarcomere compliance, is a property of the S1 heads. Assuming that the S1 portion of the crossbridges in rigor strained muscle fibres is bent, we calculated the Young's modulus of the S1 portion and found that it is about 10² MN m^–2. Because this order of magnitude is reasonable in terms of globular protein elasticity, bending is likely to be the nature of the S1 compliance in rigor muscle fibres.     

Series elastic properties of skinned muscle fibres in contraction and rigor

Summary Isometric tension of skinned fibres from the frog semitendinosus muscle is sigmoidally related to Ca²⁺ concentration betweenpCa 7 and 6. Stiffness measurements showed that the Ca²⁺-activated tension may be due to recruitment of attached cross-bridges. In the absence of ATP (rigor solution) the skinned fibre develops a rigor tension which reaches about 80–110% of the maximum Ca²⁺-activated tension.However, stiffness measurements showed that in rigor many more cross-bridges are attached to actin at any one moment than in contraction. It was concluded that the force per cross-bridge is 37% smaller in rigor than in contraction.     

Tension responses to quick length changes of glycerinated skeletal muscle fibres from the frog and tortoise

1. Twitch fibres isolated from the sartorius muscle of the frog were glycerinated (cf. Heinl, 1972) and thin fibre bundles dissected from the m. ileofibularis of the tortoise were briefly glycerinated as described by Julian (1971).2. The glycerinated fibres (length 0.3-0.5 cm) were fixed to an apparatus which performed length changes within 5 msec and recorded the time course of tension changes in the fibres.3. The fibres were suspended in a relaxing medium, containing ATP and 4 mM-EGTA. Contraction was induced by raising the calcium concentration to 4 mM-CaEGTA.4. The tension time course of activated fibres following quick length changes (0.1-1% L(0)) was studied. The tension records produced by quick releases and stretches could be resolved into four phases similar to the kind shown in Fig. 1 a.5. The phase of quick tension recovery was found to take place more rapidly in frog than in tortoise fibres: it was completed in approximately 30 msec (after stretch) and in approximately 20 msec (after release) in frog fibres (3 degrees C). The corresponding values obtained for tortoise fibres were approximately 300 and approximately 400 msec (3 degrees C).6. In tortoise fibres the size of the elastic and quick recovery phase increased with rising isometric tension (induced by raising the calcium concentration (pCa 8 to 5)), and decreased with increasing sarcomere length (2.5-4.2 mum). In fibres, in which the rigor state was induced by withdrawal of ATP, no quick tension recovery was recorded.7. It is suggested that the rotational movement of the crossbridge head on the actin filament, postulated by Huxley & Simmons (1971 b) is taking place more slowly in the tortoise than in the frog muscle. Furthermore, it is suggested that this rotational movement does not occur in the rigor state, as no quick tension recovery was recorded there.     

Cross-bridge stiffness in Ca2+-activated skinned single muscle fibres

Tension transients, in response to small and rapid length changes (completed within 40 s), were obtained from skinned single frog muscle fibres incubated in activating solutions with varying concentrations of Ca²⁺. The first 2 ms of these transients were described by a linear model in which the fibre is regarded as a rod composed of infinitesimally small, identical segments containing a mass, one undamped elastic element and in the case of relaxed fibres two damped elastic elements in series, or in the case of activated fibres three such elastic elements in series. The stiffness of activated fibres, expressed in elastic constants or apparent elastic constants, increased with increasing concentrations of Ca²⁺. All the damped elastic constants that were necessary to describe the tension responses of activated fibres were proportional to isometric tension. However, the undamped elastic constant did not increase linearly with increasing isometric tension. Equatorial X-ray diffraction patterns were obtained from single frog muscle fibres under similar conditions as under which the tension transients were obtained. The filament spacing (d ₁₀)of Ca²⁺-activated single frog muscle fibres decreased with increasing isometric force, whereas the intensity ratio (I ₁₁/I₁₀)increased linearly with increasing isometric force. From experiments in which dextran (MW 200000 Da) was added, it followed that such a change in filament spacing would modify passive stiffness. The d ₁₀value of relaxed fibres decreased and stiffness increased with increasing concentrations of the polymer dextran, whereas I ₁₁/I₁₀remained constant. The relation of stiffness and filament spacing with concentration of dextran was used to eliminate the effect of decreased filament spacing on stiffness of activated fibres. After correction for changes in filament spacing the undamped complicance C ₁, normalized to tension, was not constant, but increased with increasing isometric tension. If we assume that isometric tension is proportional to the number of force generating cross-bridges, this means that only part of the undamped complicance of activated fibres is located in the crossbridges.     

Filament overlap affects TnC extraction from skinned muscle fibres

Summary Recent studies on calcium regulation of muscle contraction selectively extract troponin C (TnC) from skinned skeletal muscle fibres with a low ionic strength rigor solution containing a Ca²⁺/Mg²⁺ chelator. As previous results from this laboratory and others demonstrate a crossbridge effect, especially rigor, on many of the properties of TnC, the effects of filament overlap on TnC extraction from skinned rabbit psoas muscle fibres were investigated. Tension-pCa relationships at a sarcomere length of 2.7 m were determined before and after a 5 min TnC extraction at sarcomere lengths of 2.3, 2.5, 2.7, 3.1, 3.3 or 3.5 m with 20 mm Tris, pH 7.8, 5 mm EDTA. The decrease in the post-extraction maximum Ca²⁺ activated tension, an indicator of the amount of TnC extracted, was linearly related to the overlap of the thick and thin filaments with decreases in tension being associated with a decrease in filament overlap. The smaller fibre diameter at the longer sarcomere length could facilitate diffusion of TnC from fibre segments. However, the wide range of measured diameters, 40–120 m, accounted for only 14% of the observed tension decrement and shrinking the fibre with polyvinylpyrrolidone did not increase the tension decrement. Increasing the sarcomere length before extraction was also found to decrease the TnC content of fibre segments along with the post-extraction maximum tension. Thus, TnC appears to be preferentially extracted from non-overlap than overlap regions of the sarcomere. These results further indicate that rigor crossbridges affect TnC other than through increased Ca²⁺ binding and that under the conditions used here, they retard its extraction.     

A study on the mechanism of phalloidin-induced tension changes in skinned rabbit psoas muscle fibres

Summary The time course of phalloidin induced changes in isometric tension of partially activated skinned rabbit psoas fibres was studied as a function of both phalloidin concentration and time of pre-incubation with phalloidin. Upon addition of phalloidin to non-pretreated (control) fibres there was a fall in tension folllowed by an increase in tension. The latency of both parts of the response was inversely related to the phalloidin concentration in the range 40–130 m phalloidin. By pre-incubating the fibres with phalloidin for varying periods of time it was possible to obtain responses which appeared to represent later portions of the control response. Thus after pre-treatment with 40 m phalloidin in either rigor or relaxing solution for 5 min (the time corresponding to minimal tension in the control response) the tension response resembled that of the control, beginning from the vicinity of the minimum. The pattern of staining of the fibres by rhodamine-phalloidin was analysed by laser confocal microscopy to relate the mechanical response to phalloidin localization. If fibres were treated with rhodamine-phalloidin for 20–25 min there was a labelling of the I-Z-I segment with intense peaks of fluorescence at the Z-line and the ends of the I filaments. If fibres were pre-incubated for 5 min with phalloidin and then labelled with rhodamine-phalloidin the fluorescence at the Z-line and at the ends of the I filaments was suppressed and the peak of the fluorescence intensity was shifted toward the middle part of the I filament. The data indicate that the decrease in tension caused by phalloidin was associated with binding of phalloidin to the pointed ends of actin filament and the Z-line region, whereas the increase in tension occurred when phalloidin was bound along entire length of the actin filament.     

Transmission phenomena and early tension recovery in skinned muscle fibres of the frog

Tension responses due to small rapid length changes completed in 50 s were obtained from segments with different length of single fibres of the ileofibularis muscle of the frog. The very early parts of the responses varied with segment length. A simulation of the early parts of the response was carried out by means of a linear model in which the fibre is regarded as a rod of infinitesimally small segments containing undamped elasticity, damped elasticity and mass in series. In the simulation corrections were included for the effects caused by the viscosity and density of the surrounding fluid and for the force transducer characteristics. The results indicate the presence of a very rapid component in the fast recovery with a time constant of 5–15 s. The undamped elasticity of the activated fibres corrected for their passive properties was such that a sudden shortening corresponding to 2.6 nm/half sarcomere would reduce active tension to zero.     

Deterioration induced by physiological concentration of calcium ions in skinned muscle fibres

Summary The deteriorating effect of m order of Ca²⁺ on skinned frog skeletal muscle fibres was studied from the view point of the digestion of proteins by calcium-activated neutral protease (CANP). Tension developed in solutions containing no MgATP (rigor solution) decreased irreversibly with the addition of Ca²⁺ in quantities of more than 0.1 m. Low temperature was seen to suppress (Q₁₀>4), and neutral pH to maximize, this decrease in tension. In rigor solution containing Ca²⁺, SDS electrophoresis indicated that a 95 k dalton component (-actinin) was released from the fibre; electron micrography showed the disappearance of Z-lines. These results suggest that one of the causes for decrease in rigor tension is the proteolytic activity of CANP, and its inhibitors were shown to be quite useful in experiments on skinned fibre.     

Localization and elasticity of connectin (titin) filaments in skinned frog muscle fibres subjected to partial depolymerization of thick filaments

Summary The localization and elasticity of connectin (titin) filaments in skinned fibres of frog skeletal muscle were examined for changes in the localization of connectin and in resting tension during partial depolymerization of thick filaments with a relaxing solution containing increased KCl concentrations. Immunoelectron microscopic studies revealed that deposites of antibodies against connectin at a sarcomere length of 3.0 m remained at about 0.8 m from the M-line, until the thick filament was depolymerized to the length of approximately 0.4 m. On further depolymerization, the bound antibodies were found to move towards the Z-line and, on complete depolymerization, were observed to be within 0.3 m of the Z-line; a marked decrease in resting tension accompanied this further depolymerization. These results suggest that connectin filament starts from the Z-line, extends to the M-line, and contributes to resting tension. After partial depolymerization of thick filaments, the distances between the anti-connectin deposits and the Z-line and between anti-connectin deposits and the M-line increased with sarcomere length, suggesting that connectin filaments are elastic along their entire length.     

A 10 μs component in the tension transients of isolated intact skeletal muscle fibres of the frog

The transients of the tension and the angle of a first order diffraction line of isolated single intact muscle fibres of theM. lumbricalis IV of the foot resulting from rapid length changes have been measured. Furthermore simulations of the tension transients, taking the resonance frequency of the force transducer, the inertia of the fibre and the influence of the surrounding fluid into account, are presented. The tension transients could be simulated by a system of continuous elements with undamped elasticity —Young's modulusE ₁=1.8×10⁸ N/m² — in series with elasticityE ₂=5.4×10⁷ N/m² parallel to a damping element — coefficient of damping =2,300 Ns/m²; yielding a relaxation time of 10 s. The Young's modulus of the undamped series elasticityE ₁ implies that a sudden shortening of 2 nm per half sarcomere should reduce the active tension to zero. The results obtained from the diffraction pattern recording show that the displacement in the fibre is in accordance with the relative length change caused by the displacement generator. It is therefore believed that the tendon and tendon-muscle junction are sufficiently stiff to allow ascribing the above-mentioned elasticity and damping to the fibre itself.     

Slip of rabbit striated muscle in rigor or AMPPNP

Summary Single glycerol-extracted rabbit psoas muscle fibres have been slowly extended either in rigor or in the unhydrolysable ATP analogue AMPPNP, and their sarcomere length, sarcomere structure and tension measured. The length of regularly arrayed sarcomeres, measured by optical diffraction, increased continuously as the muscle was stretched; the maximum sarcomere extension seen was approximately 6%. In the electron microscope sarcomeres from extended muscle fixed in rigor or AMPPNP remained regular in their internal structure, without rupture or obvious lengthening around the Z line. During steady extension at 0.024% per min the tension in the muscle fibre rose until it reached a limiting value [T _m ] when the sarcomeres had stretched by 0.8–1.6% and then remained constant with continued extension, while the sarcomeres continued to stretch. Provided that a novel form of preparation of the glycerol-extracted fibres was employed,T _m in rigor was a large fraction of the tension expected from an active isometric muscle fibre. In the presence of AMPPNPT _m was reduced by a factor of 2 to 3. Step extension by 0.08% at 5-min intervals gave the same pattern of mechanical response with similar values ofT _m . The isometric tension decay in the interval between the steps was very rapid at first and slowed continuously until the next step. The average speed of tension fall between 30 and 300 s after stretch was measured at each step and plotted relative to the tension in the muscle. The relationship approached linearity, although with a significant upward curvature at high tension. The proportionality constant of the rate of tension fall to tension was 4.5×10^–4 s^–1 in rigor and 9×10^–4 s^–1 in AMPPNP. These values are less than the apparent dissociation rate constants for acto-subfragment-1 or acto-heavy meromyosin under similar conditions (Marston, 1982). These results indicate that interfilament slip does occur in rabbit skeletal muscle both in rigor and in AMPPNP, but that it is much slower that that predicted from the behaviour of the isolated proteins, as if the myosin heads interacted so as to obstruct their detachment.     

Force and force transients in skeletal muscle fibres of the frog skinned by freeze-drying

A freeze-drying method is described by which single skinned skeletal muscle fibres or fibre bundles can readily be obtained. Skinned fibre segments of the ileofibularis and semitendinous muscles of the frog — activated by means of a rapid increase in the Ca-concentration — showed very stable and reproducible contractions. Complete activation occurred at a Ca-concentration of 1.6·10^–6 M and the mid-point of the pCa-tension curve occurred at 6.3·10^–7 M. Addition of phosphate (10^–2 M) had a depressing effect on the speed of the Ca-activated tension development as well as on the maximum tension reached.Addition of caffeine (10^–2 M) had no effect on the tension generation, indicating that the sarcoplasmic reticulum, if present, was not active. The force responses due to rapid length changes applied to the Ca-activated fibre preparations were found to be qualitatively similar to the force responses on intact tissue. This skinning technique might be employed on human biopsies, enabling the measurement of physiological parameters such as for example force and shortening velocity.     

The effects of tonicity on tension and stiffness of tetanized skeletal muscle fibres of the frog

Tension and stiffness of tetanically activated skeletal muscle fibres of the frog were studied at varied tonicity of the extracellular medium (1.7-3.2 degrees C; sarcomere length, 2.13-2.22 microns). The stiffness was measured from the change in peak tension in response to fast (0.2 ms) stretches and releases of small amplitude (0.11-0.15% of the fibre length). The bathing solution was made hypotonic by reduction of NaCl and hypertonic by addition of sucrose. The osmotic strength of the solutions tested varied from 81 to 168% of the isotonic value. Maximum tetanic tension decreased markedly with increased tonicity. The active stiffness, on the other hand, increased as the tonicity was raised, and the tension/stiffness ratio (the total extension of the undamped fibre elasticity) was thus greatly reduced under these conditions. Evidence is presented to show that the change in the tension/stiffness ratio is due neither to the development of rigor cross-bridges nor to the recruitment of passive parallel-elastic elements in response to increased tonicity. Neither are viscous-like components important for explaining the effect. A change in the tension/stiffness ratio, similar to that seen in response to increased tonicity, did not occur as fibre width was reduced by increasing the sarcomere length. This suggests that the changes in the fibre volume affect this ratio mainly by mechanisms that are unrelated to changes in lateral spacing between the myofilaments.     

Specific perforation of muscle cell membranes with preserved SR functions by saponin treatment

Summary The effects of saponin onXenopus and frog skeletal muscle fibres were examined. The twitch ofXenopus single fibres was first potentiated slightly and then irreversibly abolished by 5–10 µg/ml of saponin. Treatment with saponin at 5–10 µg/ml or higher concentrations for 30 min resulted in perforation of the muscle cell membrane, indicated by the following evidence. (i) Fibres became responsive to contractile activating solutions with a pCa-tension relationship similar to that of mechanically skinned fibres. (ii) Removal and re-introduction of MgATP became effective in bringing fibres into rigor and the relaxed state, respectively. (iii) After the saponin treatment large contractions due to Ca release from the SR could be elicited by substitution of Cl for methanesulphonate in the medium. (iv) The treatment decreased the optical path length across the fibre, indicating loss of soluble proteins. (v) The lattice spacing of myofilaments was wider after the treatment as in mechanically skinned fibres. Contractile response of mechanically skinned fibres and their SR responses such as Ca uptake, Ca-induced Ca release and Cl-induced Ca release were not affected by treatment with 50 µg/ml saponin for 30 min, while 150 µg/ml or higher concentrations severely impaired the SR functions. It is possible, therefore, to make chemically skinned skeletal muscle fibres in which the functions of the SR are preserved by applying 10–50 µg/ml saponin.     

Stiffness of frog muscle fibres during rise of tension and relaxation in fixed-end or length-clamped tetani

1. Stiffness measurements were performed during the rise, the plateau and the relaxation of tetanic contractions both in whole single muscle fibres and in tendonfree fibre segments under either fixed-end or length-clamp conditions. Fibres were isolated from the tibialis anterior muscle of the frog. Experiments were performed at 2–6° C. Changes in length of tendon-free fibre segments were monitored by means of a striation follower, an optoelectronic device which, during contraction, measured sarcomere displacement at the level of two selected regions of a fibre. 2. Fast length perturbations imposed at one tendon end of a fibre during the plateau of tetanic contractions distribute uniformly along its length. 3. During the tetanus rise stiffness led isometric tension in whole fibres under fixed conditions as well as in tendon-free fibre segments under length-clamp conditions. 4. It was confirmed that a significant part of the unlinearity ofT ₁ relations is determined by tendon compliance. 5. During the isometric phase of relaxation in fixed-end tetani, the decline of tension led that of stiffness both in whole fibres and in tendon-free fibre segments. 6. It is concluded that the shift observed between stiffness and tension during tetanus rise and relaxation represents a true specific event in the contractile process.     

Structural change of crossbridges of rabbit skeletal muscle during isometric contraction

Summary Structural changes of crossbridges during isometric contraction have been studied by electron microscopy. Chemically skinned rabbit fibres were rapidly frozen either in activating solution or in ATP-free (rigor) solution, freeze-substituted and embedded. Longitudinal sections of muscle fibres show that the number of crossbridges in active fibres (isometric contraction) is approximately the same as in rigor fibres. Crossbridges of the active and rigor states differ in their shapes, angles and manner of arrangement on the thin filaments. In rigor many crossbridges are wide near the thin filaments and narrow near the thick filament shafts; in active fibres they have more uniform width along their length. The angle of the crossbridges in active fibres is somewhat variable. The average angle is 90° to the filament axis. The crossbridges are arranged on the thin filament retaining the 14.3 nm thick filament periodicity. The crossbridges in rigor are tilted and their arrangement near the thin filament reveals the 36 nm actin periodicity. The variability in the shapes of the crossbridges in active fibres is still higher when we look at them in cross-sections of muscle fibres. The crossbridge shapes in the cross-sections were classified and the relative frequency of different shapes was determined. The shapes that are commonly observed in active fibres are similar in that the majority of the mass of the crossbridges is farther away from the thin filament than the crossbridges in rigor fibres.     

Elastic and inelastic behaviour of resting frog muscle fibres

Dynamic and static elasticity of the resting skeletal muscle of the frog have been studied as a function of the sarcomere length. Isolated intact fibres as well as glycerol extracted fibres show a resting tension starting at 2,05 m sarcomere length and increasing approximately exponentially to 2·10⁴ N/m² at 3,0 m sarcomere length. Differences between the two types of preparation were seen in the dynamic experiments. The dynamic Young's modulus ofintact fibres (recorded at 1 Hz and small amplitudes) increased from 2·10⁵ N/m² at 2,1 m sarcomere length to 2,5·10⁶ N/m² at 2,9 m while the static modulus varied from 5·10³ N/m² to 3·10⁵ N/m², the dynamic modulus at small amplitudes was equivalent to the modulus of the short range elasticity (SRE). The range of the SRE did not depend on the sarcomere length and amounted to about 5 nm per sarcomere. The dynamic modulus strongly depended on the amplitude: at large amplitudes the muscle became less stiff by a factor of 10 to 20. This tendency levelled off at about 10 Hz by a strain-induced relaxation process. The dynamic modulus of theglycerol extracted fibres was nearly of the same magnitude as the static modulus, there was neither evidence for an SRE nor for a significant amplitude-dependence of the dynamic modulus. For interpreting the results we propose to further develop the meander model of muscle (Pechhold et al. 1973 b).