Synthesis of fast myosin induced by fast ectopic innervation of rat soleus muscle is restricted to the ectopic endplate region

Skeletal muscle fibres, long multinucleated cells, arise by fusion of mononucleated myoblasts to form a myotube that matures into the adult fibre. The two major types of mature fibre, fast and slow fibres, differ physiologically in their rate of isotonic shortening. At the molecular level these type-specific physiological properties are ascribed to different isoforms of myosin, a major protein involved in shortening. Differentiation of fast and slow fibres seems to be under the control of motoneurones, and mature fibres are innervated by only one motoneurone. When rat soleus muscle (SOL, a slow muscle) is dually innervated with a fast nerve, it acquires some properties of a fast muscle, that is, low sensitivity to caffeine and high glycogen content. We report here that in dually innervated soleus muscle the foreign fast nerve induces synthesis of fast isoforms of myosin, but only in the segment of the muscle fibre that is close to the foreign endplate. The localized influence of the nerve endplates suggest that factors controlling the phenotypic expression of the muscle fibre have a short range of activity.     

Mechanical performance of scallop adductor muscle during swimming.

Mechanical performance of skeletal muscle has long been the subject of intense interest, but the details of in vivo performance of individual skeletal muscles during normal locomotion remain largely unknown. Performance in vitro has been described with considerable precision under simplified loading conditions. The force production and shortening velocity of most muscles, however, probably change continuously during natural movements. Therefore, modelling in vivo performance on the basis of in vitro contractile properties is subject to large degrees of uncertainty. Designing in vitro experiments that effectively examine the limits of mechanical performance requires increasing knowledge of precisely how muscles are used during normal movements. We report here measurements of the mechanical performance of the adductor muscle in scallops during jet-propulsion swimming. Swimming in scallops is powered solely by the striated portion of the single adductor muscle. Exploiting this simple locomotor morphology with simultaneous high-resolution measurements of pressure and flow rate, we have recorded nearly instantaneous measurements of the performance of a single skeletal muscle during normal locomotion.     

Is stepwise sarcomere shortening an artefact?

A report in 1977 raised the intriguing possibility that sarcomere shortening in muscle may occur in a stepwise fashion, in which episodes of shortening are interrupted by periods of little or no movement. This was taken by its authors to imply the synchronous activity of cross-bridges over a large volume of tissue-behaviour which cannot easily be reconciled with commonly accepted views of muscle contraction. Stepwise shortening has also been reported recently in relaxed muscle fibres on which length changes were externally imposed, and that system has allowed us to define more rigorously the circumstances in which stepwise shortening is observed. Here we report a high correlation between the frequency of 'steps' or 'pauses' and the translation velocity of the fibre past the measuring system, suggesting that stepwise shortening is not a physiological property of muscle but an instrumentation artefact.     

Fibre type composition of single motor units during synapse elimination in neonatal rat soleus muscle

Skeletal motor neurones innervate the specialized 'types' of fibres comprising most mammalian muscles in a characteristic fashion: each motor neurone forms a 'motor unit' by innervating a set of fibres all of the same type. Because the type expression of adult muscle fibres is plastic and apparently controlled by their innervation, each motor neurone is thought to impose a common type differentiation on all the fibres in its motor unit. However, the situation in developing muscles cannot be this simple. Muscle fibres in neonates receive synaptic input from several motor neurones and achieve the adult, single innervation only after a period of 'synapse elimination. Despite this polyneuronal innervation, differentiated fibre types are present in neonatal muscles. This means either that the motor neurones polyneuronally innervate fibres in a random fashion and type expression is not determined by innervation or that the polyneuronal innervation is ordered in such a way that each fibre could receive unambiguous instructions for type differentiation. We have investigated these possibilities here by determining the fibre type composition of motor units in neonatal rat soleus muscle. We find that even during the time of polyneuronal innervation each motor neurone confines its innervation to largely one of two fibre types present in the muscle. Therefore, some mechanism during early development segregates the synapses of two groups of soleus motor neurones onto two separate populations of soleus muscle fibres.     

Fast and slow myosin within single skeletal muscle fibres of adult rabbits.

There is good evidence for the coexistence of different myosin types both in developing muscles and in Purkinje cells from adult chicken hearts. In skeletal muscle fibres of adult animals, however, coexistence of fast (FM) and slow (SM) myosin has only been demonstrated after long-term electrical stimulation. The term 'promiscuity' has recently been coined to describe the coexistence of different myosin isoenzymes within a single fibre. Using novel, refined immunological methods we demonstrate here the presence of both FM and SM within single fibres of the musculus tibialis anterior of adult rabbits. Essentially identical results were also obtained with other muscles. Our findings imply that the genes coding for FM and SM can be expressed simultaneously within the same cell throughout an animal's entire life, and not only during development or after artificial electrical stimulation.     

Is resistance of a muscle to fatigue controlled by its motoneurones?

The original experiment of Buller et al. and the many subsequent confirmatory reports clearly show that the time-to-peak tension and many other speed-related parameters of slow and fast muscle fibres are dictated by the motoneurone. It has been concluded that the motoneurone exerts this control of the physiological and associated biochemical properties by the frequency at which it excites the muscle fibre. However, no studies have been reported on the fatigue properties and the associated biochemical characteristics after cross-reinnervation. Based on the 'size principle' of motoneurones, it would be reasonable to assume that a muscle fibre reinnervated by a small motoneurone would be active often and that this would be manifested biochemically as an elevated oxidative capacity. Also, it has been shown repeatedly that the mitochondrial content of a muscle fibre can be modified by daily endurance type exercise. Thus, it would seem that the motoneurone at least indirectly also controls the mitochondrial content of a muscle fibre by controlling the degree of activity. We have now tested this hypothesis using self- and cross-reinnervated muscles in cats. We found that fast- and slow-twitch muscles retained their characteristic fatigue resistance properties regardless of whether the nerve to which they had become connected had originally innervated a fatigue-resistant or relatively fatiguable muscle.     

Myosin isoenzyme redistribution in chronic heart overload.

Since the first observation by Spann et al., it has become clear that in cardiac hypertrophy induced by a mechanical overloading, the velocity of shortening of the cardiac muscle (Vmax) is reduced (see ref. 2 for review). Most authors agree that this mechanical alteration is accompanied by a decrease in the Ca2+-dependent ATPase activity of myosin (see ref. 3 for review). The molecular basis of such changes was unknown because the structural modifications of the myosin molecule were ill-defined. Nevertheless, it has recently been shown that, like skeletal muscle myosin, cardiac myosin is composed of several polymorphic forms, comparable to isoenzymes. In the skeletal muscle, new functional requirements can induce changes in both contractile activity and type of myosin isoenzyme synthesised. We now report that an increase in cardiac work produced by mechanical overloading in rats induces the preferential synthesis of a cardiac myosin isoenzyme characterised by specific immunological and electrophoretic properties and exhibiting a lower ATPase activity. This adaptive change could account for the reduced shortening speed of this hypertrophied cardiac muscle.     

Fast and slow myosin in developing muscle fibres.

Slow and fast isoenzymes of myosin coexist in all the fibres of a fast-twitch mammalian muscle during early development. They later become segregated into different populations of fibres. Slow myosin is most abundant when the speed of contraction of the muscle is slow and the fibres are multiply innervated; its synthesis in the majority of the fibres seems to be 'switched off' when the speed of contraction increases and the fibres become innervated by single motoneurones.     

'Square arrays' in the sarcolemma of human skeletal muscle fibres.

Fast twitch fibres of rat and rabbit show rectangular patterns of intramembrane particles in freeze-fracture preparations of the sarcolemma. These 'square arrays' are almost totally absent in the slow twitch coleus muscle of rat. I report here differences in the incidence of square arrays in human fetal and adult muscle and in different fibres within a human muscle. Square arrays probably classify fast and slow twitch fibres in freeze-fracture preparations of mixed muscles.     

Migration of myoblasts across basal lamina during skeletal muscle development

Basal lamina is a sheet of extracellular matrix that separates cells into topologically distinct groups during morphogenesis and is thought to form a barrier to cell migration. We have examined whether, during normal muscle development, myoblasts--mononucleate muscle precursor cells--can cross the basal lamina that surrounds each multinucleate muscle fibre. We marked myoblasts in vivo by injecting replication-defective retroviral vectors encoding LacZ into muscle tissue and analysed the fate of their progeny by the expression of beta-galactosidase. A dual labelling method with broad application to retroviral lineage-marking studies was developed to ensure that most clusters of labelled cells were clones derived from a single precursor cell. Most of the myoblasts that were infected at a late stage of rat hindlimb development, when each fibre with its satellite myoblasts is individually encased in a basal lamina sheath, gave rise to clones that contributed to several labelled fibres. Our results show that myoblasts from healthy fibres migrate across basal lamina during normal development and could contribute to the repair of fibres damaged by injury or disease.     

关于快慢两种肌纤维的形态结构及代谢特性与运动能力的研究

本文从形态结构及生理代谢角度论述了人体骨骼肌中的快慢两种肌纤维的生理机制及运动能力，并揭示出了用不同强度的练习可分别发展不同类型的肌纤维，同时也为运动员的选材提供了可靠的科学依据。     

PH值对应用肌球蛋白 ATP酶法进行梭内肌纤维分型的影响

探讨酸性和碱性预孵育处理，对应用肌球蛋白ATP酶法进行梭内肌纤维分型的影响。采用肌球蛋白ATP酶法。PH4．3和PH4．6预孵育液孵育后，梭内肌纤维中的核袋纤维染色呈阳性，核链纤维染色相对较浅；PH9．4预孵育后，核袋与核链纤维均呈阳性或强阳性；PH10．4预孵育后，核袋1纤维呈阴性，核袋2纤维呈阳性，核链纤维呈强阳性。结论是：用mATP酶法研究梭内肌纤维的分型时，预孵育液的PH值对分型结果有显著影响，应当说明并严格把握预孵育液的PH值。     

Rapid regeneration of the actin-myosin power stroke in contracting muscle.

At the molecular level, muscle contraction is the result of cyclic interaction between myosin crossbridges, which extend from the thick filament, and the thin filament, which consists mainly of actin. The energy for work done by a single crossbridge during a cycle of attachment, generation of force, shortening and detachment is believed to be coupled to the hydrolysis of one molecule of ATP. The distance the actin filament slides relative to the myosin filament in one crossbridge cycle has been estimated as 12 nm by step-length perturbation studies on single fibres from frog muscle. The 'mechanical' power stroke of the attached crossbridge can therefore be defined as 12-nm shortening with a force profile like that shown by the quick recovery of force following a length perturbation. According to this definition, power strokes cannot be repeated faster than the overall ATPase rate. Here, however, we show that the power stroke can be regenerated much faster than expected from the ATPase rate. This contradiction can be resolved if, in the shortening muscle, the free energy of ATP hydrolysis is used in several actin-myosin interactions consisting of elementary power strokes each of 5-10 nm.     

Inhibition of climbing fibres is a signal for the extinction of conditioned eyelid responses

A fundamental tenet of cerebellar learning theories asserts that climbing fibre afferents from the inferior olive provide a teaching signal that promotes the gradual adaptation of movements. Data from several forms of motor learning provide support for this tenet. In pavlovian eyelid conditioning, for example, where a tone is repeatedly paired with a reinforcing unconditioned stimulus like periorbital stimulation, the unconditioned stimulus promotes acquisition of conditioned eyelid responses by activating climbing fibres. Climbing fibre activity elicited by an unconditioned stimulus is inhibited during the expression of conditioned responses-consistent with the inhibitory projection from the cerebellum to inferior olive. Here, we show that inhibition of climbing fibres serves as a teaching signal for extinction, where learning not to respond is signalled by presenting a tone without the unconditioned stimulus. We used reversible infusion of synaptic receptor antagonists to show that blocking inhibitory input to the climbing fibres prevents extinction of the conditioned response, whereas blocking excitatory input induces extinction. These results, combined with analysis of climbing fibre activity in a computer simulation of the cerebellar-olivary system, suggest that transient inhibition of climbing fibres below their background level is the signal that drives extinction.     

Limitations in the use of actomyosin threads as model contractile systems

Recent studies have suggested that actomyosin threads may provide a useful model for studying the properties of contractile systems. The development of highly sensitive positional feedback transducers has enabled the properties of these threads to be measured reproducibly. Potential applications include such systems as ventricle, smooth muscle and non-muscle preparations, from which it is difficult to obtain suitable fibres for mechanical studies. In addition, studies with chemically modified myosins may provide new insights into the relationships between the biochemical and mechanical events in the cross-bridge cycle. However, there are indications that the mechanical properties of actomyosin threads differ from those of intact fibres in several important respects. For example, contraction velocity is proportional to isometric tension in threads, but is independent of filament density in intact fibres. We have now determined the force-velocity characteristics of actomyosin threads prepared from muscles with known differences in their physiological contraction velocities. No direct relationships could be found between the velocity characteristics of the threads and those of intact muscle. We conclude that the measured velocities of threads reflect properties of the actomyosins other than cross-bridge cycling times, thus severely limiting the usefulness of this technique for comparative purposes.     

Distribution of polymorphic forms of troponin components and tropomyosin in skeletal muscle.

Specific antibodies have been used to show that in adult skeletal muscle the slow and fast forms of the components of the troponin complex are located in type I and type II fibres respectively. alpha-Tropomyosin is restricted to type II cells. During development, as a result of changes in innervation and in certain diseased stages, both the slow and fast polymorphic forms of the troponin components are present in the same cell.     

Convergent evolution in mechanical design of lamnid sharks and tunas

The evolution of 'thunniform' body shapes in several different groups of vertebrates, including whales, ichthyosaurs and several species of large pelagic fishes supports the view that physical and hydromechanical demands provided important selection pressures to optimize body design for locomotion during vertebrate evolution. Recognition of morphological similarities between lamnid sharks (the most well known being the great white and the mako) and tunas has led to a general expectation that they also have converged in their functional design; however, no quantitative data exist on the mechanical performance of the locomotor system in lamnid sharks. Here we examine the swimming kinematics, in vivo muscle dynamics and functional morphology of the force-transmission system in a lamnid shark, and show that the evolutionary convergence in body shape and mechanical design between the distantly related lamnids and tunas is much more than skin deep; it extends to the depths of the myotendinous architecture and the mechanical basis for propulsive movements. We demonstrate that not only have lamnids and tunas converged to a much greater extent than previously known, but they have also developed morphological and functional adaptations in their locomotor systems that are unlike virtually all other fishes.     

Study on Properties of Jute Fibres Spun on Ring Spinning System

Jute fibre properties,viz. linear density,mechanical properties,and fibre staple length were analyzed in this paper. Morphology,lignin content,and composition analysis for jute fibres were investigated in order to give further understanding of the fibre properties changing during the chemical treatment and yarn producing processes. The results show that jute properties are greatly changed during the processes of chemical treatment,breaking carding,and finisher carding,whereas slight changes can be found in ...