Recordings of the change in tension in striated muscle after a sudden alteration of the length have made it possible to suggest how the force between the thick and thin muscle filaments may be generated.

Proposed Mechanism of Force Generation in Striated Muscle

Calcium ion and muscle contraction.

In the preceding paper (Buller, Eccles & Eccles, 1960) evidence was presented which suggested that the differentiation of slow muscles of the cat hind limb to a large extent failed to occur after certain operative procedures on the spinal cord. This finding indicates that in some way the central nervous system controls muscle differentiation. A more analytic investigation into this postulated influence of nerve on muscle has been accomplished by dividing and cross-uniting nerves to fast and slow muscles, so that motoneurones formerly innervating the fast muscle come to innervate the slow muscle by virtue of the regenerative outgrowth of their fibres, and vice versa for the motoneurones of the slow muscle. The effect of this crossed innervation on the speed of muscle contraction has been tested at varying times after the cross-union. These investigations have also been carried out on animals subjected to the operative procedures on the spinal cord, as previously described (Buller et al. 1960). Cross-union experiments allow in addition an investigation into the possible effects in the reverse direction, i.e. of speed of muscle contraction on the conduction velocity of the axons and the after-hyperpolarization of the motoneurones which innervate it. A preliminary account of part of this investigation has already been published (Bulier, Eccles & Eccles, 1958).

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1960.sp006395

Interactions between motoneurones and muscles in respect of the characteristic speeds of their responses.

FUNG, Y. C. B. Elasticity of soft tissues in simple elongation. Am. J. Physiol. 213(6) : 1532-l 544. 1967 .-Elasticity of living soft tissues is strongly nonlinear. Based on experimental results on rabbits’ mesentery, a theoretical framework is presented in which the elastic properties of soft tissues can be described. It is shown that the mathematical formulation works well also in reducing published data on the series element of the heart and striated muscles, and the skin. In simple elongation the tensile stress is nearly an exponential function of the strain in the lower stress range. Based on this fact, it is shown that although we are dealing with the finite deformation of highly nonlinear materials, the elastic property of soft tissues in tension can be expressed quite simply in most cases. It is necessary, however, to give up the usual practice of trying to characterize the elasticity of a tissue by a representative Young’s modulus, because this modulus varies over a very wide range, which is often zero at vanishing stress, and increases linearly as the stress increases, and therefore is meaningless unless the exact stress level is specified. New physical constants recommended are : the slope and curvature at the origin of the curve of d T/dX vs. T, where T stands for tension and X stands for the extension ratio, and the tensile stress T*, (based on the original crosssectional area) at a specific value of the extension ratio A*.

Elasticity of soft tissues in simple elongation

1. Apparatus for applying a step change of length to an isolated muscle fibre is described. The step was complete in about 0.2 ms.2. Effects of tendon compliance were eliminated by using a spot-follower device and by gripping the tendons with metal clips close to the fibre ends.3. The natural frequency of the force transducer was above 10 kHz.4. Steps of various amplitudes and in either direction were applied to isolated muscle fibres about 6 mm long from the anterior tibial muscle of Rana temporaria during tetanic stimulation. Initial sarcomere length was 2.0-2.2 mum, and temperature was 0-3 degrees C.5. The tension response to a step could be divided into four phases. The initial response was an apparently elastic change during the step itself (phase 1). After the step was completed there was a rapid partial recovery towards the original tension (phase 2, lasting 2-5 ms), followed by a slowing or reversal of recovery (phase 3, 10-50 ms), and finally a much slower return to the original tension (phase 4). Most of this paper is concerned with phases 1 and 2.6. The initial tension change (phase 1) occurred synchronously with the applied length change, indicating that the fibres possess a compliance which is almost linear and almost undamped. Its stiffness is such that an instantaneous shortening of about 4 nm per half-sarcomere would bring the tension to zero from its isometric value.7. The absence of detectable damping during phase 1 indicates that the viscosity of a stimulated fibre is substantially less than the apparent viscosity of a fibre at rest.8. The instantaneous force-extension curve approached the length axis at a sharp angle and a negative tension appeared at the force transducer when a very large step was applied. These observations suggest that the structures responsible for the stiffness of the fibre remain rigid when they are not under tension.9. During the few milliseconds after the step (phase 2) the tension recovered part of the way toward the level which existed before the step. In shortening steps the time course of this recovery was adequately fitted by the sum of four exponential terms, and was similar in steps of different amplitude but with a time scale shorter the larger the step. In stretches the slow components were relatively larger than in releases.10. The tension level, T(2), approached during phase 2 depended only on the total amplitude of the step and not on the time course of the length change, provided it was complete in 1-2 ms. The extreme tension reached during a step could thus vary widely without detectable change in T(2).11. With stretches and releases of up to about 3 nm per half-sarcomere this early recovery was almost complete, so that the curve of T(2) against step amplitude was nearly horizontal. With larger releases the line curved downwards, reaching zero in a release of about 14 nm per half-sarcomere.12. When the temperature was raised both the developed tension and the stiffness increased, but the relative increase was greater for tension than for stiffness. The amount of instantaneous shortening needed to bring tension to zero was therefore also increased.13. A set of empirical equations is given which describe adequately the first few milliseconds of the tension change in response to any imposed time course of shortening.14. The rapid elasticity and early tension recovery resemble the response of a combination of two elastic components and one viscous component. Reasons are given for preferring an interpretation in terms of an undamped compliance in series with a damped compliance (Voigt element) rather than an undamped elasticity in parallel with a series combination of viscous and elastic components (Maxwell element).15. The rapid compliance does not correspond to the ;series elastic component' of two-component theories of muscle contraction.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1977.sp011911

Tension responses to sudden length change in stimulated frog muscle fibres near slack length

J. Physiol. B. R. Jewell and D. R. Wilkie striated muscle An analysis of the mechanical components in frog's This information is current as of March 19, 2008 JournalsRights@oxon.blackwellpublishing.com reproduced without the permission of Blackwell Publishing: months after publication. No part of this article may be articles are free 12 The Journal of Physiology Online . http://jp.physoc.org/subscriptions/ go to: Physiology Online The Journal of continuously since 1878. To subscribe to The Physiological Society. It has been published is the official journal of The Journal of Physiology Online

D. R. Wilkie

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An analysis of the mechanical components in frog's striated muscle.