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A comparison of the tension response to rapid lengthening or shortening steps in isometrically contracting frog skeletal muscle Dusik, Laureen Anne


Rapid length steps of isometrically contracting single skeletal muscle fibres, or whole muscle, provide a measure of the elastic properties of a structure believed to be an integral part of the myosin cross-bridge. The experiments to be described were designed to compare the elastic properties of this structure when measured with small amplitude (2-6 nm/h-s), rapid (1ms or 500 us duration) lengthening steps versus shortening steps. All the experiments were carried out at 0°-4°C, using small bundles (2-20 fibres) or single fibres from frog semitendinosus muscle. The preparation was given a rapid stretch or release at various times during the isometric twitch and tetanus. During the initial development of tension in an isometric twitch and tetanus, the stiffness was always seen to be rising faster than the force. During the relaxation phase of the isometric twitch, the stiffness was observed to lag the tension. During the relaxation phase of an isometric tetanus prior to the 'shoulder', the change in stiffness was seen to lag the tension change. Following the shoulder, there was a rapid fall in stiffness which corresponded to a similar decline in tension. In all instances, when stiffness values at a given force, and measured with a rapid release, were compared to those obtained with stretch, rapid lengthening produced a consistently higher stiffness than a shortening step. The difference was most pronounced during the late rising phase of tension, maximum tension, and the early relaxation phase of the twitch or tetanus. Several suggestions are discussed to explain this observed difference in stiffness between stretch and release. These include the possibilities that the instantaneous elasticity of the cross-bridge may be non-linear during stretch, and detachment of cross-bridges occurs during release but not stretch.

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