Sliding Filament Theory

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Abstract Sliding filament theory of muscle contraction suggests existence of an optimal muscle length associated with maximum stimuli-generated tension. This hypothesis is tested in vitro by measuring muscle tensile response to fixed voltage stimulations under length alterations of the gastrocnemius of Xenopus laevis. Muscle length is mechanically varied by shrinking or extending and an attached force transducer records the tension generated by the fixed stimuli. Our results indicate greatest tensile response of about 3.2 N when muscle length is 30 mm. The response diminishes as deviations from this optimal length increases. These results are consistent with an optimal length of overlap, with associated cross-bridging, between actin and myosin filaments in the sarcomere as suggested by sliding filament theory. Introduction According to the sliding filament theory, (1) and (2), striated muscle contracts when cross-bridges between overlapping myosin (thick) and actin (thin) filaments initiate many independent filament micro-slides which result in shrinkage of the structural unit of the muscle fiber called the sarcomere. Evidence for this sliding theory was first observed in the constancy of the length of the sarcomere’s A-band under length variations of the sarcomere. The A-band has a region where actin and myosin filaments overlap. If all muscle structural components shrank with sarcomere contraction then the A-band should also shrink but this is not observed. Sliding theory was proposed to account for this anomaly. As the muscle length decreases or increases, the overlapping of thick and thin filaments is increased and decreased respectively. Thus, according to theory, muscle length should be a determinant of active tension generated by striated muscles. Moreover, there should be an optimal length because too much sarcomere shrinkage

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