Sarcomere and filament lengths in passive muscle fibres with wavy myofibrils

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Abstract
Longitudinal compression of isolated skeletal muscle fibres ofRana pipiens caused waves to appear sharply at a critical striation spacing which was slightly less than the slack length measured at the same point. Both slack length and critical length varied between fibres and along the length of one fibre, being shortest near the tendons. The critical length varied from 1.93 to 2.11 µm. The troponin periodicity (Pdiff) was measured in embedded material by light diffraction of calibrated electron micrographs. Comparison between the troponin periodicities in a fibre made wavy at one end and stretched at the other showed that longitudinal compression did not cause shortening of the thin filaments. Comparison betweenPdiff and the troponin periodicity of fresh muscle provided an estimate of the artefact mainly caused by shrinkage during specimen preparation. It varied from 3 to 11%. The gaps between the ends of the thin filaments in the M-line region were estimated from sarcomere length (corrected for shrinkage) and the assumedin vivo values for total thin-filament length or the length between the last troponin lines (1.975 µm and 1.925 µm respectively). The estimates were confirmed by a few direct measurements of thin-filament length and periodicity. Sarcomere length varied from fibre to fibre, from 1.91 to 2.12 µm, except at the inside of bends in wedge-shaped sarcomeres where it fell to 1.86 µm in some cases. This indicates that in one fibre the tips of the thin filaments overlapped at the level of the last troponin lines, while, at the other extreme, the tips of the thin filaments only just reached the bare zone of the thick filaments. The origin of the resistance to sliding and the force which restores an actively shortened fibre to its slack length are discussed. While there may be a well-defined barrier to sliding at the point where the troponins of opposite polarity meet, there must also be an additional length-dependent resistance to account for the appearance of waves at longer sarcomere lengths. The formation of waves is interpreted as a buckling phenomenon in which a longitudinal compressive force is applied to the myofibrils which have a finite stiffness against bending and a finite elastic restraint against lateral displacement. The bending stiffness is largely and perhaps entirely accounted for by contributions from (1) the stiffness of the individual filaments and (2) the stiffness of myofibrils calculated from their Young's modulus.