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Notes: Summary Isolated skeletal muscle fibres ofRana pipiens were shortened below their slack length by longitudinal compression in a gelatine block, and examined by light and electron microscopy. Waves appeared sharply when the striation spacing (S) reached a critical value (about 2 µm) and increased in height with further compression down toS = 1.6 µm while the resting band pattern was maintained. The waves were plane, helical or irregular, with wave lengths of 5–15 striations. The Z lines usually ran perpendicular to the direction of the myofibrils to form wedge-shaped sarcomeres. The bending occurred mainly in the I band. The thin filaments ran stiffly for about 30 nm from the Z line and then bent toward the A band. The thick filaments bent very slightly, particularly at their tips. The edges of the A band were indistinct, and there were no dense lines at the A–I junction. The appearance of the individual sarcomeres resembled those in relaxed myofibrils at slack length, with no Cm bands. The H zone was only seen occasionally in the slack and wavy fibres examined. In very thin sections the individual thin filaments were seen to end in the pseudo-H zone, and not to cross the M line. There was a single array of not more than six thin filaments round each thick one in transverse sections of the M-line region. These observations suggest that the narrowing of the bands observed in fresh wavy fibres is due mainly to the obliquity of the myofibrils, and that the sarcomere length measured parallel to their axis is longer than the striation spacing. The relationship between sarcomere length and the length of the thin-filament complex is discussed.

Notes: Summary 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 (P diff) 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 betweenP diff 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.