Structural studies of the waves in striated muscle fibres shortened passively below their slack length

Isolated skeletal muscle fibres of Rana 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 microns) and increased in height with further compression down to S = 1.6 microns 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.