Insect visceral muscle. Fine structure of the proctodeal muscle fibres

The fine structure of the longitudinal muscle fibres of the cockroach proctodeum was investigated by electron microscopy. The fibre is separated incompletely into fibrils, the resting sarcomere length is variable: about 5·8 to 7·3 μm, and the A- and I-bandings are not always clear in longitudinal sections. The ratio of thin and thick filaments at the overlapped region is about 4·1:1 when relaxed, and about 9·8:1 when fully contracted. The myofilament array is not well organized.The previously observed prolonged time course of muscle contraction seems to correlate with the present observations on the poorly developed sarcoplasmic reticulum and irregular distribution of transverse tubules. The Z-bands are irregularly aligned and discontinuous in longitudinal sections. The Z-band structure was studied in relation to the supercontractility. It was found that at maximal isotonic contraction (about 25 per cent rest length) the myofilaments pass through the expanded Z-regions.