Ultrastructure of stick insect and locust skeletal muscle in relation to excitation-contraction coupling

Abstract

The subcellular structures involved in excitation-contraction coupling have been examined in locust metathoracic extensor tibialis muscle and stick insect prothoracic flexor tibialis muscle. In both muscles the sarcomeres are of similar resting length (4·5 μm) and the fibres are invaded by sarcolemmal invaginations and an extensive transverse tubular system is present. Locust muscle has a much more extensive sarcoplasmic reticulum and more frequent dyads per unit area than stick insect muscle. The myofilament array is fairly similar in both muscles; in locust, each myosin filament is surrounded by nine to eleven actin filaments, while in stick insect twelve actin filaments usually surround each myosin filament. The striking differences in speed of contraction and relaxation between these muscles can almost certainly be ascribed to differences in development of the sarcoplasmic reticulum and the dyads, which are areas of active calcium uptake and release during the excitation-contration coupling cycle.