Attachments of phasic and tonic abdominal extensor muscles in crayfish

Abstract

The fine structure of attachments of phasic and tonic crayfish abdominal extensor muscles to the exoskeleton was studied by means of electron microscopy. Tonic muscles contract more slowly and exert more tension per unit area of cross section than phasic muscles. The muscle attachment regions show differences in structure which are correlated with the different contractile properties. In both muscles, microtubule-filled tendinous cells intervene between the end of the muscle and the exoskeleton; but in phasic muscles the tendinous cells are joined to the muscle by rather long extracellular microfibrils which in general run obliquely to the longitudinal axis of the muscle, whereas in tonic muscle, the tendinous cells and the muscle are joined together more directly by shorter microfibrils running parallel to the muscle's longitudinal axis, and also by desmosome-like structures. In addition, the extracellular materials in tonic muscles appear to be firmly attached to the Z lines of the muscle sarcomeres; such an association was not observed in phasic muscle.