Structure of membranes in crayfish muscle: comparison of phasic and tonic fibres

Membranes of two crayfish muscles with different contraction speeds were studied with freeze-fracture replicas and thin sections. A fast-contracting, short sarcomere phasic muscle, the tail flexor, and a slowly contracting long sarcomere tonic muscle, the carpopodite flexor, were chosen for this study. Membranes examined included the plasmalemma, clefts, T-system, Z-tubules and sarcoplasmic reticulum (SR). We found distinct differences in the distribution of these membranes: T-system and clefts are more elaborate in the tail flexor, while SR is more extensive in the leg flexor. The tail flexor apparently lacks Z-tubules. These differences were more obvious in freeze-fracture replicas than in thin sections. In freeze-fracture replicas, both junctional and non-junctional T-tubule membranes can be distinguished from Z-tubules by content of intramembranous particles. The junctional regions of T-system and surface membranes contain large (10-11 nm) intramembranous particles that are absent from non-junctional parts of these membranes. There is also a class of particles on the junctional SR fracture faces that differs from intramembranous particles on non-junctional SR. These junctional specialization are similar in long and short sarcomere fibres.