Fine structure of the myotendinous junction and "terminal coupling" in the skeletal muscle of the lamprey, Lampetra japonica.

The myotendinous junction in the skeletal muscle of adult lamprey Lampetra japonica was studied with an electron microscope. Numerous finger-like sarcolemmal invaginations were present at the ends of muscle fibers to form the myotendinous junction. Parietal fibers of each muscle unit showed more closely distributed sarcolemmal invaginations than central fibers. Features of the myotendinous junction generally conform to the accounts in the literature. The sarcolemmal invagination was covered on its sarcoplasmic aspect by the connecting filament layer and the dense amorphous attachment layer, and on the extracellular aspect by the intermediary layer and the external lamina with collagen fibrils arising from the myosepta. Sarcolemmal invaginations were sometimes seen to consist of a pair of sarcolemmas of adjacent muscle fibers within a muscle unit, which is characteristic to the myotendinous junction of lamprey. It is noteworthy that the connecting filament layer is much thinner than that, e. g., in the tadpole tail muscles (Nakao, '74). Furthermore, it is much thicker in the parietal fibers than in the central fibers. The sarcolemma of the terminal segment of the invagination frequently showed specific coupling with cisterns of the sarcoplasmic reticulum (terminal coupling). The external lamina is partially or completely deficient in the terminal segment of sarcolemmal invaginations which form terminal couplings so that collagen fibrils contained in the invagination appear to be in direct contact with the sarcolemma; however, definite relationships of collagen fibrils with the sarcolemma and the external lamina in the terminal segment of invagination still remain obscure. This type of coupling is considered to play a role in the coupling of excitation to contraction of muscle fibers as triads and diads.