Specialized contacts between sarcolemma and sarcoplasmic reticulum at the ends of muscle fibers in the diaphragm of the rat

Summary An electron-microscopic study of the myotendinous portion of the diaphragm in the Wistar rat has shown that at the ends of muscle fibers, longitudinally oriented invaginations and peripheral furrows of the sarcolemma establish specialized contacts with individual sacs of the sarcoplasmic reticulum. The construction of these terminal contacts is similar to that of contacts between sarcolemmic T-tubules and terminal cisternae of the sarcoplasmic reticulum, characterized by formation of triads. The contact zones of the sac membrane are undulated and bound to the adjoining sarcolemma via electron-dense profiles of varying forms. Frequently, the terminal contacts and triads are located at the same level within the muscle fiber, at the borderline between A- and I-bands of the sarcomeres. At the ends of muscle fibers combined contacts between peripheral furrows of the sarcolemma, terminal cisternae of the sarcoplasmic reticulum, and T-tubules of the triads are also disclosed. The implications of the terminal contacts for muscle contraction are discussed.