Ultrastructure of junctional folds of motor end plates in extensor digitorum longus muscles of mice.

Motor end plates of C-fibers of extensor digitorum longus muscles of mice were examined in serial sections in order to investigate the exact shape of the bottom of their junctional folds by electron microscopy. The results show that the folds can be in continuity with T-tubules forming triadic junctions with the cisterns of the sarcoplasmic reticulum. The triads are in close contact with the myofibrils below the sole plate regions. The evidence that T-tubules originate from junctional folds indicates that it is necessary to reconsider the conception of their configuration.