The structure of the junction between sarcoplasmic reticulum (SR) and transverse tubular (T) system at the triad has been studied in twitch fibers of the frog. The junction is formed by flattened surfaces of the SR lateral sacs and the T-system tubule, which face each other at a distance of 120–140 A. At periodic intervals of about 300 A, the SR membrane forms small projections, whose tips are joined to the T system membrane by some amorphous material. The SR projections and the amorphous material are here called SR feet. The feet are disposed in two parallel rows, two such rows being present on either side of the T-system tubule. The junctional area between the feet is apparently empty. The feet cover no more than 30% of the T system surface area and 3% of the total SR area. The functional significance of this interpretation of the junctional structure is discussed.

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