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Abstract

Electron micrographs of rat gastrocnemius muscle fibers show membrane-like dense structures traversing the space between the transverse system (T) and the terminal segments of sarcoplasmic reticulum (R). The periodicity of the structures, viewed in longitudinal sections of triads, varies from 150–250 Å. The distance between apposed membranes of T and R is 80–100 Å at the level of the dense structures between T and R. The distance is slightly greater than 100 Å at the level of less dense areas which alternate with the dense structures. The scalloped appearance of the R membrane, associated with differences in distance between T and R when the levels of dense structures and less dense areas are compared, leads to the suggestion that a holding force might be exerted by the dense structures. Such a holding force would support the view that the membrane-like dense structures are indeed connections between T and R. If membrane-like connections between T and R completely surround the less dense areas it is possible that a “physiological tight junction” conductive to electrotonic spread of current might exist between apposed membranes of T and R.