Structure of the subsynaptic sarcoplasm in the interfolds of the frog neuromuscular junction.

After aldehyde fixation of frog muscles, complex organelles, which appear specific to the subsynaptic sarcoplasm, were observed at the neuromuscular junction. These organelles have a cylindrical shape; their diameter ranges generally between 150 and 300 nm, and their length between 500 nm and several micrometres. They are situated between the folds formed by the postsynaptic membrane beneath the nerve terminal branches, and, like these folds and the filament bundles of the interfolds, are orientated perpendicular to the axis of the terminal branches. The organelles are not limited by a membrane, but their cylindrical form is delimited by the interfold filaments which are applied to their surface and which constitute a sort of muff. The majority of the interfolds contain only a single subneural cylinder. On occasion two, three or more may be seen in the same interfold; in this case, the cylinders remain distinct, generally being separated from each other by filaments. Each of these cylinders contain an electron-dense axial strip from which radiate tenuous trabeculae. These trabeculae make contact with the filaments surrounding the cylinder. At both ends the axial strip connects with the plasma membrane. Tubules belonging to the sarcoplasmic reticulum can be seen on the surface of the cylinder: some of these pass through the cylinders, by-passing the axial strip. Several hypotheses concerning the possible functions of these cylinders and of the subneural filaments are discussed.