Scanning electron-microscopic study on the three-dimensional structure of motor endplates of the slow (tonic) muscle fibers in the frog, Rana n. nigromaculata.

The three-dimensional organization of the motor endplates of the slow fibers of the rectus abdominis muscle in the Japanese meadow frog (Rana nigromaculata nigromaculata Hallowell) is visualized by use of a field-emission scanning electron microscope after removal of connective tissue components by HCl hydrolysis. Clusters of shallow oval depressions approximately 1-3 microns in diameter are seen in the postsynaptic membrane at intervals of about 150 microns. On the surface of these depressions, a few low bulges of postsynaptic membrane are irregularly arranged. Terminal boutons, approximately 1-3 microns in diameter, occur along the length of nerve branches and terminals and fit into the shallow oval depressions of the postsynaptic membrane. The Schwann cells covering the terminal branches exhibit a simpler organization than those in twitch fibers.