The three-dimensional structure of motor endplates in different fiber types of rat intercostal muscle. A scanning electron-microscopic study.

The three-dimensional organization of the motor end plates in the red, white and intermediate striated muscle fibers of the rat intercostal muscle was observed under a field-emission type scanning electron microscope after removal of connective tissue components by HCl hydrolysis. The motor endplate of the white fiber had terminal branches (or axon terminals), which were large, long and thin, and small but numerous nerve swellings (or terminal boutons). The motor endplate of the red fiber had terminal branches, which were small, short and thick, and had large but fewer nerve swellings. The motor endplate of the intermediate fiber was intermediate in size and structure between these two. In detached nerve-ending preparations, primary synaptic grooves with slit-like openings of the junctional folds appeared on the surface of the muscle fibers. The primary synaptic grooves were more developed in the white fiber than in the red fiber, and they were intermediate in the intermediate fiber. The numerical ratio of slit-like openings was 1:1.8:3.5 in the red, intermediate and white fiber, respectively. The Schwann cells and their processes were observed on the surface of the motor endplate, with the processes covering the upper orifices of the primary synaptic grooves and sealing the terminal branches. The number of Schwann cells was usually three in the white fiber, two in the intermediate fiber and one in the red fiber.