NEUROMUSCULAR-JUNCTIONS MADE BY NERVE-FIBERS SUPPLYING THE LONGITUDINAL MUSCLE OF THE GUINEA-PIG ILEUM

This study reports on the ultrastructure of the projections to the lon gitudinal muscle of the guinea-pig ileum from nerves of the tertiary p lexus. Reconstructions of serial electron micrographs through axons in bundles of the tertiary plexus showed that the majority of vesicle co ntaining profiles which became exposed through a gap in the Schwann ce ll formed specialized neuromuscular junctions. At these junctions, the exposed profile and the muscle cell were separated by less than 100 n m and the intervening-cleft was filled with a single layer of basal la mina. Small synaptic vesicles were aggregated towards the area of clos e contact, In this tissue two different types of neuromuscular junctio n were found. Two-thirds of junctions were similar to those found in o ther tissues. They had many small vesicles aggregated towards the area of junctional contact: some 20% of these junctions had pre-junctional membrane specializations. The remaining junctions were smaller than t hose usually found in autonomic end organs. These junctions covered a small area of membrane and contained only a few small synaptic vesicle s: pre-junctional membrane specializations were not found on these jun ctions. Results of physiological experiments have shown that neurally released transmitters activate a different subset of receptors to exte rnally applied transmitters. The morphological data presented here sug gest that specialized neuromuscular junctions exist where these recept ors may be localised.