Neuromuscular junctions made by nerve fibres supplying the longitudinal muscle of the guinea-pig ileum.

This study reports on the ultrastructure of the projections to the longitudinal muscle of the guinea-pig ileum from nerves of the tertiary plexus. Reconstructions of serial electron micrographs through axons in bundles of the tertiary plexus showed that the majority of vesicle containing profiles which became exposed through a gap in the Schwann cell formed specialized neuromuscular junctions. At these junctions, the exposed profile and the muscle cell were separated by less than 100 nm and the intervening cleft was filled with a single layer of basal lamina. Small synaptic vesicles were aggregated towards the area of close contact. In this tissue two different types of neuromuscular junction were found. Two-thirds of junctions were similar to those found in other 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 those usually found in autonomic end organs. These junctions covered a small area of membrane and contained only a few small synaptic vesicles; pre-junctional membrane specializations were not found on these junctions. Results of physiological experiments have shown that neurally released transmitters activate a different subset of receptors to externally applied transmitters. The morphological data presented here suggest that specialized neuromuscular junctions exist where these receptors may be localised.