The form of sympathetic postganglionic axons at clustered neuromuscular junctions near branch points of arterioles in the submucosa of the guinea pig ileum.

In fluorescence histochemical studies of the sympathetic innervation of the arterial vessels of the submucosa of the guinea pig ileum, we have identified clusters of varicosities overlying arteriolar branch points. These were particularly obvious on the small arterioles less than 45 microns in diameter, the non-branching regions of which generally lack much other innervation. Serial reconstruction from electron micrographs of axon bundles from the region of arteriolar branch points revealed the form of the varicose axons. Branching of axon bundles sometimes involved branching of individual axons. Within a cluster, most axons had several varicosities along lengths as short as 3 micron. The varicosities were very irregular in size and shape. Most large varicosities (greater than 1 micron in diameter) that were bare of Schwann cell covering (73%) formed neuromuscular junctions with basal lamina intervening between axon and muscle membranes. A smaller proportion (44%) of small varicosities (0.5-1.0 micron diameter) also formed junctions. Of all bare varicosities, 37% did not form contracts. In several of the large varicosities, prejunctional membrane specializations were identified over part of the neuromuscular junctions.