Frequency of neuromuscular junctions on arteries of different dimensions in the rabbit, guinea pig and rat.

The medio-adventitial border of a variety of perfusion-fixed arteries of young adult rabbits, guinea pigs and rats has been studied in the electron microscope. The arterial media in the different vessels ranged from 2 to 25 cells thick. Neuromuscular junctions, defined as axon varicosities containing synaptic vesicles closely apposed to the outer surface of smooth muscle cells, with only a single layer of basal lamina intervening between axon and muscle membranes, were identified in all three species. Junctions were found in most vessels less than 1 mm in diameter with a frequency ranging from 8,000-150,000 junctions per square millimeter of smooth muscle surface, the number generally increasing with decreasing arterial diameter. These small arteries were mostly, but not exclusively, muscular rather than elastic. In large arteries, such as abdominal aortae and some elastic arteries lying close to the heart (e.g. the carotid), no junctions were found. In a few vessels, such as guinea pig basilar (muscular) and rat and guinea pig superior mesenteric (elastic) arteries, junctions were found infrequently (1,000-4,000/mm2). The data indicate that all muscular arteries in rats and guinea pigs, and most muscular arteries in rabbits, receive innervation in the form of sympathetic neuromuscular junctions. Whilst a few elastic vessels are sparsely innervated with junctions, some are surrounded by axon bundles containing vesicle-filled varicosities. The physiological significance of the latter is obscure.