An ultrastructural analysis of the sympathetic neuromuscular junctions on arterioles of the submucosa of the guinea pig ileum.

The relationship of the varicosities of sympathetic postganglionic nerve terminals to the smooth muscle cells of arterioles in the submucosa of the guinea pig ileum has been investigated quantitatively by electron microscopy. Longitudinal sections were cut through arterioles about 50 micron in diameter after fixation in vitro or in situ under pressure. About 13% of the varicosities in individual ultrathin sections made contact with the outer surface of the smooth muscle cells. The neuromuscular junctions resembled those in skeletal muscle: the basal laminae of the axon bundle and of the smooth muscle were fused, and synaptic vesicles were accumulated close to the region of fusion. When individual varicosities were examined in serial sections, 92% and 83% in two preparations were found to form junctions of this kind. Most of the noncontacting varicosities were bare of Schwann cell toward the arteriolar surface and separated from it by less than 200 nm. Almost all axon profiles contained synaptic vesicles with electron dense cores after exposure to 5-hydroxydopamine. In electrophysiological experiments, ionophoretic application of noradrenaline to the arteriolar surface along the nerve bundles (demonstrated subsequently by fluorescence histochemistry) produced responses resembling those evoked by nerve stimulation. These anatomical and physiological data, taken together with the evidence for quantal release in this preparation (see Hirst et al., '85), suggest that neuromuscular transmission involves the rare release of a quantum of noradrenaline at discrete points on the smooth muscle membrane.