Sympathetic innervation of renal and extra-renal arterial vessels.

The innervation of arterial vessels has been analyzed using serial thin sections examined in the electron microscope. Sympathetic postganglionic noradrenergic axons have been found to make neuromuscular junctions on the outermost cells of the media of virtually all muscular arterial vessels with diameters between 20 microns and 1 mm in rat, rabbit and guinea pig. Junctions are formed by the majority of varicosities which are not covered by Schwann cell on the side towards the artery surface. The junctions resemble those on skeletal muscle fibers, having a single layer of basal lamina in the synaptic cleft. Although post-junctional specializations have not been identified, 15 to 20% of junctions show electron dense regions of the prejunctional membrane with associated clumps of synaptic vesicles. The form of the varicose axons differs, and three distinct types have been distinguished on morphological grounds. Two of these occur in the juxtaglomerular region of the kidney, with characteristic distributions of junctions which differ between the afferent and efferent arterioles. The presence of numerous close junctions has revolutionized our thinking about how sympathetic neuroeffector transmission occurs in arterial vessels.