ULTRASTRUCTURAL ANALYSIS OF SYMPATHETIC NEUROMUSCULAR-JUNCTIONS ON MESENTERIC VEINS OF THE GUINEA-PIG

This study reports on the detailed ultrastructure of sympathetic postg anglionic varicose axon terminals on mesenteric veins leading from the ileum of the guinea pig and in particular the structural arrangement of the varicosities with venous smooth muscle cells. The response to n erve stimulation in veins has a long time course and it has been sugge sted that this reflects a wide separation between the site of transmit ter release and the receptors on the effector cell membrane. The aim o f this study was to determine the distance between individual sympathe tic varicosities and smooth muscle cells in mesenteric veins. Fluoresc ent histochemical preparations of the sympathetic innervation of the d ifferent branches of mesenteric veins indicate the branching network o f varicose axons around the vessel to be relatively dense. Electron mi crographs show the innervation to be confined to the adventitia close to the medio-adventitial border and to be predominantly catecholaminer gic. A serial section ultrastructural analysis of the relationship of the varicosities with the outer smooth muscle cells showed that almost all (98%) of the exposed axon varicosities in the adventitia formed n euromuscular junctions. Three-dimensional reconstructions from serial sections of individual varicosities have shown that the junctions have structural specialisations identical to neuromuscular junctions descr ibed on arterial vessels and similar to those found at skeletal neurom uscular junctions. The density of neuromuscular junctions on the veins was found to be similar to that on the corresponding artery in the sa me animal. We suggest that in veins, noradrenaline is released focally at neuromuscular junctions. (C) 1993 Wiley-Liss, Inc.