Mf. Klemm et al., ULTRASTRUCTURAL ANALYSIS OF SYMPATHETIC NEUROMUSCULAR-JUNCTIONS ON MESENTERIC VEINS OF THE GUINEA-PIG, Journal of comparative neurology, 334(1), 1993, pp. 159-167
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.