Ultrastructure of sympathetic axons and their structural relationship with vascular smooth muscle

Abstract

This review focuses on the more recent findings of the structure of sympathetic postganglionic axons and the association of their varicose terminals with vascular smooth muscle. These studies have investigated the innervation of a wide range of vessels from different regions of the vasculature in the rat, guinea pig and rabbit and have predominantly used serial sections and computerised three-dimensional reconstructions of entire varicosities. They have shown, contrary to previous studies conducted in the 1960s and 1970s, that sympathetic axon varicosities commonly form structurally specialised neuromuscular junctions with vascular smooth muscle cells of most resistance arteries and some small veins. In addition, they have shown that most axon varicosities innervating small arterioles and small mesenteric veins form neuromuscular junctions, indicating that neurotransmitter is primarily released at such neuromuscular junctions. This review discusses the structure of sympathetic neuromuscular junctions, their development, structural diversity and distribution on vessels from different regions of the vasculature. These more recent structural findings and their possible significance for our understanding of mechanisms involved in neural transmission in blood vessels is discussed.

Specialised sympathetic neuroeffector associations in immature rat iris arterioles

“…Fluorescence histochemical estimations of varicosity numbers were also at variance with ultrastructural data, which showed that varicosity numbers were not significantly different between the 2 ages. However as suggested by Luff (1991Luff ( , 1996, the clustering of varicosities observed in the present study may lead to an underestimate of varicosities at the light microscope level. Thus, caution should be taken when using fluorescence histochemistry as the primary method of assessing nerve fibre and varicosity density (van Orden et al 1966 ;Bevan et al 1980 ;Burnstock & Griffith, 1983 ;Luff, 1991 ;Dodge et al 1994).…”

“…Like the reconstructed varicosities from arterioles of 21 d animals, the size and shape of the varicosities varied considerably, as did the size and shape of the VSM contact areas. While VSM cell contact areas were similar in size at 14 and 21 d, they were significantly smaller than those in other blood vessels (Luff, 1996 : 0.58p0.06 µm#, n l 49, from 4 different beds), although they were similar to those observed between varicosities and dilator myoepithelial cells (Hill et al 1993). The average volume of irideal varicosities from 14 d animals was 70 % that of 21 d arterioles, but this was not significant.…”

“…Such studies should be considered semiquantitative at best. Estimation of nerve fibre density using ultrastructural methods should also be considered with caution (Luff, 1996). Where 2 or more effectors are found close together in the same region, the particular fibres associated with the effector of interest may be difficult to differentiate from those associated with the other effector.…”

“…The functional implications of this are speculative. These varicosities may release transmitter, however, since there were no apparent specialised neuroeffector regions associated with these varicosities, which are suggestive of a specialised release site (see Hirst et al 1996 ;Luff, 1996), transmitter release may occur nonspecifically from all over the varicosity surface. The mixed receptor responses at 14 d may result from transmitter release at these sites.…”

“…Specialised neuroeffector associations have been described in a number of adult autonomic tissues (see Hirst et al 1996 ;Luff, 1996 ;Bennett et al 1998, for reviews). These studies have found that the majority of varicosities in autonomic tissues contain regions where the membranes of varicosities and the effector cells are closely apposed with synaptic vesicles being accumulated near these sites.…”