Ultrastructural features of the innervation and smooth muscle of the rabbit facial vein, and their relationship to function

Structural organization of hepatic portal vein in rat with special reference to musculature, intimal folds, and endothelial cell alignment

Structural organization of hepatic portal vein (HPV) was examined in adult rats by means of light and electron microscopy. Three characteristic features were found in the wall structure of rat HPV. (1) Tunica media consisted of two kinds of smooth muscle. The inner circular smooth muscle (CSM) was composed with one or two layer of smooth muscle cells, and was found in the entire length of the HPV and its tributaries. The outer longitudinal smooth muscle (LSM) was limited to a specific region of HPV; in particular it was well-developed at distal half of HPV. CSM counteracts luminal hydrostatic pressure to prevent circumferential hyperextension of venous wall, whereas LSM is likely to counteract a tractive force produced by gravity and movement of small intestine. (2) Intima of HPV showed a unique feature, intimal folds, which protruded into the lumen and were aligned almost circumferentially. Intimal folds were found only at the same region where the LSM was well-developed. Thus, LSM is presumably involved in the formation of intimal folds. (3) The endothelial cells between intimal folds were circumferentially aligned along the folds, although those in the other regions of HPV were arrayed along the longitudinal axis of HPV or the direction of blood flow as reported in other kinds of blood vessel. This finding implied that the circumferential blood flow locally occurs on the surface of intima between the intimal folds.

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

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.

Ultrastructure of the neuromuscular junction of vasomotor nerves in the microvasculature of human dental pulp

Vasomotor nerves in human dental pulp were more closely related to arterioles than to venules. Most were composed of unmyelinated fibres, which were mainly adrenergic. They appeared close to arterioles that were surrounded by a few layers of contractile smooth-muscle cells. The smaller arterioles with a diameter of 10-15 microns received a more intimate innervation by vasomotor nerves than did the larger. These vessels occasionally showed much narrower neuromuscular junctions than previously reported. Most of these nerve fibres were identified as adrenergic by the presence of chromaffin-positive synaptic vesicles detected by ultrastructural enzyme histochemistry. Their function appeared to be to regulate the blood flow and/or the blood pressure by stimulating smooth-muscle cells, resulting in contraction and a change in the calibre of the vessels. Capillaries and venules, which have a higher permeability, received weaker innervation by the vasomotor nerves than did arterioles. The intimate relation between vasomotor nerves and arterioles is related to the function of dental pulp in normal and pathological conditions.

Ultrastructural analysis of sympathetic neuromuscular junctions on mesenteric veins of the guinea pig

This study reports on the detailed ultrastructure of sympathetic postganglionic 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 nerve stimulation in veins has a long time course and it has been suggested that this reflects a wide separation between the site of transmitter release and the receptors on the effector cell membrane. The aim of this study was to determine the distance between individual sympathetic varicosities and smooth muscle cells in mesenteric veins. Fluorescent histochemical preparations of the sympathetic innervation of the different branches of mesenteric veins indicate the branching network of varicose axons around the vessel to be relatively dense. Electron micrographs show the innervation to be confined to the adventitia close to the medio-adventitial border and to be predominantly catecholaminergic. 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 neuromuscular junctions. Three-dimensional reconstructions from serial sections of individual varicosities have shown that the junctions have structural specialisations identical to neuromuscular junctions described on arterial vessels and similar to those found at skeletal neuromuscular junctions. The density of neuromuscular junctions on the veins was found to be similar to that on the corresponding artery in the same animal. We suggest that in veins, noradrenaline is released focally at neuromuscular junctions.

Mechanisms of inhibitory noradrenergic transmission in the rabbit facial vein

In isolated buccal segment of the rabbit facial vein, electrical responses produced by perivascular nerve stimulation and exogenously applied noradrenaline (NA) were recorded from the smooth muscle cells using microelectrode. Perivascular nerve stimulation hyperpolarized the smooth muscle cell membrane. The hyperpolarization was converted to depolarization after application of the beta-adrenoceptor antagonist, propranolol, and the depolarization was blocked by alpha 2-adrenoceptor antagonists, yohimbine. These responses elicited by nerve stimulation were blocked by tetrodotoxin or guanethidine, but not by atropine. Exogenously applied NA mimicked the responses elicited by nerve stimulation. The amplitude of the beta-adrenoceptor-mediated hyperpolarization was increased in low potassium solution, decreased in high potassium solution, but unaltered by low sodium or low chloride solution, i.e., the hyperpolarization may be generated by an increase in potassium conductance of the membrane. An involvement of the apamin-sensitive (Ca-dependent) potassium channel or sodium-potassium ATPase in the hyperpolarization was ruled out.

The human adrenergic neurovascular mechanism

Although evidence is limited and derived from many studies with many different objectives on a variety of blood vessels under varying experimental conditions, it appears that the human adrenergic vascular neuroeffector mechanism is qualitatively similar to that defined in studies on animals. The characteristics of the alpha-adrenoceptor based upon pA2 measurements and other pharmacological characteristics show it to be similar to adrenoreceptors defined in animals. The sensitivity of vascular smooth muscle to norepinephrine is of the same order throughout the human vasculature and is similar to that seen in most animal vessels. In most blood vessels the alpha-adrenoreceptor is the dominant type. By contrast the beta-adrenoceptor component is usually small with a high threshold except in the coronary artery and facial vein. Sympathetic frequency-response relationships of both arteries and veins are similar to those from animal studies. The human alpha-adrenoceptor is coupled both to sequestered calcium sites and slow calcium channels, and these two mechanisms are responsible for the biphasic response of human blood vessels to norepinephrine. The slow calcium channel but not the sequestered calcium system is sensitive to calcium antagonists. There is evidence that the human adrenergic mechanism is altered in disease.