Electron microscopic studies of the pressoreceptor fields of the carotid sinus of the dog

Abstract

In the dog, pressosensitive endings of the sinus nerve extend along the border between the adventitia and media of the carotid sinus wall. The axon endings, containing a great number of mitochondria, can be divided into small (600-2,000 nm) and large (6,000-8,000 nm) end swellings. In the terminal region the pressosensitive fibers are surrounded by ramified and highly structured Schwann "terminal cells". The topographic location in relation to elastic and collagenous tissue indicates a functional connection between receptors and efferent nerve endings in the immediate surroundings has been discussed in this report. Several axon endings contain variable amounts of glycogen which is regarded as an indication for the inactive metabolic state of the ending. Axonal swellings demonstrate considerable modification in structure, such as loss of structural integrity in mitochondria, the formation of lamellar fields, vesicular irregularities and disintegration of axoplasm, all of which are considered as the morphological expression of "wearing out", degeneration and possibly regeneration.

Observations on the Structure and Innervation of the Presumptive Carotid Sinus Area in the Giraffe (<i>Giraffa camelopardalis</i>)

“…lOe). Similar mitochondria-rich varicosities have been described by previous workers in the carotid sinus walls of other animals and are believed to be the terminal endings of the sensory nerves distributed to this area [/?ees, 1967, 1968 ;Chiba, 1972;Bock and Gorgas, 1976;Knoche and Addicks, 1976;Knoche et al, 1980]. The axonal varicosities observed here are closely related to Schwann cells, but also show large areas denuded of Schwann cell coverings ( fig.…”

“…Furthermore, in some species sensory nerve fibres are distributed in large numbers not only to the carotid dilatation itself, but also to other confluent vascular areas of the carotid bifurcation region [Rees, 1968]. In all cases, however, the densely innervated arterial walls correspond to the parts of the carotid bifurcation, having a high density of elastic tissue in the tunica media [Muratori, 1966;Rees, 1967;Rees and Jepson, 1970;Bagshaw and Fisher, 1971;Bock and Gorgas, 1976;Knoche and Addicks, 1976;Knoche et al, 1980]. The elastic structure of the arterial wall at the carotid bifurcation area appears, therefore, to be a reliable morphological feature for identifying the baroreceptor areas.…”

Firing characteristics of single-fiber carotid sinus baroreceptors.

“…Baroreceptors have been reported to be closely associated with elastin, collagen, and smooth muscle within the sinus wall. [30][31][32][33][34] If baroreceptors are mechanically coupled to and monitor deformation of sinus wall elements, variations in distensibility of wall elements might lead to differential activation of receptors at different degrees of sinus distension.34 Thus, variations in coupling to different wall elements may contribute to differences in discharge patterns of type I versus type II baroreceptors and also to the differences seen in firing characteristics within each type of receptor. For example, the observation that both Fsat and slope are inversely correlated with Pth for type I receptors suggests that, within this group, those baroreceptors with high Pth are associated predominantly with lessdistensible collagen and that those with lower Pth are coupled to more-distensible smooth muscle or elastin.…”

Endothelium-mediated and direct actions of acetylcholine on rabbit aortic baroreceptors.

“…[15][16][17][18] This anatomic association suggests a possible efferent mechanism for modulating baroreceptor afferent activity. Conceivably, acetylcholine released from efferent varicosities could stimulate the aortic baroreceptors directly or through changes in vasoactive tone.…”