Ultrastructure of baroreceptors in the carotid sinus of the rabbit

Abstract

In the rabbit, baroreceptive intercalated and terminal receptor segments of the sinus nerve extend in the border region of adventitia and media of the carotid sinus wall. These receptors measuring 600–6,500 nm harbour a great number of mitochondria. They are either round, ovoid or elongated. Round receptors are present over the whole area of the carotid sinus. Large receptors appear to be missing in the upper region, where only very elongated receptors are present, which are arranged in circular array in relation to the vascular axis. The baroreceptors are enveloped by special organelle-rich terminal cells of Schwann cell origin. The receptors have a special orientation in regard to the muscle-elastic system and to collagen. Receptors contain osmiophilic and laminated bodies, signs of degeneration and vesicles of different sizes. Many tubuli as well as glycogen granules are apparent. Some of these axoplasmatic changes appear to be related to metabolic processes (turnover) occurring in young and old animals.

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

“…Therefore, it is confirmed that the area equivalent to a carotid sinus in the giraffe is situated at the carotico-occipital junction. This area is characterised by a preponderantly elastic tunica media and a rich sensory innervation through a branch of the glossopharyngeal nerve in conformity with the findings of other workers [Muratori, 1966;Rees, 1967Rees, , 1968Rees and Jepson, 1970;Bagshaw and Fisher, 1971;Bock and Gorgas, 1976;Knoche and Addicks, 1976;Knoche et al, 1980], However, it is pointed out that when locating the carotid baroreceptor area in the giraffe, the occipital artery -wrongly named internal carotid artery by Lawrence and Rewell [1948] -is the branch of the terminal carotid segment to be noted, rather than the anastomotic branch between the carotid and the vertebral arteries -erroneously named the occipital artery by Lawrence and Rewell [1948]. The carotid baroreceptor area involves the origin of the caroticovertebral anastomotic branch only when this artery shares a common origin with the occipital artery ( fig.…”

“…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.…”

“…Conceivably, therefore, as the function of elastic tissue has been consi dered to be that of controlling the rate of change in the form of a structure [Hass, 1939;Ayer, 1964;Rodbard, 1970], then its preponderance at the baroreceptor area modulates the distension of the intimomedial layer in systole and its prompt resump tion of normal morphology in diastole. In this way the arterial wall maintains a smoothly graded and intermittend deforma tion to which the baroreceptor nerves have been shown to be more responsive [Ead et al, 1952;Heymans et al, 1953;Gero and Gerova, 1967] The Connective Tissue/Nerve Relationships Previous ultrastructural studies of the mammalian carotid sinus wall have shown that the terminal sensory endings are located mainly in the media-adventitial bor der and deep adventitia where they are closely related to the connective tissue ele ments and the adventitial smooth muscle cells [Rees, 1967[Rees, , 1968Chiba, 1972;Bock and Gorgas, 1976;Knoche and Addicks, 1976;Knoche et al, 1980]. The receptor nerve terminals located in the outer media are normally sandwiched between two par allel elastic lamellae [Knoche et al, 1980], while those situated at the media-adventitial border are frequently applied to the adven titial surface of the external elastic lamina [Rees, 1967], The receptors located in the deep adventitia are generally related to the isolated adventitial smooth muscle cells and the elastic fibres found in this region [Rees, 1967;Knoche et al, 1980], Receptors located within the outer adventitia are mostly surrounded by bundles of collagen fibres taking a multidirectional course [Rees, 1967;Chiba, 1972;Knoche and Addicks, 1976;Knoche ex al., 1980], The problems faced in this study when obtaining and processing material for elec tron microscopy from a large animal makes it relatively difficult to determine the pat terns of distribution of the sensory nerves in the presumptive carotid baroreceptor area.…”

“…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.…”

Cholinesterase activity in the carotid sinus baroreceptor.

“…By electron microscopy it was evident that the reaction products were The presumptive baroreceptors in the carotid sinuses of various animals have been studied by light (1,2,14,18) and electron microscopy (3,4,10,11,12,15). In general, such baroreceptors are located in the regions between the adventitia and the media in the internal carotid artery wall, and consist of axon terminals with intermittent enlargements along their courses and Schwann cells investing such axon terminals.…”