Responses to circulatory pressures, and conduction velocity, of pulmocutaneous baroreceptors in Bufo marinus.

Abstract

SUMMARY1. Baroreceptor activity was recorded within the recurrent laryngeal branch of the toad vagus in forty-four preparations. The receptive fields of the receptors were located in the pulmocutaneous artery (p.c.a.), generally within 5 mm of its separation from the truncus. However, the most easily recorded afferents in this nerve were mechanoreceptors which responded to punctate stimulation of the lip of the glottis.2. The conduction velocities of p.c.a. baroreceptor and mechanoreceptive glottal afferents recorded in the recurrent laryngeal nerve ranged from 0 3-07 (0-5+041) m s-1 and 2-2-14-0 (6 8+0 8) m s-1 respectively, which suggests that baroreceptor afferent fibres are non-myelinated and that glottal afferent fibres are myelinated.3. The p.c.a. baroreceptor discharge was largely confined to a period of systole in which systemic and p.c.a. arterial pressure profiles were identical. The maximum discharge frequency, number of spikes per cycle, and the duration of discharge increased, and the discharge latency decreased, as p.c.a. pressures were elevated. The latency of the discharge was pronounced at low p.c.a. pressures, and could be partially accounted for by the conduction time of the baroreceptor afferents to the electrodes (up to 100 ms).4. Carotid and aortic arterial baroreceptor and pharyngeal afferents were recorded in two pharyngeal branches of the vagus nerve which were closely associated with the carotid and aortic arterial arches.5. It is suggested that baroreceptor populations with their receptive fields in the third-(carotid) and sixth-(p.c.a. or pulmonary) arch arteries should be considered homologous in anurans and mammals, but that those of the fourth-(aorta) arch artery should not, since the vagal branches in which their afferents are carried do not appear to be equivalent in anurans and mammals.

Response characteristics of pulmocutaneous arterial baroreceptors in the toad, Bufo marinus.

“…The distribution is similar to that found in mammals (Van Vliet & West, 1987), and is consistent with the suggestion that baroreceptors are restricted to the vascular remnants of the embryonic branchial circulation (Koch, cited by Heymans & Neil, 1958;Adams, 1958). In terms of their demonstrated effects on heart rate, the dominant baroreceptor population in the toad appears to be that located in the pulmocutaneous artery (p.c.a.)…”

“…Arterial baroreceptor endings are distributed among each of the three central arterial arches in anuran amphibians (Neil, Strom & Zotterman, 1950;Ishii, Honda & Ishii, 1966;Ishii & Ishii, 1978; Ishii, Ishii & Kusakabe, 1985a;Van Vliet & West, 1987). The distribution is similar to that found in mammals (Van Vliet & West, 1987), and is consistent with the suggestion that baroreceptors are restricted to the vascular remnants of the embryonic branchial circulation (Koch, cited by Heymans & Neil, 1958;Adams, 1958).…”

“…baroreceptors are active within the physiological range of arterial pressures though their discharge is sparse at pressures approaching those found in resting conscious toads (Ishii & Ishii, 1978;Van Vliet & West, 1987). Recently we reported that p.c.a.…”

“…Recently we reported that p.c.a. baroreceptors were connected to slowly conducting afferents which exhibited conduction velocities typical of non-myelinated fibres (Van Vliet & West, 1987). The presence or absence of myelination in mammalian baroreceptors is associated with marked differences in the structure of the baroreceptive endings, the response characteristics of the receptors, and the baroreflexes which they mediate (Kirchheim, 1976; Thoren, 1981).…”

“…(Smith, Berger & Evans, 1981;West & Van Vliet, 1983). This population may be considered to be homologous with the pulmonary arterial baroreceptor population in mammals (Van Vliet & West, 1987).…”