The function of baroreceptor C fibres in the rabbit's aortic nerve

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Abstract
The participation of aortic nerve C fibers in the baroreflex was investigated by recording changes in renal nerve activity in response to acute increases in arterial pressure, in 2 experimental situations: with the A and C fibers of the left aortic nerve intact, and with the A fibers temporarily blocked by a hyperpolarizing current (anodal block). Pressure was increased by manual inflation of an intra-aortic balloon, and interference from other baroreceptor areas was avoided by carotid occlusion and sectioning of the right arotic nerve. Rises in mean arterial pressure exceeding 20 mmHg, to levels above 110 mmHg, were needed to trigger sympathetic inhibition via C fibers. A rise of 45 mmHg caused 50% reduction in sympathetic activity, equal to stimulation of aortic nerve C fibers at about 3 Hz. In contrast, a rise of 20-30 mmHg evoked more than 60% reduction in sympathetic activity when the A fibers were operative. Judged by these studies of peak reflex responses to brief pressure rises, baroreceptors with C fibers in the aortic nerve have a much higher threshold to pressure than their myelinated counterparts; the C fibers contribute to the baroreflex inhibition of sympathetic discharge only when pressure is increased well above normal resting levels.