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

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