The relationship between aortic baroreceptor activity and arterial pressure is not monotonic.

Previous reports indicate that when aortic pressure (AP) falls below the threshold (P (th)) for baroreceptor sensitivity, activity in the aortic depressor nerve (ADN) may increase. To quantify and explain this anomalous behaviour, we analysed curves describing the relationship of baroreceptor fibre activity in rabbit left ADN to AP. Data were obtained in anaesthetised New Zealand White rabbits. Occlusion and release of cuffs around the inferior vena cava and descending aorta generated AP ramps (25-140 mmHg). Response curves were obtained for 173 fibres in 26 animals. Thirty percent of curves had a nadir (J-shaped curve), and in 40% activity was always present. In fibres showing activity below P (th), firing was predominantly diastolic, switching to systolic firing at P (th). The unusual behaviour of a substantial fraction of aortic baroreceptors below P (th) accounts for the J-shaped response curve of the whole ADN. We suggest that fibres that fire during diastole at pressures below P (th) may have sensory endings close to the origin of the left subclavian artery. As a consequence of this anatomical location, low pressures can impose strain on these receptors, which is then relieved by the systolic pulse.