Comparison of afferent activity of canine aortic and sinus nerves.

Pressure-dependent changes in afferent activity in the aortic and the sinus nerves were studied in 17 anesthetized dogs that were artificially ventilated with oxygen. Systemic arterial blood pressure was varied either continuously or in steps over the range of mean aortic blood pressure from 220 to 50 mm Hg with a pressurized reservoir connected to the abdominal aorta. Multifiber preparations from 20 aortic and 8 sinus nerves were used, and stimulusresponse curves were defined by measuring the mean impulse frequency at the various levels of pressure. Heart rate and pulse pressure were similar during recordings from both nerves. The two curves were S-shaped, with that for the aortic nerve being to the right of that for the sinus nerve. The change in activity was approximately linear from 200 to 120 mm Hg (mean aortic blood pressure) in the aortic nerve and from 180 to 80 mm Hg in the sinus nerve; there was no significant change in impulse frequency below mean blood pressures of 100 and 70 mm Hg, respectively. The threshold systolic blood pressure was 95 ± 3 (SE) mm Hg for the aortic receptors and 62 ± 4 mm Hg for the carotid sinus receptors. Thus, in the dog, the aortic baroreceptors are involved mainly in the control of high blood pressure, whereas at lower pressures the major control occurs through the carotid sinus baroreflex.