Vasomotor inhibition in rabbits by vagal nonmedullated fibers from cardiopulmonary area

In anesthetized rabbits with sinus and aortic nerves cut, when the cervical vagal nerves were cooled to 12, 8, 6, and 0°C, there were progressive increases in aortic blood pressure of 7 ± 1, 15 ± 2, 25 ± 1, and 41 ± 2 mmHg (SE), respectively. For comparison, during cooling of the aortic and vagal nerves, at 12°C there was a decrease in firing in the afferent fibers from aortic baroreceptors (48 ± 4%) and pulmonary stretch receptors (57 ± 5%), and at 6°C all activity was abolished. Thus, at 6°C the activity in medullated fibers from the aortic baroreceptors and pulmonary stretch receptors is blocked, but the increase in aortic blood pressure with vagal cooling is only 60% of that with cooling to 0°C. This demonstrates that cardiopulmonary receptors with nonmedullated vagal afferents (C fibers) contribute to the tonic inhibition of the vasomotor center. Because of overlap in sensitivity of different vagal fibers to cooling, the total contribution of these C fibers cannot be evaluated.