Characteristics of rat aortic baroreceptors with nonmedullated afferent nerve fibers.

Nervous activity was recorded from nonmedullated afferent baroreceptor fibers in the left aortic nerve of normotensive rats. The aortic arch was perfused in vitro with oxygenated Krebs-Henseleit solution. Two different patterns of aortic C fiber discharge were found. One type (5 of 29 fibers) had a regular discharge at constant pressure whereas the other type had an irregular discharge. The differences between the two types were independent of the mean steady state rates of discharge. The average threshold pressure for the receptors with the regular discharge was 87 mm Hg (range, 65-125); it was 152 mm Hg (range, 85-260) for the irregularly discharging receptors. During a pressure step the receptors in either group normally showed an initial high frequency burst of activity and a sustained discharge which had a lower frequency. Furthermore, the peak discharge and the sustained discharge were lower on the average in the receptors with the irregular type of firing. We also found that addition of norepinephrine to the perfusate lowered threshold. Since unmyelinated axons are the predominant fiber type in the rat aortic nerve and since some receptors are activated at physiological pressures they may contribute to the tonic control of blood pressure. Their effects should be particularly important when the blood pressure is increased.