Abstract
Summary
The maximum decrease in blood pressure and heart rate in response to electrical stimulation of the aortic (AN) and carotid sinus nerves (CSN) of the dog was essentially the same in both cases. Thus, reported difference in the effectiveness of the reflexes served by these nerves must be due to the baroreceptor discharge characteristics in response to a rise in pressure in the respective regions. Both nerves consist of at least two groups of depressor afferents and one fiber group which appears to arise from chemoreceptors. These fiber groups are distinguishable by their threshold to electrical stimulation. The major group of depressor fibers in the AN have a low threshold to stimulation while the major group of depressor fibers in the CSN are of higher threshold and presumably smaller in diameter. Maximal depressor effects to stimulation of the CSN are attained at lower stimulus frequencies (10-20 Hz) than is the case during AN stimulation (60-80 Hz). The chemosensory fibers in both nerves have a higher threshold than either group of depressor fibers. Stimulation of these fibers in the AN causes a rise in pressure and cardiac acceleration which nearly offsets the effects of stimulating the depressor fibers. However, little increase in breathing is observed. Stimulation of the chemosensory fibers in the CSN causes a marked hyperventilation but has relatively little effect on blood pressure and heart rate.
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