Reflex cardiovascular responses to graded stimulations of low- and high-threshold afferents in the carotid sinus and aortic nerves in the cat.

Reflex responses to electrical stimulation of low-threshold and high-threshold afferents in the aortic and carotid sinus nerves were investigated in chloralose-anesthetized cats. Changes in blood pressure, heart rate and muscle resistance were recorded. Low-threshold stimulation induced depressor responses of moderate magnitude. When stimulation intensity was increased to engage also the high threshold aortic nerve afferents a more pronounced reflex response was seen even with stimulation frequencies less than 10 Hz. Furthermore, stimulation of low- and high-threshold aortic nerve afferents induced different reflex patterns. For a given fall in blood pressure, the low-threshold afferents induced a greater reflex decrease in muscle vascular resistance compared with the high-threshold afferents, which reduced heart rate more. The effects seen at low frequency stimulation of high-threshold aortic nerve afferents were interpreted to reflect activation of the non-medullated baroreceptor afferents, while the depressor reflexes at low-threshold stimulation were induced by medullated baroreceptor afferents. Also the reflex effects of continuous and intermittent stimulation patterns of the various afferents were compared. At the same number of imp/s an intermittent stimulation of the low-threshold afferents induced greater reflex changes in blood pressure and muscle vascular resistance but smaller changes in heart rate. No such difference was observed when high-threshold afferents were similarly stimulated.