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

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Abstract
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 < 10 Hz. 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. The reflex effects of continuous and intermittent stimulation patterns of the various afferents were also compared. At the same number of impulses per second 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.