Convergence of noxious and non-noxious cutaneous afferents and baroreceptor afferents onto single adrenal sympathetic neurons in anesthetized rats

Single units of the adrenal sympathetic nerve (n = 46) were dissected and characterized with respect to tonic discharge and response to cutaneous and baroreceptor stimulation. The frequency of tonic discharge averaged 1.6 Hz and cardiovascular rhythmic modulation was observed in 53% of the units. The stimuli employed in the present study included phenylephrine-induced increases in blood pressure and pinching or brushing of lower chest skin. Mean unit activity increased 27% on lower chest pinching stimulation, decreased 12% on lower chest brushing stimulation and decreased 62% on phenylephrine-induced baroreceptor stimulation. Although there was a tendency for units with higher tonic firing frequency to have a greater response to stimulation, this relationship was not significant for pinching or brushing of lower chest skin. The close correlation between tonic activity and response to phenylephrine was explicable on the basis of a near total depression of many units, which resulted in a larger decrease in firing frequency for units with initially high spontaneous discharge rates. As might be expected, units with cardiovascular rhythmicity manifested greater responses to baroreceptor activation. This correlation was independent of tonic rate of discharge since rhythmic and non-rhythmic units did not significantly differ in tonic activity. While a majority of units responded in a typical fashion to all three stimuli (i.e. with increases to pinching and decreases to brushing and phenylephrine administration), there was little correlation between the response magnitude of individual units to any two of the stimuli employed. We conclude, therefore, that most adrenal sympathetic units receive convergent reflex input from cutaneous noxious and non-noxious afferents as well as from baroreceptor afferents, although for any individual unit the quantitative significance of each input varies.