Central interaction between reflex responses to activity in aortic nerve A- and C-fibres

The question of whether or not activity in myelinated (A) and non-myelinated (C) afferents in a baroreceptor nerve shows interaction of their reflex effects, was investigated in 10 anesthetized rabbits. The central end of the cut left aortic nerve was stimulated by two sets of electrodes, one for selective high-frequency excitation of A-fibres (A-stimulation) and one for low-frequency activation of C- (and A-) fibres by another pulse generator (C-stimulation). The pulse rates in C-stimulations were too low to evoke reflex effects via A-fibres. The hypotensive response to combined stimulations of A- and C-fibres (AC-stimulation) was found to exceed the sum of responses to separate A- and C-stimulations in 21 of 22 stimulation series. For sympathetic activity to the kidney, a greater than additive effect was observed in 16 of 24 series (P = 0.08), while in 6 of the series, the responses were equal. Median values of the ratio AC/(A + C) were 1.28 for the reflex changes in pressure and 1.11 for the effects on renal nerve activity. Similarly, C-stimulation, which in comparison to A-stimulation affected sympathetic activity relatively more than blood pressure, in 13 of 16 series (P = 0.01) produced a greater sympathetic inhibition when added to a background of A-fibre activity than when alone. These results suggest that a synergistic interaction exists between central effects of afferent discharge in aortic nerve A- and C-fibres.