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

Abstract

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 2 sets of electrodes, 1 for selective high-frequency excitation of A-fibers (A-stimulation) and 1 for low-frequency activation of C- (and A-) fibers by another pulse generator (C-stimulation). The pulse rates in C-stimulations were too low to evoke reflex effects via A-fibers. The hypotensive response to combined stimulations of A- and C-fibers (AC-stimulation) exceeded 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. 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-fiber activity than when alone. A synergistic interaction evidently exists between central effects of afferent discharge in aortic nerve A- and C-fibers.