Graded and dynamic reflex summation of myelinated and unmyelinated rat aortic baroreceptors.

Unmyelinated (C) and myelinated (A) baroreceptor (BR) axons are present in rat aortic depressor nerve (ADN). With graded ADN electrical activation and anodal conduction blockade, reflex responses in anesthetized rats were assessed as changes in mean arterial pressure (MAP) and heart rate (HR). We tested the hypothesis that C-type BR inputs are effective at low frequencies because they outnumber A-type. Anodal current (Ian) reversibly eliminated all MAP and HR responses to A-selective stimuli. High intensities activated all ADN axons (A+C) and decreased MAP at lower frequencies (<10 Hz) than were effective with A-selective stimulation. I(an) reduced only MAP responses to >10-Hz ADN stimulation. Burst patterns significantly augmented A- but not C-selective reflex responses despite identical numbers of shocks per second. A-selective stimuli failed to evoke significant bradycardia even at 200 Hz. Maximum intensity stimuli plus Ian (C selective) evoked less bradycardia than without I(an) (A+C), indicating supra-additive summation unlike the occlusive summation for MAP responses. However, activation of reduced numbers of C-type BRs with all A-type BRs suggests a strong A to C interaction in reflex bradycardia responses. Surprisingly, Ian block of A-type conduction eliminated all reflex bradycardia at such submaximal intensities despite C conduction and depressor responses. A- and C-type BRs act synergistically, and A-type activity is absolutely required in cardiac but not in depressor pathways. Thus greater numbers do not appear to account for C-type BR efficacy, and critical interactions between these two sensory subtypes appear to occur differentially across cardiac and systemic baroreflex effector pathways.