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 ass essed as changes in mean arterial pressure (MAP) and heart rate (HR). We te sted the hypothesis that C-type BR inputs are effective at low frequencies because they outnumber A-type. Anodal current (I-an) reversibly eliminated all MAP and HR responses to A-selective stimuli. High intensities activated all ADN axone (A + C) and decreased MAP at lower frequencies (<10 Hz) than were effective with A-selective stimulation. I-an reduced only MAP respons es to >10-Hz ADN stimulation. Burst patterns significantly augmented A- but not C-selective reflex responses despite identical numbers of shocks per s econd. A-selective stimuli failed to evoke significant bradycardia even at 200 Hz. Maximum intensity stimuli plus I-an (C selective) evoked less brady cardia than without I-an (A+C), indicating supra-additive summation unlike the occlusive summation for MAP responses. However, activation of reduced n umbers of C-type BRs with all A-type BRs suggests a strong A to C interacti on in reflex bradycardia responses. Surprisingly, I-an block of A-type cond uction eliminated all reflex bradycardia at such submaximal intensities des pite C conduction and depressor responses. A- and C-type BRs act synergisti cally, and A-type activity is absolutely required in cardiac but not in dep ressor pathways. Thus greater numbers do not appear to account for C-type B R efficacy, and critical interactions between these two sensory subtypes ap pear to occur differentially across cardiac and systemic baroreflex effecto r pathways.