System arterial baroreceptors located in the carotid sinus, right subclavian-brachycephalic angle and aortic arch are well known for their important regulatory effects on cardiovascular, renal, and ventilatory functions. Although it has been assumed that baroreflex responses result from the stimulation of baroreceptors with myelinated afferent fibers (A-fibers), it has become clear in recent years that another type of baroreceptor with nonmyelinated afferent fibers (C-fibers) exists and may contribute to baroreflexes. In addition, a second group or arterial C-fibers endings (chemosensitive endings) have been found which are of interest not only for their response to pressure, but also for their response to endogenous substances (extra-cellular ions, hormones, catecholamines, endothelial factors) that play a role in circulatory adjustments. The functional importance of C-fiber baroreceptors and chemosensitive endings is not yet known. A systemic study of the distribution, afferent properties, and reflex functions of these arterial C-fibers in the carotid sinus, right subclavian angle, and aortic arch will be performed in which the visuosensory regions will be vascularly isolated and distended with controlled pressures. The distribution of C-fiber baroreceptors and chemosensitive endings and their afferent responses to changes in arterial pressure, to short term hypertensive resetting, to changes in firing of sympathetic nerves supplying the baroreceptor area, and to endogenous chemical substances will be determined by recording impulses from single nerve fibers in each of the visuosensory regions. The reflex effects of arterial C-fibers will be determined by stimulating baroreceptors in response to increased pressure or chemosensitive endings in response to administration of chemicals when afferent A-fibers are blocked selectively by cooling the appropriate visuosensory nerve. These studies will define reflex effects on the regional distribution of blood flow and sympathetic nerve activity. In other studies, the contribution of the afferent input from arterial C-fibers to central resetting of the baroreflex will be determined. By elucidating both the conditions in which arterial C-fibers are stimulated and their reflex effects, these studies will provide insights into the functional importance of C-fiber baroreceptors and chemosensitive endings in the reflex regulation blood pressure under a variety of physiological and pathophysiological conditions.
