AFFERENT AND REFLEX PROPERTIES OF ARTERIAL C FIBERS

Systemic arterial baroreceptors located in the carotid sinus, right
subclavian-brachiocephalic 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 of arterial
C-fiber 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 such as bradykinin and prostaglandins
that may play a role in circulatory adjustments. The functional importance
of C-fiber baroreceptors and chemosensitive endings is not yet known. A
systematic 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 anesthetized dogs in which the
vasosensory 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 (catecholamines, bradykinin, prostaglandins) will be
determined by recording impulses from single nerve fibers in each of the
vasosensory 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
vasosensory nerve. These studies will define reflex effects on heart rate,
cardiac output, arterial pressure, vascular resistance, renal nerve
activity, plasma vasopressin concentration and renin activity, minute
ventilation, and airway resistance. 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 of the
heart, peripheral circulation, kidney, and respiratory system under a
variety of physiological and pathophysiological conditions.