THE ROLE OF ARTERIAL BARORECEPTORS IN THE UNDIVIDED CIRCULATION OF ANURAN AMPHIBIANS

Baroreceptors with afferent fibers in branches of the glossopharyngeal nerve or the vagus have been identified in all three arterial arches in anuran amphibians. Baroreceptors in the pulmocutaneous arch (PCA) a ppear to provide the principal feedback of arterial blood pressure to the cardiovascular centers of the medulla. An additional contribution arises from receptors within the aortic arch. The receptors are slowly adapting mechanoreceptors and, at least in the case of the PCA barore ceptors, transmit baroreceptive information via nonmyelinated afferent s. Pressure thresholds coincide with the range of systolic arterial pr essures associated with undisturbed toads. Therefore, increases in sys tolic pressure above resting result in both increases in firing freque ncy and the recruitment of previously inactive receptors. Denervation experiments demonstrate that feedback from PCA baroreceptors tonically inhibits the cardiovascular system even at rest. Experimental elevati on of PCA pressure above resting values causes bradycardia and reduces systemic vascular resistance, resulting in a compensatory reduction i n systemic arterial pressure. In addition, PCA resistance increases, w hich probably reflects a constriction of the extrinsic pulmonary arter y. This response opposes the negative feedback regulation of arterial pressure but may protect the pulmonary microvasculature by increasing upstream resistance, thereby reducing intravascular pressures in pulmo nary vessels. This is consistent with the view that the primary role o f anuran PCA baroreceptors is to protect the delicate vasculature of t he gas-exchange organ against high central driving pressures transmitt ed by the incompletely divided anuran circulation. Future studies shou ld be directed toward an investigation of the central pathways involve d in the PCA baroreflex.