Phylogenetic Trends in the Baroreceptor Control of Arterial Blood Pressure

This article attempts to draw out current themes in the comparative physiology of baroreceptors in order to point out potential areas of future research. The first theme concerns the distribution of baroreceptive zones in the central cardiovascular system. Baroreceptor populations are widely distributed among jawed vertebrates. Multiple baroreceptive zones commonly occur within segments of the central cardiovascular system derived from the embryonic visceral arch arteries and are innervated by the corresponding visceral arch nerves. Aortic (fourth arch) and pulmonary (sixth arch), but not carotid, baroreceptors are the most conspicuous populations, at least in terms of the consistency with which they have been functionally identified. A second theme that links baroreceptor structure to function has emerged recently. Unmyelinated baroreceptors have been identiied in amphibians, reptiles, and mammals. Such receptors have thresholds ranging from near to well above normal arterial pressures and act predominantly to mediate reflex responses to increases in arterial blood pressure above the normal range. In contrast, myelinated baroreceptors with thresholds well below normal arterial pressures have so far only been positively identiied in mammals. Quantitative data suggest that the reflex regulation of arterial pressure in nonmammalian vertebrates is of physiological importance. In the toad, open-loop analysis of the pulmocutaneous baroreflex suggests that the peak capacity of this reflex to regulate arterial blood pressure is of the same order of magnitude as that of the carotid sinus baroreflex in the dog. However, as only unmyelinated baroreceptors are present in the toad, expression of this reflex is largely restricted to arterial pressures well above the normal range. Finally, it is appealing to speculate that differences in the structure and function of baroreceptors between vertebrate classes are linked to corresponding differences in vascular anatomy and metabolic rate. Unmyelinated baroreceptors may be widely distributed in vertebrates, which would reflect a widespread need to protect the circulation from damaging increases in arterial pressure. In vertebrates with incompletely divided hearts, baroreflexes may be of particular importance in protecting the circulation of the gas exchanger from excessive pressures. Low-threshold myelinated baroreceptors may represent a relatively specialized addition topressure-regulating systems in endothermic vertebrates that have poor tolerance to even transient hypotension.