The Reptilian Baroreceptor and Its Role in Cardiovascular Control

Abstract

Anatomical evidence for the existence of a depressor nerve in reptiles was first reported 100 years ago. Electrical stimulation of the central end of the cut nerve, however, did not produce an unequivocal fall in heart rate and blood pressure, so it was thought not to function as a depressor nerve. This remained the state of knowledge for fifty years when Marco Fedele performed a superb anatomical and physiological study of the depressor nerves of turtles and lizards. He demonstrated that there were two depressor nerves from each vagus; the superior nerve originated from the jugular ganglion or superior laryngeal nerve in turtles and from the superior laryngeal nerve or vagus in lizards; the inferior nerve originated from the nodose ganglion or slightly caudad of this ganglion. The nerves were shown to terminate in the proximal truncus arteriosus. Unlike the earlier workers, Fedele obtained a clear depressor effect on stimulating the depressor nerves. In more recent times baroreflexes have been demonstrated in response to hemorrhage and body tilting in reptiles, with snakes receiving particular attention. The evidence indicates that aquatic snakes are less effective at maintaining blood pressure than terrestrial and arboreal forms. The sensitivity (gain) of the baroreceptor-heart rate reflex, when it is expressed as a percentage change in heart rate per unit pressure change, is approximately the same in reptiles, amphibians, and mammals. In addition, the ultrastructural appearance of the baroreceptors of lizards is similar to that of mammals. A quantitative assessment of the ability of reptiles to correct disturbances in blood pressure has not yet been made, but techniques for obtaining this information are now available.