Blood Pressure Regulation by Aortic Baroreceptors in Birds

This review summarizes current knowledge of avian baroreceptor location, innervation pattern, and function. Birds have one set of arterial baroreceptors associated with the root of the aortic arch and innervated bilaterally by the aortic nerves, branches of the vagus originating from the nodose ganglia. There is a tonic level of baroreceptor input to the baroregulatory mechanisms in the brain stem, and this baroreceptor input is necessary for the maintenance of normotensive pressure in birds. Arterial blood pressure rises acutely and remains chronically elevated after barodenervation. Few studies of baroreceptor or baroreflex function have been made in birds. Baroreceptors discharge in systole with properties similar to high-threshold, slowly adapting mammalian baroreceptors. Evaluating the results of studies of baroreflex function in birds is problematic, since these studies were inconsistent in methodology. They were done in either awake or anesthetized animals, and several different methods for varying arterial pressure were used. It is recommended that baroreflex function be assessed by examining the correlation between spontaneously occurring values of heart rate or cardiac output and arterial pressure measured simultaneously in awake, unrestrained animals. The contribution of baroreceptors to cardiovascular control during exercise and submersion, and possible roles of the baroreflex in homeostasis, are discussed This review emphasizes the lack of data about the nature of baroreceptors and their participation in the integrated control of the circulation in birds.