Chemoreceptor modulation of endogenous respiratory rhythms in vertebrates.

The relative contributions of O2- and CO2-sensitive chemoreceptor information to centrally generated respiratory patterns have changed dramatically during vertebrate evolution. Chemoafferent input from branchial O2 chemoreceptors modulates centrally generated respiratory patterns but is not critical for respiratory rhythmogenesis in fishes. In air-breathing fishes, branchial O2 chemoreceptors monitoring internal and external stimuli control the relative contributions of the gills and air-breathing organ to net ventilation, and chemoafferent input is necessary for initiating air breathing. In the transition from water to air breathing by amphibious vertebrates, rhythmic patterns of branchial ventilation are completely replaced by arrhythmic and intermittent patterns of air breathing, and there is progressive dependence on CO2 as a source of respiratory drive. Periodic initiation of air breathing in resting animals appears to depend on attaining a threshold level of afferent activity from O2- and CO2/pH-sensitive chemoreceptors, since hyperoxia and/or hypocapnia can abolish air breathing in all air-breathing vertebrates. Conversely, chemoreceptor stimulation in amphibians and reptiles converts intermittent to more continuous air breathing patterns, suggesting that adequate biasing input from chemoreceptors activates a central rhythm generator. Chemoafferent input in homeotherms serves as one of several sources of drive for rhythmic breathing and supplies feedback for blood gas homeostasis in the face of metabolic or environmental change.