THE ROLE OF CO2 PH CHEMORECEPTORS IN VENTILATORY CONTROL

It now appears that at least some members of all classes of vertebrate s exhibit ventilatory responses to changes in CO2/pH per se, including fishes. With the transition from aquatic to aerial respiration, there is an increase in the sensitivity of animals to this complex of stimu li, an increase in the variety of putative receptors possibly involved in eliciting ventilatory responses and an increase in the relative im portance of this complex of stimuli in the genesis of resting ventilat ion. The variety of CO2-sensitive chemoreceptors present in air-breath ing lower vertebrates adds considerable complexity to experimental stu dies of ventilatory responses to CO2/pH. Because or the locations, dis charge characteristics and reflex effects of the different receptor gr oups, most air-breathing lower vertebrates show different responses to increases in CO2/[H+] due to cerebral ischemia, anoxia, metabolic aci dosis and environmental hypercarbia. In some cases the differences are only quantitative, while in other cases the responses are qualitative ly very different. These differences appear to reflect differences in the relative strength of the reflexes elicited by the various receptor groups and the net sum of their modulating influences when CO2/pH are altered via different routes. Although the situation is simpler in th e higher vertebrates, in all cases the input from all of the CO2/[H+]- sensitive receptors appears to act as a biasing input which summates w ith other afferent information to modulate respiratory motor output, e ven in those species that breathe intermittently.