Phylogeny of respiratory chemoreceptor function in vertebrates

All sites of O-2 chemoreception for breathing in vertebrates are peripheral to the central nervous system. With the progression from aquatic to aerial breathing, and subsequent loss of intra-cardiac shunting, there is a reduc tion in the number of sites for O-2 chemoreception as well as a restriction of chemoreceptors to internal sites. Oxygen receptors associated with the first gill arch and homologous structures become the primary respiratory O- 2 receptor. O-2 sensing by respiratory chemoreceptors appears to be a uniqu e feature of glomus type cells and may involve a heme-like protein that tig htly couples ventilatory responses to changes in the hemoglobin carrying ca pacity of the blood. With the switch from water to air as a respiratory med ium there is an increase in the sensitivity of animals to CO2/pH rather tha n O-2 as the primary respiratory stimulus. CO2/pH chemoreception is not a u nique feature of any one cell type and multiple receptor sites for sensing CO2/pH are found in all species. CO2/pH receptors associated with the first gill arch and homologous structures become the primary peripheral arterial receptor. Central CO2/pH chemoreception arises with air breathing (phyloge netically and ontogenetically) and, once present, central chemoreceptors ta ke on the dominant role in producing steady state responses to changes in C O2/pH. Intrapulmonary chemoreceptors for CO2/pH are present in diapsid rept iles and birds but their function is unknown. Along with nasal receptors se nsitive to CO2 in some amphibia and reptiles (whose function is also unknow n), they act to inhibit, rather than stimulate, breathing. In all vertebrat es, however, chemoreceptor input acts as a modulating input to a conditiona l central rhythm generator for breathing but the periodic breathing pattern s of amphibia and reptiles appear more highly dependent on this input than the continuous breathing patterns of fish, birds and mammals.