Pulmonary receptors in reptiles: discharge patterns of receptor populations in snakes versus turtles

This study examines the effects of lung inflation/deflation with and withou t CO2 on the entire population of pulmonary receptors in the vagus nerve in two species of snakes and two species of turtles. We asked the question, " how does the response of the entire mixed population of pulmonary stretch r eceptors (PSR) and intrapulmonary chemoreceptors (IPC) in species possessin g both differ from that in species with only PSR"? This was studied under c onditions of artificial ventilation with the secondary goal of extending ob servations on the presence/absence of IPC to a further three species. Our r esults indirectly illustrate the presence of IPC in the Burmese python and South American rattlesnake but not the side necked turtle, adding support t o the hypothesis that IPC first arose in diapsid reptiles. In both species of snake, CO2-sensitive discharge (presumably from IPC) predominated almost to the exclusion of CO2-insensitive discharge (presumably arising from PSR ) while the opposite was true for both species of turtle. The data suggest that for animals breathing air under conditions of normal metabolism there is little to distinguish between the discharge profiles of the total popula tion of receptors arising from the lungs in the different groups. Interesti ngly, however, under conditions of elevated environmental CO2 most volume-r elated feedback from the lungs is abolished in the two species of snakes, w hile under conditions of elevated metabolic CO2, it is estimated that volum e feedback from the lungs would be enhanced in these same species.