Are reptilian pulmonary receptors mechano- or chemosensitive?

Abstract

BOTH birds and mammals possess pulmonary receptors which increase discharge in phase with each inspiration^1–5. In mammals the adequate stimulus of these receptors seems to be the total transpulmonary pressure which varies with both inflation volume and rate and sign of the volume change⁶. The lungs of birds, however, consist of a complex system of air tubes^7,8 and unlike those of mammals are relatively inexpansible. Many avian pulmonary receptors have no mechanosensitivity, the adequate stimulus being the changes in airway CO₂ throughout the breathing cycle^9–12. So rate and degree of inflation of the respiratory system is signalled by different sensory modalities in birds and mammals. The demonstration that mammalian pulmonary receptors are partially inhibited by high alveolar CO₂ (refs 13–15) and that some avian receptors may be mechanosensitive^16,17 suggests that in the more phylogenetically ancient vertebrate classes it might be possible to locate a pulmonary receptor which has distinct mechano- and chemosensitive properties, thereby allowing speculation on the evolution of the avian and mammalian receptor types.