Mechanisms of CO2 Sensing in Avian Intrapulmonary Chemoreceptors

Avian intrapulmonary chemoreceptors (IPC) are CO₂ sensitive vagal afferents that help control breathing in birds. IPC provide important phasic and tonic sensory feedback that helps control breathing depth and rate and CO₂ homeostasis in birds. IPC have some unusual properties compared to many other commonly studied respiratory chemoreceptors, mostnotably their “backwards” response to CO₂ and extremely rapid response to CO₂. When IPC experience an abrupt increase or decrease in CO₂ stimuli, they often show an exaggerated discharge response immediately following the stimulus change, followed by partial spike frequency adaptation. Recordings of IPC discharge from prenatal, paranatal, and postnatal ducklings indicate that IPC undergo development and maturation, likely triggered by the conversion from chorioallantoic to pulmonary respiration. IPC are highly responsive airway CO₂ chemoreceptors with no O₂ sensitivity, while neuroepithelial bodie, at least in mammals, are O₂ chemoreceptors or perhaps mechanosensors, with negligible CO₂ sensitivity.