Open-loop respiratory chemosensitivity in chickens and ducks.

In order to study the behavior of the avian respiratory center under open-loop conditions, recordings of respiratory motor discharges were made from an intercostal nerve in a unidirectionally ventilated paralyzed preparation during normoxic hypercapnia, hypoxic normocapnia and asphyxia, and under control conditions. Respiratory motor burst rate (RMBR) in ducks showed a maximum at 40 mmHg Paco2 with depression at higher and lower values, both in the steady state and during rapid transients. RMBR was apparently independent of Paco2 in chickens, and of Pao2 in both species. During asphyxia, RMBR in ducks fell steadily while the number of spikes in each burst did not change. RMBR during asphyxia bore the same relationship to Paco2 as during unidirectional ventilation. Chickens did not respond consistently to asphyxia, but RMBR tended to increase and, in some cases, the number of spikes per burst also increased. Possible mechanisms and functions of the asphyxic respiratory depression observed in ducks are discussed.