The alteration of CO2 respiratory sensitivity in chickens by thoracic visceral denervation

We measured respiratory movements in nine groups of six cockerels, 20-24 weeks of age. We opened the thorax and all air sacs, and unidirectionally ventilated each lung separately. The right lung received constant P(CO2), while the P(CO2) was altered to the left lung. There were only small differences in response to P(CO2) alterations whether both pulmonary circulations were intact, the left pulmonary circulation was blocked, or the left lung was denervated and the right pulmonary circulation blocked, suggesting (1) that the extrapulonary and pulmonary P(CO2) -sensitive afferents (in one lung) have equivalent influence, and (2) the influences of the two afferent systems are not additive. Respiratory sensitivity after bilateral vagotomy is small despite pulmonary innervation by CO2 -sensitive spinal afferents, perhaps one reason for abnormal respiration after vagotomy. The respiratory influences of pulmonary vagal and spinal CO2-sensitive afferents are also non-additive, suggesting that non-additive interactions among afferents controlling respiration may be common in the chicken. Rates of response to altered intrapulmonary P(CO2) are determined by central mechanisms and not the time for CO2 distribution or receptor response.