Lung deflation stimulates fictive ventilation in decerebrated and unidirectionally ventilated toads

We describe how the degree of lung inflation and hypercapnia influenced fictive ventilation in five toads (Bufo marinus) that were decerebrated and paralysed with roccuronium. Both lungs were unidirectionally ventilated and the degree of lung inflation was determined by controlling the outflow resistance of these catheters, while ventilatory motor output was assessed on the basis of nervous activity in the mandibular branch of the Vth cranial nerve. The pattern of the recorded activity ('fictive ventilation') resembled the ventilatory patterns previously described for conscious toads. Increasing the fraction of CO2 in the gas mixture used for unidirectional ventilation from 0.00 to 0.05 stimulated fictive breathing. Fictive ventilation was also greatly stimulated, at all CO2 levels, by reduced lung volume, while complete inflation of the lungs abated fictive ventilation at all levels of CO2. Stimulation of CO2 sensitive chemoreceptors and pulmonary stretch receptors appear to have interactive effects on the central generation of ventilatory output in toads.