Role of pulmonary stretch receptor feedback in control of episodic breathing in the bullfrog.

This study compared the "fictive" breathing patterns of decerebrate, paralyzed, unidirectionally ventilated bullfrogs in which pulmonary stretch receptor (PSR) feedback was either absent bilateral vagotomy), maintained constant at different levels (tonic) or oscillated with each fictive breath (phasic) under different levels of hypoxic or CO2-related respiratory drive. Tonic and phasic PSR feedback had identical effects on the fictive breathing pattern; decreasing PSR feedback increased the peak integrated trigeminal electroneurogram recordings and decreased breathing frequency. The effects of bilateral vagotomy and lung deflation to 0 cmH2O on breathing pattern were identical. Although hypoxia (fractional concentration of O2 in air = 0.06) had no significant effect on fictive breathing, ventilating frogs with increasing CO2 levels (fractional CO2 concentration in inspired air range: 0.00-0.03) increased the number of breaths in each fictive breathing episode, and this effect was potentiated by PSR feedback. Whenever respiratory drive was increased, regardless of the method (increase in PSR feedback or chemoreceptor drive), occasional single breaths were replaced by breathing episodes, indicating that the mechanisms responsible for the clustering of the breaths and the onset/termination of breathing episodes are not dependent on either input alone.