A role of pulmonary rapidly adapting receptors in control of breathing.

We analysed the breathing pattern of anaesthetised rabbits during unloaded breathing when breathing was accelerated by inspired CO2 and when they breathed against positive or negative pressures before and during block of pulmonary stretch receptors by SO2, and after bilateral vagotomy. Before block moderate steps of inflation or deflation (0.5 kPa) produced relatively larger changes in duration of expiration than in duration of inspiration, indicating the relative sensitivities of the two phases. With stretch receptors, blocked inflation or deflation shortened expiration, demonstrating the influence of rapidly adapting receptors on that phase of breathing. If pulmonary stretch receptors were the major determinants of the duration of inspiration, we would have expected inspiratory duration in the stretch receptor blocked and vagotomised states to be almost identical. They were not, inspiratory duration being less in the blocked than in the vagotomised state. Possibly vagal afferent activity other than that of stretch receptors shortens inspiratory duration. However, we have found that rapidly adapting receptor activity (and any unmyelinated fibre activity provoked by rapid inflation or deflation of the lungs) never directly shortened inspiration. We therefore propose a mechanism whereby rapidly adapting receptors may indirectly affect duration of inspiration.