Discharge of vagal pulmonary receptors differentially alters neural activities during various stages of expiration in the cat.

1. The purpose was to evaluate the hypothesis that neural expiration is composed of two phases: I, a post inspiratory period; and II, the period at which expiratory activities of spinal nerves reach peak values. We hypothesized that the discharge of pulmonary stretch receptors might differentially alter neural activities during these two phases. 2. Activities of the phrenic nerve, intercostal nerve and nerves innervating the thyroarytenoid muscle of the larynx and triangularis sterni muscle of the chest wall were recorded in decerebrate and paralysed cats. 3. The experimental animals were ventilated with a servo-respirator which produced changes in tracheal pressure, and lung volume, in parallel with alterations in integrated activity of the phrenic nerve. 4. In order to assess the influence of the discharge of slowly adapting pulmonary stretch receptors upon neural activities during expiration, lung volume was held at end-expiratory or end-inspiratory levels for individual respiratory cycles. 5. When pulmonary inflation was prevented, phrenic activity increased, as did activity of the thyroarytenoid nerve during early expiration. In contrast, activities of the triangularis sterni and intercostal nerves during mid- to late expiration declined. 6. Holding the lungs at end-inspiratory levels caused a reduction of thyroarytenoid activity and increases in peak triangularis sterni and intercostal activities. Neural expiration typically continued as long as the lungs were maintained at the end-inspiratory level. 7. Responses were qualitatively similar in hypocapnia, normocapnia and hypercapnia, but the magnitude of changes in neural activities was typically augmented with elevations in end-tidal fractional concentrations of CO2. 8. We conclude that the discharge of slowly adapting pulmonary stretch receptors inhibits neural activities during early expiration and augments activities during mid-to late expiration. Hence, our data support the concept that neural expiration is composed of two stages in which neural activities may be differentially controlled.