Pulmonary receptor discharge and expiratory muscle activity

Expiratory activity in abdominal muscles can be elicited when animals breathe against a continuous positive airway pressure (CPAP). To evaluate possible pulmonary receptor mechanisms underlying this effect, graded bilateral vagal cooling was used to inhibit expiratory discharge of abdominal muscles in anesthetized adult opossums during CPAP (6-8 cm H2O); then the discharge patterns in fibers from slowly adapting pulmonary stretch receptors and rapidly adapting pulmonary receptors were assessed when cut vagus nerves were cooled to temperatures similar to those required to inhibit expiratory muscles. Expiratory motor responses were completely inhibited by cooling the vagi to a mean temperature of 9.6 degrees C (range: 7-12 degrees C), but concurrently observed augmentation of amplitude and duration of inspiratory (diaphragm) discharge continued with further lowering of vagal temperatures. Peak inspiratory and tonic expiratory-phased neural activity in fibers from slowly adapting receptors were typically reduced at vagal temperatures between 7 and 12 degrees C. Those units with the highest rates of discharge were maximally affected by cooling. Most rapidly adapting receptors, in contrast, did not maintain a reliable breath-by-breath activation during CPAP; those fibers that could be used for cooling studies showed relatively little inhibition because of their low discharge rates. These data are consistent with the possibility that high levels of discharge from slowly adapting pulmonary stretch receptors elicit expiratory activity in abdominal muscles.