Responses of pulmonary slowly adapting receptors to airway occlusion in cat

To determine if phasic pulmonary slowly adapting stretch receptor (SAR) activity is abolished by the no-inflation test, we monitored the discharge patterns of individual SAR during respiratory cycles with and without lung inflation. In spontaneously breathing, anesthetized cats, the airway was occluded at end-expiration at both control functional residual capacity (FRC) and an end-expiratory lung volume elevated with an expiratory threshold load (ETL). We recorded from 67 SAR at FRC and from 32 of these while on the ETL. At FRC, 29 (43%) continued to fire during occluded inspiratory efforts. Of 20 afferents which did not fire during occlusions at FRC, 13 discharged during occlusions on ETL. At FRC, 39% of SAR had modulation indices (MI; difference between peak and minimum discharge frequencies during occlusion expressed as a fraction of the same change during a non-occluded breath) greater than 0.2; on ETL, 72% of SAR had MI greater than 0.2. Identification of medullary inspiratory neurons as ones with (I beta) and without (I alpha) SAR input depends on vagally-mediated respiratory drive to the airway smooth muscle in which SAR are located, the response characteristics of SAR projecting to that neuron, and end-expiratory lung volume.