Characteristics of lung mechanoreceptors in spotted gar, Lepisosteus oculatus.

Single unit and whole nerve activity were recorded in situ from pulmonary mechanoreceptors in Lepisosteus oculatus in response to step inflation and ramp or flow through ventilation of the lung with air and varying levels of CO2 in air. Slowly adapting receptors (SAR), rapidly adapting receptors (RAR), and CO2-sensitive SAR were identified. Whole nerve activity was often present when transpulmonary pressure was 0 cmH2O and increased due to recruitment and elevated discharge of already-active fibers as lung volume rose. SAR became tonically active once the lung exceeded their threshold volume and demonstrated a rate-sensitive burst of activity on inflation and a rate-sensitive inhibition of activity after deflation of the lung. RAR responded to lung inflation or deflation with a burst of activity. Six of eleven SAR were inhibited by ventilation of the lung with from 6 to 10% CO2 in air, even when lung pressure and volume were kept constant. These receptor discharge characteristics, which were similar to those found for lungfish and amphibians, may account for the reflex responses of gar to lung volume changes.