Afferent vagal activity during hyperthermic polypnea in the pigeon

Respiration-modulated activity in afferent vagal fibers was recorded in 10 pigeons during euthermic breathing and thermal panting. Of these fibers, 13 were identified as intra-pulmonary chemoreceptors (IPCs), that increased discharge with diminishing lung gas PCO2, and 13 as mechanoreceptors, that increased firing with lung inflation. Two types of IPC were observed that were distinct by their firing pattern during panting. Phasic IPCs displayed phasic discharge within the respiratory cycle, even at respiratory frequencies (fresp) as high as 400 min-1. Tonic IPC fired tonically and increased their discharge as fresp increased. Several IPCs were silent during euthermic breathing, but discharged tonically as fresp increased with thermal polypnea. Discharge of neither type of IPC was consistently related to PaCO2. Discharge from mechanoreceptors was phasic with respiration, up to values of fresp as high as 350 min-1. However, the average number of impulses per breath decreased as fresp increased. We conclude that discharge from phasic intrapulmonary chemoreceptors and mechanoreceptors may contribute to setting the respiratory pattern during hyperthermic polypnea.