INFLUENCE OF INTRALARYNGEAL CO2 ON THE RESPONSE OF LARYNGEAL AFFERENTS TO UPPER AIRWAY NEGATIVE-PRESSURE

Effects of intralaryngeal CO2 on the response of superior laryngeal af ferents to negative pressure were investigated in 20 anesthetized spon taneously breathing adult cats. Single-fiber action potentials were re corded from the peripheral cut end of the superior laryngeal nerve. Th e larynx was exposed to negative pressure during inspiration when the animal breathed against an occluded upper airway. Among the 99 recepto rs evaluated, 54 were respiratory modulated and 45 were nonmodulated e ndings. The effect of intralaryngeal CO2 on the response of 39 recepto rs responding to negative pressure was determined by exposure of the l arynx to CO2 or air for 1 min followed immediately by upper airway occ lusion. The mean discharge frequency of 22 fibers inhibited by negativ e pressure was 32.4 +/- 2.6 Hz during air trials compared with 29.9 +/ - 2.6 Hz during CO2 trials (P < 0.005). During occlusion of the upper airway after the warm humidified air trial, the discharge frequency of these endings decreased to 24.2 +/- 2.3 Hz compared with 17.5 +/- 2.2 Hz after CO2 trial (P < 0.001). The mean discharge frequencies of 17 fibers stimulated by negative pressure were 3.7 +/- 2.6 and 4.4 +/- 1. 8 Hz, respectively, during air and CO2 trials. The mean frequencies in creased to 14.7 +/- 3.5 Hz (air) and 18.6 +/- 4.0 Hz (CO2) during uppe r airway occlusions (P < 0.01). We conclude that intralaryngeal CO2 ca n alter the response of pressure-sensitive laryngeal afferents, thereb y having a role in the maintenance of upper airway patency. It may hav e a greater role in restoring airway patency during obstructive apnea, because the reflex effects of CO2 on the activity of upper airway dil ating muscles are enhanced by decreased vagal volume feedback.