The effects of hypercapnia and hypoxia on single hypoglossal nerve fiber activity.

The respiratory related modulation of hypoglossal nerve activity has been studied at the single fiber level in cats under hyperoxic hypercapnia and hypoxic conditions and their conduction velocities determined. Changes in fiber activity were compared to simultaneous changes occurring in phrenic activity. Three different kinds of discharge patterns were observed: (a) inspiratory, (b) phasic activity during both inspiration and expiration, and (c) continuous random activity with no respiratory modulation. These fibers could be grouped into three categories according to their pattern of discharge during CO2 breathing. Type I fibers, mean conduction velocity of 30.0 m/sec, exhibited only an inspiratory phasic discharge during 100% O2 breathing. Their discharge frequency increased rapidly with higher levels of CO2 and hypoxia. Type II fibers, mean conduction velocity of 36.7 m/sec, had three different kinds of inspiratory-expiratory discharge patterns during 100% O2 breathing. With increasing hypercapnia or hypoxia fibers of this group discharged phasically during inspiration and discharge at low frequency during expiration. Type III fibers had a non phasic discharge pattern at 100% O2 breathing and at all levels of CO2 tested (up to 10%). Discharge frequency rose during CO2 rebreathing and hypoxia, but the rate of increase was much less than Type I and Type II fibers. Their mean conduction velocity was 41.3 m/sec. The inspiratory activity of Type I and II fibers increased their activity more than the phrenic during hypercapnia and hypoxia. Type II and Type III fibers are responsible at least in part for the tonic activity of the nerve.