Differences in respiratory neural activities between vagal (superior laryngeal), hypoglossal, and phrenic nerves in the anesthetized rat.

Abstract

Respiratory neural activities were recorded from the efferent vagal (superior laryngeal (Xsl)), hypoglossal (XII), and phrenic nerves in spontaneously breathing rats anesthetized with halothane. The onset of inspiratory discharges in the cranial nerves was slightly earlier (5-60 msec) but more gradual than that of phrenic bursts. When the anesthesia was deepened by increasing the concentration of halothane or by injection of pentobarbital, inspiratory discharges in the three nerves were well maintained although there was a progressive decrease in respiratory frequency and a prolongation of the delay from the start of Xsl or XII inspiratory activities to the onset of phrenic bursts. Inhalation of CO₂ increased respiratory frequency and augmented the peak phrenic activity whereas the peak inspiratory activities in the cranial nerves remained unchanged under elevated end-tidal P_CO2. Both in deeper anesthesia and in hypercapnia, changes in respiratory frequency were due mainly to alterations in the duration of expiration. The results indicated that in the rat, 1) overall inspiratory activities in various nerves innervating the diaphragm and accessory respiratory muscles in the upper airway are quite resistant to depressing actions of halothane or halothane-pentobarbital anesthesia, although the mechanism controlling respiratory frequency is strongly affected, and 2) excitatory signals elicited by an elevated P_CO2, via respiratory chemosensors preferentially augment inspiratory activities in the phrenic nerve. Factors influencing the temporal difference in the onset of inspiratory activities in the cranial and phrenic nerves are discussed.