STUDY OF EXPIRATORY LARYNGEAL MOTONEURONS

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  • 1 January 1976
    • journal article
    • research article
    • Vol. 72  (3) , 311-343
Abstract
The activity of the whole recurrent nerve branch to expiratory laryngeal muscles, along with unitary activity of fibers from this branch were studied in cats. Conduction velocities of fibers ranged between 19.5-93 m/s. Two types of preparations were used: low encephale isole (at C7 level) cat with concomitant light Dial anesthesia or without any anesthesia, or cats deeply anesthetized with Dial or chloralose-urethane. The vagus nerves were preserved. Under normocapnic conditions, the delay of recruitment (i.e., the lag between the interruption of phrenic nerve activity and the start of unitary expiratory recurrent nerve activity) increased as the residual phrenic nerve activity increased; in fact, the delay was nearly zero with unanesthetized or lightly anesthetized preparations (small residual phrenic activity), but could extend up to 800 ms with deeply anesthetized preparations (large and long-lasting residual activity). Recurrent nerve units showed a maximum firing rate at the beginning of a discharge. The effect of pulmonary inflation depended on the moment when it occurred; if inflation occurred during or just before a phrenic nerve discharge (with an inhibitory effect as consequence), the succeeding laryngeal expiratory discharge was reduced. Usually, the discharge of the expiratory laryngeal motoneurons (ELM) did not overlap the following inspiratory activity which built up during the late phase of expiratory pause (i.e., activity exhibited by the early inspiratory laryngeal motoneurons (ILM)). During the establishment of progressive hypocapnia, the duration of discharge of ELM gradually lengthened, up to the expiratory pause duration. When hypocapnic apnea was reached, ELM discharge without any interruption, but at low frequency. An explanation may be that ELM receive a continuous excitatory input from tonic neurons belonging to the neighboring expiratory centers; ELM would be phasically inhibited by neurons belonging to the system which drives the early ILM; a rebound phenomenon consecutive to this inhibition would explain the fact that ELM immediately reach their maximum discharge frequency.