Membrane potential trajectories and discharge characteristics were measured intracellularly in 29 phrenic motoneurons of anesthetized, paralyzed and artificially ventilated cats during hypercapnic respiration and the aspiration reflex. Fifteen 'active' cells discharged spontaneously during inspiration, and the remaining 14 'quiescent' cells exhibited no discharge in spite of strong central respiratory drive. The mean membrane potential of the quiescent cells during inspiration (-62 ± 4 mV) was significantly lower than the threshold level determined for the active cells (-52 ± 4 mV). The mean axonal conduction velocity was slower for the active (60.4 ± 8.7 m/s) than quiescent cells (67.4 ± 6.9 m/s). All phrenic motoneurons discharged during the aspiration reflex with maximum instantaneous frequencies ranging from 6 to 357 Hz. No differences were found for the maximum discharge frequency during the reflex between the active and quiescent cells. Although there were differences in the slopes of the depolarization during inspiration between the groups of cells, no such difference existed in the slopes during the aspiration reflex. The threshold level for the first spike during the reflex was the same as that during inspiration but the level for successive spikes became progressively less negative while spike amplitude decreased and duration increased. Stimulation of the nasopharynx to elicit the aspiration reflex was found to alter the timing of the subsequent respiratory cycles. © 1989.
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