Medullary inspiratory neurons with stable respiratory rhythm and little correlation to phrenic high-frequency...

Respiratory neurons with a highly stable rhythm have been proposed to be involved in the central neural mechanism responsible for the respiratory rhythmogenesis. In the present study it was examined whether the stably discharging inspiratory neurons are related to the high-frequency oscillation (HFO) in the phrenic nerve activity. Experiments were performed on 27 vagotomized rabbits anesthetized with diethyl ether, immobilized with gallamine triethiodide, and ventilated artificially. Spike-triggered averaging was used to evaluate the degree of the correlation between inspiratory unit spikes and the phrenic HFO. Twenty out of the 35 inspiratory units examined were related to HFO and were located in the medullary reticular formation (n = 19; n, number of units) and in the vicinity of the nucleus tractus solitarius (n = 1). The correlation to HFO decreased after the intravenous injection of thiamylal. The inspiratory units with little correlation to HFO were located in the reticular formation of the pons (n = 3) and medulla (n = 12). The inspiratory neurons with a stable respiratory rhythm (n = 4) had little correlation to HFO and located in the lateral region of the medullary reticular formation. These results suggest that the central neural mechanisms responsible for phrenic HFO are not directly related to the respiratory rhythmogenesis.