Patterns of bulbar respiratory neurons during and after artifical hyperventilation.

Single bulbar respiratory unit activity was recorded in urethane-anesthetized rabbits. During artificial ventilation, reflex contractions of the diaphragm were elicited by the forced deflations of the respirator. Out of 44 neurons, four I, one E and two EI units exhibited a shift of their discharge relative to the contractile phase of the diaphragm, compared to normal respiration. During hypocapnic apnea, some I, IE and E neurons became silent. Other cells belonging to all phase types, however, continued to fire tonically at a rate less than the peak rate in normal bursts. When rhythmic respiration resumed after termination of the apneic pause, five (I, IE and E) units exhibited a transient phase shift of their burst discharge compared to normal respiration. The results of simultaneous recordings of two neurons belonging to different phase types are compatible with the assumption that inspiratory units are periodically inhibited by inspiratory-expiratory cells.