Phase resetting and dysrhythmic responses of the respiratory oscillator

This study explores resetting of respiratory rhythm by facilitatory perturbations. The midbrain reticular formation and periaqueductal gray matter were electrically stimulated to evoke facilitation of phrenic nerve activity in nine anesthetized, vagotomized, and glomectomized adult cats. The animals were paralyzed and servo-ventilated to keep end-tidal PCO2 constant. Brief midbrain stimuli were given at various times in the respiratory cycle and the times of onset of rescheduled breaths after stimulation were measured. A plot of phase resetting as a function of stimulus strength and time of delivery defined a helicoid surface. The axis of this helicoid identified a unique stimulus which, when given at the inspiratory-expiratory transition, resulted in unpredictable resetting of respiratory rhythm. This stimulus had a strength that was intermediate to that which identified types 1 and 0 resetting. In one experiment, the singular stimulus often initiated a breath having prolonged inspiratory activity; resumption of the normal rhythm was delayed significantly (P less than 0.01). We conclude that the dysrhythmias observed in this study represent the respiratory oscillator's phase singularity.