Do descending influences alternate to produce episodic breathing?

This study examines the episodic breathing patterns of three disparate groups of vertebrates. In an in vitro bullfrog brainstem-spinal cord preparation, episodic breathing was replaced by uniformly spaced breaths following transection caudal to the optic chiasma. The same effect was produced in hibernating squirrels by inhalation of mild anesthesia. Preliminary data suggest that a similar conversion is also produced in hibernating squirrels by vagotomy, in conjunction with blockade of central NMDA-type glutamate receptors. In all cases, even though overall breathing frequency increased, due to elimination of periods of apnea, instantaneous breathing frequency slowed. Seals breathe episodically in sleep and when these animals awaken after the start of a breathing episode, breathing also immediately slows. The data presented here are consistent with the suggestion that in all vertebrates, higher centres can modulate the central rhythm generator for breathing, in both a positive and a negative fashion. During episodic breathing, in the species studied here, these modulating influences alternate in a fashion that produces periods of apnea alternating with periods of relatively high frequency ventilation.