The influence of descending inputs on breathing pattern formation in the isolated bullfrog brainstem-spinal cord.

This study used in vitro brainstem-spinal cord preparations from the American bullfrog, Rana catesbeiana, to examine the influence of central descending inputs on breathing pattern formation. In preparations with an episodic pattern of fictive breathing, a transection slightly caudal to the optic chiasma produced a continuous breathing pattern and increased the overall frequency of fictive breathing. Following a transection to isolate the medulla, the frequency of fictive breathing decreased and the incidence of other forms of motor output increased. Further transections between the trigeminal and vagus nerve roots resulted in variable and asynchronous discharge from each nerve. These results suggest that a primary respiratory rhythm is produced within the medulla but descending influences stimulate breathing and promote episodic breathing. It would appear that multiple elements of the respiratory control system, including tegmental and medullary sites, play a role in shaping the burst pattern of motor output associated with each breath and that slower rhythms of longer burst duration are generated by more caudal hindbrain sites.