Pontile axonal projections of medullary respiratory neurons

Our purpose was to determine if any medullary respiratory neurons have axonal projections to the rostral pons. In decerebrate, vagotomized, paralyzed and ventilated cats, we monitored efferent phrenic nerve activity and activities of single respiratory and non-respiratory neurons in the regions of the dorsal (DRN) and ventral (VRN) medullary respiratory nuclei. Neurons were classified as bulbospinal, vagal or bulbopontile if stimulation of the spinal cord, vagus nerve or pons elicited antidromic action potentials. If no such action potentials were elicited, the neurons were designated as 'not antidromically activated'. Approximately ten percent of respiratory neurons in both the DRN and VRN were bulbopontile. The great majority of these exhibited tonic or phasic phase-spanning patterns of activity. Effective stimulation loci in pons extended from the area of the trigeminal motor nuclei to the Kolliker-Fuse and parabrachial nuclei. At these same loci, antidromic action potentials were also elicited for some non-respiratory medullary neurons. The possible role of bulbopontile respiratory neurons in the control of automatic ventilatory activity is discussed.