Interactions between rostral pontine and ventral medullary respiratory neurons

Lesioning studies have demonstrated that the respiratory rhythm is generated within the brain stem and that connections between the pons and the medulla must be intact for the generation of eupneic breathing in the decerebrate or anesthetized vagotomized cat. However, the nature of proposed functional connections between pontine and medullary respiratory neurons is not well understood. The possibility of interactions between respiratory neurons of the rostral pons (n. parabrachialis medialis, Kölliker-Fuse nucleus) and the ipsilateral ventral respiratory group (VRG; n. retroambigualis, n. ambiguus, retrofacial nucleus) was investigated because of neuroanatomical and electrophysiological evidence for such connections. Phrenic nerve activity and pontine and medullary single-unit respiratory related activities were recorded extracellularly in 44 decerebrate, vagotomized, paralyzed, and artificially ventilated cats. Cross-correlation analysis was employed to detect and evaluate functional associations of pairs of cells. Eighteen (7%) of the 255 pairs of respiratory neurons analyzed showed evidence of short time scale correlations indicative of a functional interaction. The interpretations of the detected correlations suggest that some cell pairs were correlated due to mono- or paucisynaptic connections, while others were correlated due to the influence of an unobserved shared input. The interpretations for 11 of the 15 cell pairs for which a monosynaptic connection may be postulated involve a projection from a tonically active respiratory neuron. Twelve of the 18 positive correlations involved neurons whose maximum rates of discharge occurred during different parts of the respiratory cycle. The results of this study provide the first evidence of functional connections among pontine and medullary respiratory neurons based on the evaluation of simultaneously recorded spike trains and suggest that the role of the rostral pontine respiratory neurons in the control of the respiratory rhythm may be mediated by various types of interactions. When considered with the results of other studies, our data suggest that monosynaptic interactions between VRG and rostral pontine respiratory neurons play a limited role in the control of the respiratory cycle in the decerebrate vagotomized cat. It is likely that the influence of the pons on ventral medullary neurons (and vice-versa) is also exerted via polysynaptic pathways and/or via brain stem neurons not sampled in this study.