Pathways involved in synchronization of sympathetic nerve discharge to lung inflation

In urethane-anesthetized cats, we tested the hypothesis that sympathetic nerve discharge (SND) is synchronized to lung inflation, in part, over a vagal afferent pathway that bypasses the central respiratory oscillator. For this purpose, partial coherence analysis was used to mathematically remove the portion of the relationship between intratracheal pressure (ITP; index of lung inflation) and inferior cardiac SND that was common to that between ITP and phrenic nerve activity (PNA; index of central respiratory cycle). ITP-SND partial coherence (PNA-related component removed) at the frequency of breathing should be significantly different from zero if a bypass pathway exists. This was found to be the case in paralyzed, artificially ventilated cats when lung inflation and rhythmic PNA were coupled in various ratios (e.g. 1:1, 3:1) and in spontaneously breathing cats. As expected, surgical vagotomy eliminated ITP-SND coherence in artificially ventilated cats. These findings support the view that a portion of the relationship between ITP and SND is independent of the influences of pulmonary stretch receptors on central respiratory networks.