Dissociation between respiratory phase switching and phasic phrenic response on low-intensity stimulation of pneumotaxic complex and nearby structures.

Excitatory and inhibitory phasic phrenic responses (ePPR and iPPR) and inspiratory on-switch and off-switch (I-ON-S and I-OFF-S) were compared on stimulation of the pneumotaxic complex and neighboring reticular formation in cats anesthetized with nitrous oxide, curarized and spinalized at the C7 level. Stimulation consisted of a single pulse or train of 2 to 5 shocks whose intensity ranged between 100 and 300 microA. Single shock stimulation of the Kölliker-Fuse (KF) nucleus and of a region 1 mm just caudal evoked a pure iPPR during the phrenic burst. With single pulse of high intensity delivered in the second half of inspiration (I), the iPPR was associated with I-OFF-S. Single shock stimulation of a wider brain stem area including the medial part of the nucleus parabrachialis medialis (NPBM), nucleus parabrachialis lateralis and adjacent lateral and central tegmental fields evoked early iPPR combined with a later ePPR when delivered in I, and ePPR alone during expiration (E). Train stimulation of much more restricted areas could evoke I-ON-S or I-OFF-S depending on the site of stimulation and time of delivery in I or E. Definite dissociations occurred between iPPR and I-OFF-S according to the place of stimulation. A maximal iPPR, obtained from a region ventral to the NPBM, was never found to be associated with I-OFF-S. The excitability of I-ON-S, and ePPR amplitude showed opposite time-courses during E and had different latencies.(ABSTRACT TRUNCATED AT 250 WORDS)