LONG-TERM FACILITATION OF PHRENIC-NERVE ACTIVITY IN CATS - RESPONSES AND SHORT-TIME SCALE CORRELATIONS OF MEDULLARY NEURONS

1. Stimulation of either peripheral chemoreceptors or nucleus raphe ob scurus results in long-term facilitation of phrenic motoneurone activi ty. The first objective of this work was to measure the concurrent res ponses of neurones in the nucleus raphe obscurus, the nucleus tractus solitarii, and the regions of the retrofacial nucleus, nucleus ambiguu s and nucleus retroambigualis during induction of long-term facilitati on. A second goal was to assess functional relationships of the chemor esponsive raphe neurones with neurones in the other monitored location s and with phrenic motoneurones. 2. Up to thirty single medullary neur ones and phrenic nerve efferent activity were recorded simultaneously in fifteen anaesthetized, paralysed, vagotomized, artificially ventila ted adult cats. Carotid chemoreceptors were stimulated by close arteri al injection of 200 mu l of CO2-saturated saline solution. Spike train s were analysed with cycle-triggered histograms and two statistical te sts for respiratory modulation. Peristimulus-time histograms and cumul ative sum histograms were used to assess responses to stimulation. Cro ss-correlation was used to test for non-random temporal relationships between spike trains. Spike-triggered average histograms provided evid ence for functional associations with phrenic motoneurones. 3. One hun dred and thirteen of 348 neurones were monitored in the nucleus raphe obscurus. The firing rates of twenty-nine raphe neurones increased dur ing stimulation; eighteen decreased. In twenty-one pairs of concurrent ly monitored raphe neurones, the firing rate of one increased its acti vity during stimulation then decreased, while the other showed an incr ease that began as the rate of the former declined. Eighteen chemoresp onsive raphe neurones had short time scale features in their phrenic s pike-triggered averages. Short time scale features a ere found in cros s-correlograms from 184 of 1407 neurone pairs. 4. The data suggest par allel routes by which carotid chemoreceptors influence medullary raphe neurones and support the hypotheses that mid-line respiratory-related neuronal assemblies transform information from those receptors and re gulate the gain of respiratory motor output.