REFLEX RECRUITMENT OF MEDULLARY GASPING MECHANISMS IN EUPNOEA BY PHARYNGEAL STIMULATION IN CATS

1. Mechanical stimulation of the naso- and oropharynx causes the repla cement of the eupnoeic ventilatory pattern by a brief, but large, burs t of activity of the phrenic nerve. Our purpose was to define whether these changes in phrenic activity represent a switch to gasping. 2. In decerebrate, vagotomized, paralysed and ventilated cats, mechanical s timulation of the pharynx was performed during eupnoea, apneusis and g asping. The latter two ventilatory patterns were produced by ventilati ng the experimental animal with 1.0% carbon monoxide in air or with 10 0% nitrogen. Eupnoea could be re-established by a recommencement of ve ntilation with oxygen. 3. The rate of rise of phrenic activity and its peak height were much greater following mechanical stimulation of the pharynx than the phrenic bursts of eupnoea or apneusis. The durations of phrenic burst and the period between these were much less followin g pharyngeal stimulation. In contrast, these variables of phrenic acti vity were the same during pharyngeal stimulation and in gasping. 4. Pr evious studies had established that activity within a region of the la teral tegmental field of medulla is critical for the manifestation of gasping. Hence, electrical stimulation of this region during gasping e licits premature gasps whereas its ablation irreversibly eliminates ga sping. 5. We positioned a multibarrelled pipette in the critical medul lary region for gasping. Its location was verified, once gasping was e stablished in hypoxia or anoxia, by the elicitation of premature gasps following electrical stimulation. Neurons in this region were destroy ed by microinjections of the neurotoxin kainic acid; in a few experime nts the region was destroyed by electrolytic lesions. 6. Following des truction of the region of the lateral tegmental field, gasping could n o longer be provoked in anoxia. In contrast, the eupnoeic pattern of p hrenic activity continued. However, mechanical stimulation of the phar ynx no longer caused any changes in the on-going pattern of phrenic ac tivity. 7. We conclude that mechanical stimulation of the pharynx elic its a powerful reflex by which eupnoea is suppressed and gasping is el icited. Stated differently, the changes in phrenic activity during thi s pharyngeal stimulation in fact represent gasps. 8. Gasps are depende nt upon activity within a region of the lateral tegmental field of the medulla. This region plays no role in the neurogenesis of eupnoea. He nce, our results provide additional support for the concept that there are multiple sites for ventilatory neurogenesis in the mammalian brai nstem.