Parapyramidal rostroventromedial medulla as a respiratory rhythm modulator

After inhalation of 15% CO2, immunoreactions to glutamate and glutamic acid decarboxylase were found in some c-Fos or c-Jun-labeled neurons distributed in the reticular region just dorsal to the pyramidal tract in the rostroventromedial medulla (parapyramidal RVMM). This region forms vertically the narrow strip between the nucleus raphe pallidus and nucleus parapyramidalis superficialis, and extends rostrocaudally from the level just ahead of the inferior olivary complex to the level just behind the nucleus of the trapezoid body. When we placed lesions with kainate in the parapyramidal RVMM, hyperpneic and tachypneic responses to brief inhalation of 15% CO2 were completely abolished, and the eupneic rhythm changed into the gasping rhythm. This study suggests that the parapyramidal RVMM consists of neuronal substrates that subserve as the respiratory rhythm modulator.