The Midbrain and Medullary Control of Post-inspiratory Activity of the Crural and Costal Diaphragm in Vivo

Studies on brainstem respiratory neurons suggest that eupnea consists of three phases; inspiration, post-inspiration and expiration. However it is not well understood how post-inspiration is organized in the diaphragm, i.e. whether post-inspiration differs in the crural and costal segments of the diaphragm and what the influence is of post-inspiratory neurons on diaphragm function during eupnea. In this in-vivo study we investigated the post-inspiratory activity of the two diaphragm segments during eupnea and the changes in diaphragm function following modulation of eupnea. Post-inspiratory neurons in the medulla were stereotaxically localized extracellularly and neurochemically stimulated. We used three types of preparations; precollicularly-decerebrated unanesthetized cats and rats and anesthetized rats. In all preparations, during eupnea, post-inspiratory activity was found in the crural but not in the costal diaphragm. When eupnea was discontinued, in decerebrate cats in which stimulation in the nucleus retroambiguus induced activation of laryngeal or abdominal muscles, all post-inspiratory activity in the crural diaphragm was abolished. In decerebrate rats stimulation of the midbrain periaqueductal gray abolished post-inspiration in the crural, but induced activation in the costal diaphragm. In anesthetized rats, stimulation of medullary post-inspiratory neurons abolished the post-inspiratory activity of the crural diaphragm. Vagal nerve stimulation in these rats increased the intensity of post-inspiratory neuronal discharge in the solitary nucleus leading to decreased activity of the crural diaphragm. These data demonstrate that three-phase breathing in the crural diaphragm during eupnea exists in vivo and that post-inspiratory neurons have an inhibitory effect on crural diaphragm function.