[Ontogeny of respiratory muscle control. Evidence from the amphibian model]

The mechanisms which control the cyclic contraction of the respiratory muscles remain uncertain. The ontogenic approach is attractive but difficult in mammals. The bullfrog larva, the tadpole, provides a model easier to study as its brainstem can be isolated in vitro. The tadpole undergoes a development from exclusive gill ventilation to bimodal breathing and, eventually, to exclusive lung ventilation. During the metamorphic stages, lung ventilation is not dependent upon inhibitory mechanisms, suggesting that pacemaker neurones may possibly drive the lung rhythm. This is not the case for the gill ventilation. However, inhibitory mechanisms are probably involved in the shape of the lung discharge in the adult frog. During the development of the tadpole, the regions of the brainstem responsible for the lung rhythm migrate rostrally, the sensitivity to hypoxia shifts from gill ventilation to lung ventilation and the sensitivity to hypercapnia develops with the lung ventilation. The ontogenic particularities of tadpole breathing and the study of its isolated brainstem provide a unique model for the investigation of the ventilatory rhythm generation.