Studying rhythmogenesis of breathing: comparison of in vivo and in vitro models.

In all mammalian species, breathing is controlled by a neuronal network within the lower brainstem. A component known as the ventral respiratory group produces rhythmic activity, which is transmitted to spinal motoneurons to produce a periodic contraction of respiratory muscles. A dispute about the mechanisms of 'normal' respiratory rhythm generation arose from the differences between experimental preparations that have been used to dissect the process. It is, therefore, essential to compare the various experimental approaches and to discuss the differences between experimental data. We conclude that the various preparations all have great value, but that they define different operational conditions of the network, including maturation of neurons and synaptic processes. We have taken note of these in formulating a 'maturational network-burster model' for rhythm generation that includes most features of the existing models of respiratory rhythm generation.