Anatomical Organization of Central Respiratory Neurons

Most of our currently held views on the organization of central mechanisms are based on electrophysiological studies of respiratory neu­ rons, i.e. neurons that Salmoiraghi & Burns (66, 67) described as produc­ ing periodic bursts of activity which are locked constantly to some phase of activity of the diaphragm. Neurons that show no sign of rhythmicity or have irregular bursting patterns that may play a part in the genesis of rhythm are excluded by such a definition. Recently, the search for these related units-RRUs (15)-has been intense and much of the conceptual framework of the organization of the respira­ tory centers is derived from these studies. Electrophysiological studies, however, have limitations. For instance, during extracellular stimulation and recording it is not possible to know the exact morphology or location of the neuron being studied. Furthermore, since only spontaneously active units are sampled, it is possible to miss neurons that are silent under the condition of the experiment and that could have been activated under other conditions. Combinations of electrophysio­ logical and neuroanatomical mapping techniques might result in a more meaningful map of the centers. However, it is rarely possible to do definitive anatomical mapping of control mechanisms be­ cause of heterogeneity in the neuronal populations involved.