The respiratory motoneuron and apneusis

A brief review is given of experimental studies on 'apneusis' (Lumsden) and the role these have played in the conceptual development of the ideas relating to the nervous mechanism of breathing. It is suggested that the terms "apneustic," "pneumotaxic," and "respiratory" (inspiratory or expiratory) centers should be abandoned in view of the convoluted epistemology of the concepts they represent. New experiments on expiratory apneusis are described, involving sagittal incisions of the brain stem in the vicinity of the obex, which have allowed study of the brain stem-mediated effects of CO2 on expiratory motoneurons, under steady-state conditions. These experiments reveal the existence of a CO2-dependent tonic excitation of expiratory motoneurons that, it is argued, depends on a simple pathway between peripheral and central chemoreceptors and expiratory bulbospinal neurons. The possible synaptic mechanisms underlying the production of CO2-dependent appneusis are discussed in relation to peripheral chemoreceptors and illustrated in relation to a model motoneuron.