Pulmonary afferent and central influences on respiratory phase-switching in the cat

A striking feature of the mammalian respiratory rhythm generator is the abruptness of the transitions between the inspiratory (I) and expiratory (E) phases ("phase-switching"). Although reciprocal inhibitory actions between I and E neurons are important for maintaining the alternation of respiratory phases, phase-switching actions are probably produced by recurrent inhibitory neuronal loops acting within each phase. The operation of such negative feedback circuits has been studied using two types of experimental input: (a) activation of pulmonary stretch receptors by lung inflation, which shortens the I phase and prolongs the E phase, and (b) electrical stimulation in the rostral pontine "pneumotaxic center" region, which can produce premature I leads to E or E leads to I switching, depending on stimulation site. These inputs produce diverse effects on discharges of various types of medullary respiratory neuron; analysis of such observations has led to hypotheses about the possible roles of different neuron populations in phase-switching.