How is respiratory rhythm generated?

The time course of respiratory activity, e.g., the the abrupt transitions between the inspiratory (1) and expiratory (E) phases, can be explained by several types of interaction between brainstem respiratory neurons. 1) Reciprocal inhibition: Inhibition of E neurons by I neurons, and of I neurons by E neurons, has been demonstrated, a mechanism that prevents occurrence of activity in one set of populations during the period of activity of another set of populations. 2) Recurrent inhibition: During each respiratory phase, activity in recurrent loops gradually rises to levels sufficient to inhibit ongoing activity and produce abrupt phase-switching. a) I leads to E phase-switching: By use of lung inflation tests, several categories of I neurons in the region of the ventrolateral nucleus of tractus solitarius (NTS) have been specified; in particular, a population of late-I neurons has been found that may function to terminate the I phase. b) E leads to I phase-switching: Several populations of medullary neurons (early-E, late-E, and EI) have been found with responses to lung inflation that suggest that they may play a role in determining E phase duration and E leads to I switching; in particular, a population of early-E neurons in the NTS has been found whose discharge properties suggest that they may function to delay the onset of the succeeding I phase. 3) Recurrent excitation: There is evidence for the existence of recurrent excitatory actions among I neurons that may produce the augmenting pattern of I neurons' discharges.