The central neural mechanisms underlying the generation of the respiratory rhythm are still speculative (see Chapter 10). However, the spatial distribution of those neurons which organize the respiratory motor output and regulate the production of respiratory movements is quite well known (reviewed in Reference 11). Breathing in mammals relies on a neuronal network located within the brainstem. This network constitutes the central pattern generator (CPG) which elaborates a respiratory cycle essentially composed of two stable phases: inspiration (I) and expiration (E), during which thoracic and abdominal muscles alternatively contract, inflating and deflating the lung. The resistance to air flow is simultaneously controlled by oro-laryngeal-pharyngeal muscles (see Chapter 5) driven by cranial motoneurons which also present a respiratory pattern of activity in either inspiration or expiration (Figure 1).