Figure 2.

Distribution of the major groups of respiratory neurons in the brainstem and upper spinal cord, as seen in a dorsal view. Brainstem respiratory neurons form longitudinal columns, of which the most prominent one is the VRG located in the ventrolateral medullary reticular formation. The rostralmost part of the VRG, the Bötzinger complex, contains mainly late expiratory neurons. Next region caudally, the pre-Bötzinger complex, contains pacemaker neurons that, at least in neonatal animals, are capable of producing basic respiratory rhythm under in vitro conditions. Farther caudal is a large group of mainly inspiratory-modulated neurons, of which many send axons to spinal motoneurons that innervate the diaphragm and external intercostal muscles. Nucleus ambiguus runs parallel to this part of VRG and contains cell bodies of laryngeal motoneurons. Caudal to the inspiratory part of the VRG is an expiratory region whose neurons send axons to spinal expiratory motoneurons that control internal intercostal and abdominal muscles. A spinal extension of the VRG, the C2 and C3 group, again contains inspiratory neurons whose function may be to reinforce the actions of the VRG. The dorsal respiratory group is located in the viscerosensory nucleus of the solitary tract. It contains mostly inspiratory-modulated neurons, of which some have connections with spinal motoneurons and some receive input from pulmonary and laryngeal receptors. The pontine respiratory group located in the dorsolateral pons comprises cells with different patterns of respiratory modulation. These neurons integrate peripheral and central respiratory and nonrespiratory inputs and have descending projections to medullary respiratory neurons. (Modified from Fig. 2 in Ref. 266 and republished with permission from the American Academy of Sleep Medicine.)