Figure 1: Ventral aspect of the brainstem with pons and medulla: The nuclei of the respiratory neurons in the ventral respiratory group (VRG) in vicinity of the Ncl. ambiguous, the dorsal respiratory group (DRG) close to the solitari tract nucleus (NTS), and the pontine respiratory group (PRG) are marked. The areas on the ventral medullary surface serve the central CO 2 chemosensitivity (from animal experiments).