A functional collection of nerve cells in the brainstem that control ventilatory activity. The cells are linked together to form a neural net that includes the motorneurons innervating respiratory muscles. Three distinct neuronal complexes comprise the respiratory center in the brainstem: dorsal respiratory group (DRG), ventral respiratory group (VRG), and the pontine respiratory group (PRG). The spinal respiratory group (SRG) is located in the spinal cord.