Final answer:
During breathing, the external intercostal muscles and diaphragm must contract simultaneously. The DRG region of the medulla oblongata stimulates the contraction of these muscles. These contractions allow for expansion of the thoracic cavity and the flow of air into the lungs.
Step-by-step explanation:
During breathing, the external intercostal muscles and diaphragm must contract simultaneously. The external intercostal muscles raise the rib cage, allowing for expansion of the thoracic cavity, while the diaphragm contracts and flattens, increasing the volume of the pleural cavities. This creates a decrease in pressure, causing air to flow into the lungs. The neurons that must be active to initiate the contraction of these muscles are located in the dorsal respiratory group (DRG) region of the medulla oblongata.