There are four respiratory centers in the brain. One pair is aneustic and pneumotaxic located in the pons and the other pair is the inspiratory center and expiratory center located in the medulla oblongata.
Anaesthetic center is stimulated in the presence of CO2 in the blood. This stimulus reaches the respiratory center (Dorsal Respiratory Group-DRG). Then the stimulus from the respiratory center comes simultaneously to the diaphragm, intercostal muscles and the pneumotactic center. As the stimulus reaches the diaphragm and intercostal muscles, the diaphragm and peritoneum contract and respiration begins. This increases the volume of the thoracic cavity, allows air to enter the lungs, and inflates the lungs.
Then at the same time nerve impulses come directly from the pneumotactic center and from the inflated lungs via the vagus nerve to the aneustic center. As a result, the anesthetic center becomes depressed and respiration stops. On the other hand, at the same time the nerve impulse from the pneumotactic center reaches the respiratory center (Ventral Respiratory Group-VRG) and respiration begins. As a result, air escapes from the lungs.
Nerve impulses from the pneumotactic center reach the inspiratory center and the expiratory center simultaneously, resulting in simultaneous cessation of inspiration and initiation of inspiration. During inspiration, the contractile stimulus of the lung does not reach the aneustic center. So the anesthetic center is re-stimulated and the stimulus is transmitted to the respiratory center and respiration starts. In this way breathing is regulated by repetition of events in phases.