The basic rhythm of breathing is established by

Nerve impulses generated in the DRG establish the basic rhythm of breathing. While the DRG is active, it generates nerve impulses for about 2 seconds. When these impulses reach the diaphragm and external intercostal muscles, the muscles contract, and inhalation occurs. At the end of 2 seconds, the DRG becomes inactive, and the diaphragm and external intercostal muscles relax for about 3 seconds, allowing passive elastic recoil of the lungs and thoracic wall. Then, the cycle repeats. The neurons of the VRG remain inactive during quiet breathing. During forceful breathing, nerve impulses from the DRG activate the VRG. Impulses from the VRG cause forceful breathing (both inspiration and expiration).