The dorsal respiratory group (DRG) located in the medulla primarily initiates inspiration by generating rhythmic bursts of neural activity that stimulate the diaphragm and external intercostal muscles. It receives sensory input from peripheral chemoreceptors and mechanoreceptors, allowing it to adjust the respiratory rate accordingly. The DRG sets the basic rhythm of breathing by activating inspiratory motor neurons, unlike the ventral respiratory group which is more involved in forced expiration and inspiration. Therefore, the DRG is the key neuronal group responsible for starting each inspiratory phase.

Reference: Guyton and Hall, Textbook of Medical Physiology, 14th Edition.