Dorsal Respiratory Group

The DRG contains almost entirely inspiratory neurons. One kind of inspiratory neuron, the Ia neuron, like the phrenic motor neurons, demonstrates an augmenting pattern of firing that peaks at end-inspiration. Axons from these cells decussate in the medulla at or immediately rostral to the obex and connect with phrenic and inspiratory intercostal motor neurons in the spinal cord. Collaterals are sent to the ipsilateral inspiratory neurons in the VRG, but a few also are distributed to the expiratory neurons in the VRG.
The firing of Ib neurons, as of Ia neurons, occurs primarily in inspiration but, in contrast to the discharge of Ia neurons, is augmented by inputs from pulmonary stretch receptors. In animals, the discharge peak of Ib is not as sharp and the decline in activity during expiration is slower than for Ia. In the absence of excitatory input from vagal stretch receptors, however, the discharge patterns of Ia and Ib neurons are similar.
Like Ia neurons, some of the Ib neurons project to the spinal cord. Those Ib neurons that do not project to the cord appear to undergo extensive axonal arborization in the NTS. Ib neurons seem to be responsible for the shortening of inspiratory time induced by lung inflation. Their responsiveness to stretch-receptor input is less during expiration than during inspiration.
The DRG also contains late-onset inspiratory neurons that reach their peak firing rate in the transition from inspiration to expiration. Their activity, like that of Ib neurons, is facilitated by stretch-receptor activity. These neurons may participate in the short phase of graded inhibition of inspiratory activity seen with volume changes occurring toward the terminal portion of inspiration.
Recently, a small number of early-expiratory neurons have been observed in the DRG (and possibly also in the VRG) intermingled with inspiratory cells. They begin their firing shortly before the end of inspiration and reach peak discharge rates quickly, and then their activity slowly diminishes during expiration, disappearing before inspiration begins. Increases in lung volume slow the rate of decline in activity of these cells, whereas prevention of lung inflation does the opposite. The activity of these neurons is related to postinspiration inspiratory activity (PIIA), which occurs in the diaphragm and intercostal and laryngeal muscles and retards expiratory flow and the rate of lung deflation.