Ventral Respiratory Group

The VRG in the medulla comprises several anatomically and probably functionally distinct populations (Fig. 1). One classification divides the neurons of the VRG into three aggregates: the nucleus retroambigualis (NRA), the nucleus para-ambigualis (NPA), and the nucleus retrofacialis (NRF).
The nucleus ambigualis (NA) is composed primarily of subnuclei of motor neurons innervating the laryngeal, pharyngeal, and facial muscles. This nucleus also contains the vagal motor neurons innervating the bronchial smooth muscles and the smooth muscles of the thoracic and abdominal viscera. These neurons are almost completely inactive during deep anesthesia, suggesting that they are not essential to respiratory rhythmogenesis.
The NPA is located in the region medial to the NA and 1 mm caudal to 3.5 mm rostral to the obex. The NPA is composed primarily of premotor inspiratory neurons, but some expiratory cells are present.
Most of the inspiratory neurons of the NPA (like Ia cells in the DRG) fire in a ramplike fashion, with peak activity occurring at the conclusion of the inspiratory phase. The NPA also contains a few inspiratory propriobulbar, so-called early-burst neurons. These cells begin to discharge slightly before the onset of the phrenic discharge, peak rapidly, and then demonstrate a decline and disappearance of activity in the latter half of inspiration. They send no projections to spinal motor neurons, but they have a rich pattern of arborization with expiratory neurons in the contralateral NRA, whose activity they appear to inhibit.
The activity of other neurons in the VRG (located in the NRA and NRF) is mainly directed to expiration. The expiratory neurons in the NRA demonstrate a slowly augmenting pattern of activity, with peak discharge late in expiration. Input from pulmonary stretch receptors prolongs the time of firing of these neurons. Hypercapnia causes these neurons to discharge earlier in inspiration and increases the steepness with which their rate of discharge rises. Lesioning experiments indicate that neurons in the NRA are the sole source of expiratory premotor neurons but are not of fundamental importance in generating the respiratory rhythm.
Respiratory neurons in the NRF (also called the Bötzinger complex) and in an area immediately rostral to it, called the pre-Bötzinger complex, have been described. Bötzinger neurons discharge mainly in expiration with a slowly augmenting firing pattern that peaks at end-expiration. They send projections to the DRG on the opposite side and seem to inhibit the inspiratory neurons located there. On the other hand, pre-Bötzinger neurons fire during inspiration, demonstrate pacemaker-like activity, and appear to be exclusively propriobulbar in type. In the neonatal rat, lesions in this pre-Bötzinger complex eliminate respiratory rhythmogenesis. Some pharyngeal motor neurons also can be found in the NRF.

Fig. 1