Failure of any of these conditions to occur can lead to apnea. The nervous system breathing apparatus generates and coordinates the contractions of the diaphragm, upper airway respiratory muscles, and intercostal muscles. Pivotal to the function of the nervous system breathing apparatus are the dorsal and ventral respiratory groups. The dorsal and ventral respiratory groups are in the medulla. The neurons of the dorsal respiratory group are intermingled with the neurons of the tractus solitarius. The neurons of the ventral respiratory group are intermingled with the neurons of the nucleus ambiguus and retroambigualis.
The dorsal and ventral respiratory groups have efferent and afferent connections. The efferent connections of the respiratory groups are with the phrenic center, alpha motor neurons of the intercostal muscles, and the cranial nerve motor neurons of the upper airway muscles. The dorsal and ventral respiratory groups generate discharges that lead to the contractions of upper airway muscles and the intercostal muscle 100 milliseconds before the onset of diaphragmatic contractions. The upper airway muscle and intercostal muscle contractions prevent the narrowing of the upper airway and the collapse of the chest wall that would otherwise occur due to the negative intrathoracic pressure generated by the diaphragmatic contraction.