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.