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bronchi, bronchioles, and alveoli. The latter three components are housed
within the lungs and along with the trachea constitute the lower airway. The
remaining components comprise the upper airway. The mechanical act of
breathing is accomplished via rhythmic contraction of primary and accessory
respiratory muscles including the diaphragm and intercostal muscles. Con-
traction of the diaphragm creates negative pressure with the chest cavity all-
owing O 2 -rich air to pass into the respiratory tract. Following gas exchange
within the alveoli, diaphragmatic relaxation increases pressure within the
chest cavity and allows CO 2 -laden air to be expelled from the respiratory
tract through the mouth and nose. Rhythm-dictating input to these muscles
originates in the respiratory pattern generator (RPG) within the caudal
medulla. 1 The RPG will be described in more detail shortly. Output from
the RPG is transmitted to anterior horn motor neurons of the cervical and
thoracic spinal cord, which in turn contract the diaphragm and intercostal
muscles, respectively. 2 While breathing occurs automatically—from birth
to death for most animals including humans—several factors can influence
respiratory rate (RR) and rhythm. The most notable among these are the
respiratory chemical stimuli, hypoxia, and hypercapnia. 3,4 A schematic
overview of the respiratory system is given in Fig. 8.1 . Respiratory pattern
generation, airway control, and mechanisms of respiratory rhythm modula-
tion will now be discussed in greater detail.
2.1. Respiratory pattern generation
Unlike the heart that has intrinsic pacemakers, 5 the lungs do not have self-
contained pacemaking capabilities. For the respiratory system, generation of
the respiratory rhythm is relegated to the medullary brainstem. 1,6 Within the
medulla are two bilateral neuronal populations, the dorsal and ventral respi-
ratory groups (DRG and VRG, respectively), which are involved in respi-
ration. The DRG is primarily comprised of neurons that are most active
during inspiration and that are situated within the ventrolateral subgroup
of the nucleus tractus solitarius (NTS). 7 The VRG is subdivided into a rostral
part comprised largely of inspiratory premotor neurons 8 and a caudal part
containing primarily expiratory premotor neurons. 9 Rostral to the VRG
are the B¨ tzinger complex (B¨ tC), the pre-B¨ tzinger complex (pre-B¨ tC),
and the parafacial nucleus. The B ¨ tC contains inhibitory expiratory neurons
and cranial motor neurons. 10 Generation of the basic respiratory rhythm is
thought to be the responsibility of the pre-B¨ tC 11 and parafacial respiratory
group, 12,13 which includes the retrotrapezoid nucleus (RTN). 6,14 The
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