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acidosis increases breathing in vivo . 37 Mice with genetic deletion of hypo-
thalamic orexin neurons have an attenuated HCVR, and this can be partially
restored by exogenous orexin. 77 Treatment with an orexin receptor antag-
onist attenuates the HCVR in WT mice. 78
It has been proposed that central chemoreception is a widely distributed
function of neurons in many brainstem nuclei. 25 This possibility is supported
by studies showing that focal acidosis in many of the nuclei discussed above
causes an increase in ventilation in vivo . 79 It has been further suggested that
these sites may not be equally important under normal physiological condi-
tions, but rather play an important role under specific conditions, such as
during development, under anesthesia, during sleep, or in various patholog-
ical states. 80
2.3.2 Influence of other factors on breathing
In addition to being subject to chemical regulation, breathing can also be
modulated by a number of other factors including conscious control (e.g.,
voluntary modulation), limbic factors (e.g., stress), cardiac factors (e.g.,
stress, exertion), body temperature (e.g.,
fever), and physical exertion
(e.g., increased oxygen demand).
The respiratory control system receives descending input from cortical
structures, allowing for voluntary modulation of RR, depth of ventilation,
and respiratory pattern. For instance, an individual can voluntarily hold his/
her breath when submerging the head in water or encountering a noxious
olfactory stimulus. An individual can also voluntarily hyperventilate. There
are, of course, built-in failsafe mechanisms to prevent an individual from
accidentally injuring themselves from these maneuvers. For instance, if
breath is held too long, the individual will pass out, thus, releasing the cor-
tical activation and the RPG will resume its preprogrammed rate and
rhythm to maintain breathing and normal blood gases. Similarly, if the indi-
vidual hyperventilates too long, they will experience dyspnea, and this
uncomfortable sensation will be relayed to the cortex and cause it to reduce
its excitatory input to the respiratory control system.
Afferent signals from Golgi tendon organs contained within the dia-
phragm feedback to respiratory centers and inhibit respiratory activity.
Afferents from muscle spindle fibers embedded within chest wall intercostal
muscles relay information regarding chest wall expansion and contraction to
higher centers and are involved with reflex control of respiratory activity.
These muscle mechanoreceptors allow sampling of volume (length) and
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