834
UNIT 4
Maintenance of the Body
22
Control of Respiration
Although our tidelike breathing seems so beautifully simple, its
control is more complex than you might think. Higher brain
centers, chemoreceptors, and other reflexes all modify the basic
respiratory rhythms generated in the brain stem.
Neural Mechanisms
Describe the neural controls of respiration.
Control of respiration primarily involves neurons in the reticu-
lar formation of the medulla and pons. Because the medulla sets
the respiratory rhythm, we will begin there.
Medullary Respiratory Centers
Clustered neurons in two areas of the medulla oblongata appear
to be critically important in respiration
(Figure 22.23)
. Tese are:
Te
ventral respiratory group (VRG)
, a network of neurons
that extends in the ventral brain stem from the spinal cord to
the pons-medulla junction
Te
dorsal respiratory group (DRG)
, located dorsally near
the root of cranial nerve IX
Ventral Respiratory Group (VRG)
Te VRG appears to be a
rhythm-generating and integrative center. It contains groups of
neurons that fire during inspiration and others that fire during
expiration in a dance of mutual inhibition.
When its inspiratory neurons fire, a burst of impulses travels
along the
phrenic
and
intercostal nerves
to excite the diaphragm
and external intercostal muscles, respectively (Figure 22.23). As a
result, the thorax expands and air rushes into the lungs. When the
VRG’s expiratory neurons fire, the output stops, and expiration
occurs passively as the inspiratory muscles relax and the lungs
recoil.
Tis cyclic on/off activity of the inspiratory and expiratory
neurons repeats continuously and produces a respiratory rate of
12–15 breaths per minute, with inspiratory phases lasting about
2 seconds followed by expiratory phases lasting about 3 sec-
onds. Tis normal respiratory rate and rhythm is called
eupnea
(ūp-ne
9
ah;
eu
5
good,
pne
5
breath).
During severe hypoxia, VRG networks generate gasping
(perhaps in a last-ditch effort to restore O
2
to the brain). Res-
piration stops when a certain cluster of VRG neurons is com-
pletely suppressed, as by an overdose of morphine or alcohol.
Dorsal Respiratory Group (DRG)
Until recently, it was thought
that the DRG acts as an inspiratory center, performing many of
the tasks now known to be performed by the VRG. In almost all
mammals, including humans, the DRG integrates input from
peripheral stretch and chemoreceptors (which we will describe
shortly) and communicates this information to the VRG. It may
seem surprising, but many of the details of this system so essen-
tial to life are still being worked out.
Pontine Respiratory Centers
Although the VRG generates the basic respiratory rhythm, the
pontine respiratory centers
influence and modify the activity
of medullary neurons. For example, pontine centers appear to
smooth out the transitions from inspiration to expiration, and vice
versa. When lesions are made in its superior region, inspirations
become very prolonged, a phenomenon called
apneustic breathing
.
Te
pontine respiratory group
and other pontine centers
transmit impulses to the VRG of the medulla (Figure 22.23).
Tis input modifies and fine-tunes the breathing rhythms gen-
erated by the VRG during certain activities such as vocalization,
sleep, and exercise. As you would expect from these functions,
Pons
Pons
Ventral respiratory group (VRG)
contains rhythm generators
whose output drives respiration.
Pontine respiratory centers
interact with medullary
respiratory centers to smooth
the respiratory pattern.
Medulla
Medulla
To inspiratory muscles
External intercostal
muscles
Diaphragm
Dorsal respiratory group (DRG)
integrates peripheral sensory
input and modifies the rhythms
generated by the VRG.
Figure 22.23
Locations of respiratory centers and their
postulated connections.
The efferent pathway shown here is
incomplete. Medullary neurons communicate with lower motor
neurons in the spinal cord, but these are omitted for simplicity.
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