Chapter 22
The Respiratory System
835
22
the pontine respiratory centers, like the DRG, receive input
from higher brain centers and from various sensory receptors
in the periphery.
Generation of the Respiratory Rhythm
Tere is little question that breathing is rhythmic, but we still
cannot fully explain the origin of its rhythm. One hypothesis is
that there are
pacemaker neurons,
which have intrinsic (auto-
matic) rhythmicity like the pacemaker cells found in the heart.
Pacemaker-like activity has been demonstrated in certain VRG
neurons, but suppressing their activity does not abolish breathing.
Tis leads us to the second (and more widely accepted) hypoth-
esis: Normal respiratory rhythm results from reciprocal inhibition
of interconnected neuronal networks in the medulla. Rather than
a single set of pacemaker neurons, there are two sets that inhibit
each other and cycle their activity to generate the rhythm.
Factors Influencing Breathing Rate
and Depth
Compare and contrast the influences of arterial pH, arterial
partial pressures of oxygen and carbon dioxide, lung
reflexes, volition, and emotions on respiratory rate and
depth.
Inspiratory depth is determined by how actively the respiratory
center stimulates the motor neurons serving the respiratory
muscles. Te greater the stimulation, the greater the number of
motor units excited and the greater the force of respiratory mus-
cle contractions. Respiratory rate is determined by how long the
inspiratory center is active or how quickly it is switched off.
Changing body demands can modify depth and rate of
breathing. Te respiratory centers in the medulla and pons are
sensitive to both excitatory and inhibitory stimuli. We describe
these stimuli next and they are summarized in
Figure 22.24
.
Chemical Factors
Among the factors that influence breathing rate and depth, the
most important are changing levels of CO
2
, O
2
, and H
1
in arte-
rial blood. Sensors responding to such chemical fluctuations,
called
chemoreceptors
, are found in two major body locations:
Central chemoreceptors
are located throughout the brain
stem, including the ventrolateral medulla.
Peripheral chemoreceptors
are found in the aortic arch and
carotid arteries.
Influence of P
CO
2
Of all the chemicals influencing respiration,
CO
2
is the most potent and the most closely controlled. Normally,
arterial P
CO
2
is 40 mm Hg and is maintained within
6
3 mm Hg of
Higher brain centers
(cerebral cortex—voluntary
control over breathing)
Other receptors (e.g., pain)
and emotional stimuli acting
through the hypothalamus
Peripheral
chemoreceptors
O
2
,
CO
2
,
H
+
Receptors in
muscles and joints
Irritant
receptors
Stretch receptors
in lungs
Respiratory centers
(medulla and pons)
CO
2
,
H
+
+
+
+
+
+
Central
chemoreceptors
Figure 22.24
Neural and chemical
influences on brain stem respiratory
centers.
Excitatory influences (
1
) increase
the frequency of impulses sent to the muscles
of respiration and recruit additional motor
units, resulting in deeper, faster breathing.
Inhibitory influences (
2
) have the reverse
effect. In some cases, the impulses may be
excitatory or inhibitory (
6
), depending
on which receptors or brain regions are
activated. The cerebral cortex also directly
innervates respiratory muscle motor neurons
(not shown).
previous page 869 Human Anatomy and Physiology (9th ed ) 2012 read online next page 871 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off