C h a p t e r 2 1
Control of Respiratory Function
535
Control of Breathing
Unlike the heart, which has inherent rhythmic proper-
ties and can beat independently of the nervous system,
the muscles that control respiration require continu-
ous input from the nervous system. Movement of the
diaphragm, intercostal muscles, sternocleidomastoid,
and other accessory muscles that control ventilation is
integrated by neurons located in the pons and medulla.
These neurons are collectively referred to as the
respira-
tory center
(Fig. 21-18).
Respiratory Center
The respiratory center consists of two dense, bilateral
aggregates of respiratory neurons involved in initiating
inspiration and expiration and incorporating afferent
impulses into motor responses of the respiratory mus-
cles. The first, or dorsal, group of neurons is concerned
primarily with inspiration. These neurons control the
activity of the phrenic nerves that innervate the dia-
phragm and drive the second, or ventral, group of respi-
ratory neurons. They are thought to integrate sensory
input from the lungs and airways into the ventilatory
response. The second group of neurons, which contains
inspiratory and expiratory neurons, controls the spinal
motor neurons of the intercostal and abdominal muscles.
The pacemaker properties of the respiratory center
in the medulla result from the cycling of two groups of
neurons: the
pneumotaxic center
in the upper pons and
the
apneustic center
in the lower pons (see Fig. 21-18).
The apneustic center has an excitatory effect on inspira-
tion, tending to prolong inspiration. The pneumotaxic
center switches inspiration off, assisting in the control
of the respiratory rate and inspiratory volume. Brain
injury, which damages the connections between the
pneumotaxic and apneustic centers, results in an irregu-
lar breathing pattern that consists of prolonged inspira-
tory gasps interrupted by expiratory efforts.
Axons from the neurons in the respiratory center
cross in the midline and descend in the ventrolateral col-
umns of the spinal cord. The tracts that control expira-
tion and inspiration are spatially separated in the cord,
as are the tracts that transmit specialized reflexes (i.e.,
coughing and hiccupping) and voluntary control of ven-
tilation. Only at the level of the spinal cord are the respi-
ratory impulses integrated to produce a reflex response.
Trachea
and bronchi
Lungs
Diaphragm
Pons
Pneumotaxic center
Apneustic center
Inspiratory center
Medulla
Cervical
nerves
Thoracic
nerves
Expiratory center
Impulses from higher
brain centers
Carotid
arteries
and aorta
Nose
and
throat
Intercostal
muscles
FIGURE 21-18.
Schematic representation of activity
in the respiratory center. Impulses traveling over
afferent neurons (dashed lines) communicate with
central neurons, which activate efferent neurons
that supply the muscles of respiration. Respiratory
movements can be altered by a variety of stimuli.
■■
Carbon dioxide is transported in the blood:
(1) as the dissolved gas (10%), (2) attached to
hemoglobin (30%), and (3) as bicarbonate (60%).
The reversible action of carbon dioxide with water
to form bicarbonate is catalyzed by the enzyme
carbonic anhydrase within red cells and is the
major pathway for generation of bicarbonate.
