Page 906 - Fundamentals of anatomy physiology

Chapter 23 The Respiratory System 893

Figure 23–24 Basic Regulatory Patterns of Respiration.

a Quiet Breathing b Forced Breathing

INHALATION INHALATION
(2 seconds)
Diaphragm and external Muscles of inhalation
intercostal muscles contract, and opposing
contract and inhalation muscles relax.
occurs. Inhalation occurs.

Dorsal Dorsal DRG and DRG and
respiratory respiratory inspiratory center inspiratory
group active group inhibited of VRG are center of VRG
active. Expiratory are inhibited.
center of VRG is Expiratory
inhibited. center of VRG
is active.

Diaphragm and external Muscles of inhalation
intercostal muscles relax and muscles of
relax and passive exhalation contract.
exhalation occurs. Exhalation occurs.

EXHALATION EXHALATION
(3 seconds)

The Apneustic and Pneumotaxic Centers of the Pons at a time. Intervening exhalations are brief, and little pulmo-
nary ventilation occurs.
The apneustic centers and pneumotaxic centers of the pons
regulate the depth and rate of respiration in response to The CNS regions involved with respiratory control are dia-
sensory stimuli or input from other centers in the brain. grammed in Spotlight Figure 23–25. Interactions between the
Each apneustic center provides continuous stimulation to DRG and the VRG establish the basic pace and depth of respira-
the DRG on that side of the brain stem. During quiet breath- tion. The pneumotaxic centers modify that pace: An increase in
ing, stimulation from the apneustic center helps increase the pneumotaxic output quickens the pace of respiration by short-
intensity of inhalation over the next 2 seconds. Under normal ening the duration of each inhalation. A decrease in pneumo-
conditions, after 2 seconds the apneustic center is inhibited
by signals from the pneumotaxic center on that side. Dur- 23taxic output slows the respiratory pace, but increases the depth
ing forced breathing, the apneustic centers also respond to
sensory input from the vagus nerves regarding the amount of of respiration, because the apneustic centers are more active.
lung inflation. Sudden infant death syndrome (SIDS), also known as

The pneumotaxic centers inhibit the apneustic centers and crib death, is the leading cause of death for babies 1–12 months
promote passive or active exhalation. Centers in the hypothala- old and kills an estimated 2250 infants each year in the United
mus and cerebrum can alter the activity of the pneumotaxic States alone. Most crib deaths occur between midnight and
centers, as well as the respiratory rate and depth. However, 9:00 a.m., in the late fall or winter, and involve infants two to
essentially normal respiratory cycles continue even if the brain four months old. Eyewitness accounts indicate that the sleeping
stem superior to the pons has been severely damaged. infant suddenly stops breathing, turns blue, and relaxes. Ge-
netic factors appear to be involved, but controversy remains as
In some cases, the inhibitory output of the pneumotaxic to the relative importance of other factors. The age at the time of
centers is cut off by a stroke or other damage to the brain stem, death corresponds with a period when the pacemaker complex
or sensory innervation from the lungs is eliminated due to and respiratory centers are establishing connections with other
damage to the vagus nerves. In these cases, the person inhales to portions of the brain. It has been suggested that SIDS results
maximum capacity and maintains that state for 10–20 seconds from a problem in the interconnection process that disrupts the
reflexive respiratory pattern.