Page 905 - Fundamentals of anatomy physiology
P. 905
892 Unit 5 Environmental Exchange
of conditions and activity levels. As blood flows toward the al- The DRG functions in every respiratory cycle, whether quiet
veolar capillaries, it is directed toward lobules with a relatively or forced. The DRG’s inspiratory center contains neurons that
high PO2. This movement takes place because alveolar capil- control lower motor neurons innervating the external intercos-
laries constrict when the local PO2 is low. (This response is the tal muscles and the diaphragm.
opposite of that seen in peripheral tissues, as we noted in
Chapter 21. p. 778) The VRG functions only during forced breathing. It has
an expiratory center consisting of neurons that innervate lower
Also in the lungs, smooth muscles in the walls of bronchi- motor neurons controlling accessory respiratory muscles in-
oles are sensitive to the PCO2 of the air they contain. When the volved in active exhalation. Its inspiratory center contains neu-
PCO2 goes up, the bronchioles increase in diameter (broncho- rons involved in maximal inhalation, such as gasping.
dilation). When the PCO2 declines, the bronchioles constrict
(bronchoconstriction). Airflow is therefore directed to lobules Reciprocal inhibition takes place between the neurons in-
with a high PCO2. These lobules get their carbon dioxide from volved with inhalation and exhalation. p. 481 When the
blood and are actively engaged in gas exchange. The response inspiratory neurons are active, the expiratory neurons are in-
of the bronchioles to PCO2 is especially important, because im- hibited, and vice versa. The pattern of interaction between these
provements in airflow to functional alveoli can at least partially groups differs between quiet breathing and forced breathing.
compensate for damage to pulmonary lobules. During quiet breathing (Figure 23–24a):
Local adjustments improve the efficiency of gas transport Activity in the DRG increases over a period of about 2 sec-
by directing blood flow to alveoli with low CO2 levels and
increasing airflow to alveoli with high CO2 levels. These adjust- onds, stimulating the inspiratory muscles. Over this period,
ments in alveolar blood flow and bronchiole diameter take inhalation takes place.
place automatically. When activity levels increase and the de-
mand for oxygen rises, the cardiac output and respiratory rates After 2 seconds, the DRG neurons become inactive. They
increase under neural control.
remain quiet for the next 3 seconds and allow the inspira-
The Respiratory Centers of the Brain tory muscles to relax. Over this period, passive exhalation
takes place.
23 Respiratory control has both involuntary and voluntary com-
ponents. Your brain’s involuntary centers regulate the activities During forced breathing (Figure 23–24b):
of the respiratory muscles. These centers control the respiratory
minute volume by adjusting the frequency and depth of pul- Increases in the level of activity in the DRG stimulate
monary ventilation. They make these adjustments in response
to sensory information arriving from your lungs and other neurons of the VRG that activate the accessory muscles in-
parts of the respiratory tract, as well as from a variety of other volved in inhalation.
sites.
After each inhalation, active exhalation takes place as the
The voluntary control of respiration reflects activity in the
cerebral cortex. This activity affects the output of either the re- neurons of the expiratory center stimulate the appropriate
spiratory centers in the medulla oblongata and pons or motor accessory muscles.
neurons in the spinal cord that control respiratory muscles.
The respiratory centers are three pairs of nuclei in the reticu- The basic pattern of respiration thus reflects a cyclic inter-
lar formation of the medulla oblongata and pons. The motor action between the DRG and the VRG. However, the timing of
neurons in the spinal cord are generally controlled by respira- this interaction is highly variable, and current research suggests
tory reflexes, but they can also be controlled voluntarily through that respiratory rate and rhythm cannot be attributed to a single
commands delivered by the corticospinal pathway. p. 550 area. The nuclei involved in respiratory control form a complex
network that includes (1) the rhythmicity centers, (2) centers in
Respiratory Centers in the Medulla Oblongata the pons that are modulated by peripheral and higher central
inputs, and (3) an area in the ventrolateral medulla, known as
We introduced the paired respiratory rhythmicity centers of the the central pattern generator or the pre-Bötzinger complex, that is
medulla oblongata in Chapter 14; these centers play a key essential to all forms of breathing. There are reciprocal path-
role in establishing the pace of respiration. p. 496 Each ways between all these nuclei and deciphering the commu-
respiratory rhythmicity center can be subdivided into a dorsal nication is extremely difficult and the patterns change with
respiratory group (DRG) and a ventral respiratory group (VRG). The different situations. Central nervous system stimulants, such as
DRG is chiefly concerned with inspiration whereas the VRG is amphetamines or even caffeine, increase the respiratory rate by
primarily associated with expiration. facilitating the respiratory centers. These actions are opposed by
CNS depressants, such as barbiturates or opiates.
Respiratory control is an area of intense study and our
understanding of the interactions and mechanisms remains
incomplete. Due to the brain stem locations, it is nearly impos-
sible to study this mechanism in humans, and much of the
work has been done in cats and newborn rodents.
