Respiratory physiology lecture 5 Dr. Amjed H. Abbas
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Nervous and Chemical Control of Respiration
Nervous Control
Inspiratory muscles, diaphragm and intercostal muscles, composed of skeletal
muscle and must be stimulated to contract, two phrenic nerves responsible for
diaphragm contraction originate at the 3
rd
, 4
th
, and 5
th
cervical spinal nerves, 11
pairs of intercostal nerves originate 1- 11
th
thoracic spinal nerves.
Respiratory Areas in Brainstem
These centers are responsible for automatic basic rhythm of respiration, located
bilaterally in the reticular formation of the brain stem (which consists from
medulla oblongata, pons and midbrain).
The primary portions of the brainstem that control ventilation are the medulla
oblongata and the pons.
A. Medullary respiratory center: consists of dorsal groups which stimulate the
diaphragm (inspiratory center) and ventral groups (expiratory center) which
stimulate the intercostal and abdominal muscles.
B. Pontine respiratory group
It is involved with switching between inspiration and expiration, it consists of
pneumotaxic and apneustic centers.
Respiratory physiology lecture 5 Dr. Amjed H. Abbas
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Medullary respiratory centers:
The medulla oblongata is the primary respiratory control center. Its main function
is to send signals to the muscles that control respiration to cause breathing to
occur. There are two regions in the medulla that control respiration:
The dorsal respiratory group stimulates inspiratory movements.
The ventral respiratory group stimulates expiratory movements.
Respiratory physiology lecture 5 Dr. Amjed H. Abbas
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The medulla also controls the reflexes for non-respiratory air movements, such as
coughing and sneezing reflexes.
Inspiratory Center or Dorsal Respiratory Group (DRG)
- Basic rhythmic breathing, it sends signal to the Phrenic nerve ----> Intercostal
nerves ---> Diaphragm + external inter-costals
it containing inspiratory neurons
It sets the basic respiratory rate, stimulates the inspiratory muscles to contract
(diaphragm). The signals it sends for inspiration start weakly and steadily
increase for ~ 2 sec. This is called a ramp and produces a gradual inspiration.
The ramp then stops abruptly for ~ 3 sec and the diaphragm relaxes.
Ventral respiratory group (VRG): The neurons in the VRG remain almost
inactive during normal quiet respiration. There is no evidence that VRG
participates in the basic rhythmical oscillation that controls respiration.
When the respiratory drive for increased pulmonary ventilation becomes more
than normal as in exercise, respiratory signals spill over into VRG from the basic
Respiratory physiology lecture 5 Dr. Amjed H. Abbas
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oscillatory mechanisms of the DRG area. Then the VRG contribute to the
respiratory drive. VRG area is very important in providing powerful expiratory
signals to abdominal muscles during expiration. The VRG area operates as an
overdrive mechanism when high levels of pulmonary ventilation are required.
Pontine respiratory groups
The pons is the other respiratory center and is located above the medulla. Its main
function is to control the rate or speed of involuntary respiration. It has two main
functional regions that perform this role:
1.Pneumotaxic center
It is located in upper part of the pons, slightly inhibits medulla, it has inhibitory
effect on inspiration causes shorter, shallower, quicker breaths.
The pnuemotaxic center sends signals to inhibit inspiration that allows it to finely
control the respiratory rate. Its signals limit the activity of the phrenic nerve and
inhibits the signals of the apneustic center. It decreases tidal volume.
when activity of inspiratory center stops, inhibitory impulses cease from
pneumotaxic center and inspiratory impulses initiated.
2.Apneustic center
It is located in lower portion of pons, stimulates the medulla, causes longer,
deeper, slower breaths (prevent switch off), it has stimulatory effect on
inspiratory center and inhibitory on expiratory center. Its activity is modulated on
and off by pneumotaxic center. It is intermittently inhibited by vagal discharge
arise from lung which appear during inflation of the lung and disappear during
deflation of the lung.
Respiratory physiology lecture 5 Dr. Amjed H. Abbas
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The apneustic and pnuemotaxic centers work against each other together to
control the respiratory rate.
Mechanoreceptors: These receptors are placed in the walls of bronchi and
bronchioles of the lung and the main function of these receptors is to prevent the
over-inflation of the lungs. Inflation of the lungs activates these receptors and
activation of the stretch receptors in turn inhibits the neurons in inspiratory center
via vagus nerve.
When the expiration starts activation of the stretch receptors gradually ceases
allowing neurons in the inspiratory neurons become active again. This
phenomenon is called Hering-Breuer Reflex. It is particularly important for
infants. In adults it is functional only during exercise when the tidal volume is
larger than normal.