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CONTROL OF RESPIRATION
Lecture - 7
Dr. Zahoor Ali Shaikh
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Control Of Respiration
• Respiratory process is involuntary process, but
under voluntary control as we can stop
breathing.
• Respiratory center is in the brain stem. It causes
rhythmic breathing pattern of inspiration and
expiration.
• Inspiratory and Expiratory muscles are skeletal
muscles and contract only when stimulated by
their nerve supply.
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Neural Control Of Respiration
We will discuss
1. Center that generate inspiration and
expiration.
2. Factors that regulate rate and depth of
respiration .
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Respiratory Center
 In Medulla
- Inspiratory center
- Expiratory center
These are neuronal cells that provide output to
respiratory muscles for inspiration and expiration.
 In Pons
- Pneumotaxic center – upper pons
- Apneustic center – lower pons
Pontine Center influence the output from medullary
centers.
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Respiratory Center
• Inspiratory and Expiratory neurons in the
medullary center.
• We are breathing rhythmically in and out during
quiet breathing because of alternate contraction
and relaxation of inspiratory muscles
[diaphragm and External-intercostal muscles]
supplied by phrenic nerve [C-3,4,5] and
intercostal nerves .
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Respiratory Center
• Order comes from medullary center to spinal
cord motor neuron cell bodies [anterior horn
cells].
• When these motor neurons are activated, they
stimulate the inspiratory muscles leading to
inspiration.
• When these neurons are not firing, the
inspiratory muscles relax and expiration takes
place.
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Respiratory Center
Medullary respiratory center
• It has two neuronal groups:
1. Dorsal Respiratory Group [DRG] –
Inspiratory neuron.
2. Ventral Respiratory Group [VRG] –
Expiratory neuron.
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Respiratory Center
 Dorsal Respiratory Group [DRG]
• It consist of mostly inspiratory neurons, when DRG
fire, inspiration takes place, when they stop firing,
expiration takes place.
• DRG has important connection with VRG.
 Ventral Respiratory Group [VRG]
• It is composed of both inspiratory and expiratory
neurons.
• VRG remain inactive during normal quiet breathing.
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Respiratory Center
• VRG plays role during forceful breathing that is
during active expiration [remember normal
expiration is passive].
• Only during active expiration, expiratory neuron
fire from VRG and stimulate expiratory muscles.
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Respiratory Center
Generation of respiratory rhythm
• Before it was thought that DRG generates the
respiratory rhythm.
• Now it is believed that rhythm is generated by
Pre – Botzinger Complex. It displays pacemaker activity causing self induced action
potential.
• It is located near the respiratory center.
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Respiratory Center
Pontine Center
‘Pneumotaxic & Apneustic Centers’
 Pneumotaxic Center [Upper pons]
• It sends message to DRG neurons to stop inspiration, so
that expiration can take place.
 Apneustic center [Lower pons]
• It causes deep inspiration when Pneumotaxic center is
damaged, Apneusis occurs [Deep Inspiration] as
Apneustic center is free to act in absence of Pneumotaxic
center.
• Apneusis is seen in brain damage [Pneumotaxic center
damage].
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Respiratory Center
‘Summary’
• Inspiratory center [DRG] – Inspiration
• Expiratory center [VRG] – used during forced
Expiration
• Pneumotaxic center – acts on inspiratory center
to stop inspiration therefore regulates
inspiration and expiration.
• Apneustic center – causes Apneusis [deep
inspiration] when Pneumotaxic center is
damaged.
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Hering – Breuer Reflex
• When tidal volume is large, more than 1 liter e.g.
during exercise, then Hering Breuer Reflex is
triggered to prevent over inflation of the lungs.
How ?
• There are pulmonary receptors in the lungs, they are
stretched by large tidal volume.
• Action Potential from stretched receptor go via
afferent X cranial nerve ( vagus ) to medullary
center and inhibit inspiratory neuron.
• This negative feedback mechanism helps to cut
inspiration before lungs are over inflated.
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Chemical Control Of Breathing
• Chemical factors which affect the ventilation are
-PO2
-P
-H+ ion
• Their effect is mediated via respiratory
chemoreceptor.
• We will study chemoreceptors first .
CO2
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CHEMORECEPTORS
• There are two types of Chemoreceptors
1. Peripheral Chemoreceptors
2. Central Chemoreceptors
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Peripheral Chemoreceptors
Peripheral Chemoreceptors are Carotid bodies &
Aortic bodies.
Carotid Bodies
Carotid body is present near the carotid artery
bifurcation on each side.
They contain cells which can sense the level of PO2,
P , H+ ion.
CO2
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Peripheral Chemoreceptors
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•
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Carotid bodies [cont]
Carotid body sends impulse to respiratory center
in medulla via IX cranial nerve
[glassophyrangeal].
Aortic bodies
These receptors are situated in the aortic arch .
They also sense the O2, CO2, and H+ ion changes
in the blood.
Aortic body sends impulse to respiratory center
in medulla via X cranial nerve [vagus].
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Central Chemoreceptors
• They are located in the medulla near the respiratory
center .
• These central chemoreceptors monitor the effect of
PO2, P , and H+ ion.
• This H+ ion is generated by CO2 in the Extra
Cellular Fluid [ECF] of the brain which surrounds
the central chemoreceptors.
• When CO2 increases, we get:
CO2 + H2O  H+ + HCO3• Increased H+ directly stimulates the central
chemoreceptors.
CO2
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Effect of PO , P , and H+ ion On
Peripheral & Central Chemoreceptors
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CO2
Effect On Peripheral Chemoreceptors
• Decreased PO in the arterial blood – stimulates
peripheral chemoreceptors when arterial PO
falls below 60mm Hg (strong effect).
• Increased PCO2 in the arterial blood – weakly
stimulates peripheral chemoreceptors.
• Increased H+ ion in the arterial blood –
stimulates peripheral chemoreceptors.
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Effect of PO , P , and H+ ion On
Peripheral & Central Chemoreceptors
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CO2
Effect On Central Chemoreceptors
• Decreased PO in the arterial blood – depresses
the central chemoreceptors when arterial PO
falls below 60mm Hg.
• Increased PCO2 in the arterial blood and
[increased H+ in the brain ECF] – strongly
stimulates central chemoreceptors. It is
dominant control of ventilation.
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Effect of PO , P , and H+ ion On
Peripheral & Central Chemoreceptors
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CO2
IMPORTANT - PCO2 level more than 70 80mmHg directly depresses the central
chemoreceptors and respiratory center.
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‘Summary’
• Decreased PO2, increased PCO2, increased H+ ion
concentration in arterial blood stimulates Peripheral
Chemoreceptors. Most important stimulating factor
is decreased PO2 on peripheral chemoreceptors.
• Increased PCO2 in the arterial blood and increased H+
ion in the brain ECF strongly stimulates the central
chemoreceptors and dominant control of
ventilation.
-Decreased PO in the arterial blood – depresses the
central chemoreceptors.
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What Happens When We Hold The
Breath Voluntarily?
• When we hold breath, there is increased CO2 and
increased H+ ion in the ECF of brain.
• It stimulates the central chemoreceptors – which
stimulates respiratory center in medulla,
therefore, we have to break the breath.
• During this period of holding, PO2 does not fall
below 60mmHg to cause stimulation of
peripheral chemoreceptors, therefore, it is
central effect.
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What You Should Know From This
Lecture
• Neural Control of Respiration
• Name of Respiratory centers in the Brain stem
• Role of Inspiratory, Expiratory, Pneumotaxic and
Apneustic centers in control of breathing
• Pre-Botzinger complex [Pace-maker for respiration]
• Hering Breuer Reflex
• Chemical Control of Breathing
• Peripheral Chemoreceptors
• Central Chemoreceptors
• Effect of decreased PO2, increased PCO2 and H+ on
Peripheral and Central Chemoreceptors
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Thank you