NEURAL AND CHEMICAL REGULATION OF
RESPIRATION
LEARNING OBJECTIVES
At the end of lecture the student should be able to know
• About control of breathing
• How blood gases affect the respiratory center of
brain?
• How respiratory center control blood gases and pH?
• About Nervous regulation (autonomic control).
– How brain stem regulates respiration
– The other stimuli that modify the respiratory
rhythm and the pathways that these signals take
to the brain stem.
– Voluntary control (cerebral cortex)
REGULATION OF RESPIRATION
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Control of ventilation (respiration)
– refers to the physiological mechanisms involved in
the control of breathing
– Gas exchange (exchange of oxygen & carbon
dioxide)
• primarily controls the rate of respiration
• is the most important function of breathing
– So, the control of respiration is based primarily on
how well this is achieved by the lungs
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REGULATION OF RESPIRATION BY CNS
Adjust the rate of alveolar ventilation according to the
demands of body
PO2 and PCO2 in the arterial blood hardly altered even
during respiratory distress
Lungs can maintain the pao2 and paco2 within the
normal range, even under widely varying conditions
by regulation from respiratory centre
Respiratory center
– Those areas of the brain that stimulate the
• Contraction of the diaphragm &
• intercostal muscles
REGULATION OF RESPIRATION BY CNS
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Breathing
– an involuntary , rhythmic act
• Rhythmicity produced by orderly discharge of
neurons supplying the respiratory muscles
• initiated from the respiratory areas present in
medulla oblongata and pons
neurons in the brainstem
– automatic control of unconscious breathing
neurons in the motor cortex of the cerebrum
– voluntary control
Respiratory Structures in
Brainstem
RESPIRATORY CONTROL
SYSTEM IN CNS
There are four components
to this control system:
1- Control centers for breathing in the brain stem
(medulla oblongata and pons)
2- Chemoreceptor for O2 and CO2
3- Mechanoreceptor in the lungs and joints
4- The respiratory muscles, whose activity is directed by
the brain stem centers
BRAIN STEM CONTROL OF BREATHING
• Controlled by the medulla and pons of the brain stem
• three groups of neurons or
brain stem centers
– The medullary rhythmicity
center
– The apneustic center
– The Pneumotaxic center
Function
– Control Frequency of normal, involuntary
breathing
MEDULLARY RESPIRATORY CENTER
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Located in the reticular formation of brain
Composed of two groups of neurons distinguished by
their anatomic location:
– The inspiratory (I) neurons (dorsal respiratory
group)
– The expiratory (E) neurons (ventral respiratory
group).
Function
– Control the basic rhythm of respiration
INPUT TO INSPIRATORY CENTER
•Receives sensory input mainly from:
• Peripheral chemoreceptor via
Glossopharyngeal nerve and vagus nerve
• Mechanoreceptors in the lungs via vagus nerve.
Also receive information from:
–Stretch receptors in the lungs via cranial nerve x.
–From peripheral chemoreceptor in the region where
cranial nerve ix and x leave the brainstem.
– From ph and pco2 receptors in the bone-duraarachnoid CSF space.
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EFFERENTS FROM INSPIRATORY CENTER
Efferent fibers go to spinal cord synapse with lower
motor neuron in cervical and thoracic region
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Efferents from spinal cord
– phrenic nerves to the diaphragm
– intercostal nerves to the intercostal muscles.
Final Response:
contraction of diaphragm and intercostal muscle
Inspiration occurs.
FUNCTION OF INSPIRATORY CENTER
• Establish the basic rhythm of breathing
• During quiet breathing
– inspiration last for 2
seconds
– Expiration last for 3
sec.
• Also called central control
pattern generator b/c it
plays most fundamental
role in control of
respiration
EXPIRATORY CENTER
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Located in the ventral respiratory neurons
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Responsible primarily for expiration
Long column of neurons located in the nucleus
ambigus rostrally and retroambiguus caudally
Consists of two types of neurons:
– I neurons in its mid region
– E neurons at its rostral and caudal ends
These neurons are inactive during quiet breathing.
Expiration center is activated when activity and
requirements are increased e.g. Exercise
APNEUSTIC CENTER
Location:
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Reticular formation center located in the lower pons
Function:
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Causes apnea (cessation of breathing).
Coordinates the transition b/w inhalation and
exhalation
Facilitates inspiration.
Send signals to the dorsal respiratory group of
neurons to prevent the switch off inspiratory ramp
signals.
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Stimulation of this center increased the duration of
inspiration this result in a deeper and more
prolonged inspiratory effort
The rate of respiration becomes slow and depth of
respiration is increased.
PNEUMOTAXIC CENTER
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Located in the upper pons
Sends inhibitory impulses to the inspiratory area to
turn off inspiration,
Limiting the burst of action potentials in the phrenic
nerve.
It limits the size of tidal volume
It regulates the respiratory rate.
Area controls the other two centers
Primarily to limit the inspiration
This has secondary effect of increasing the rate of
breathing b/c limitation of inspiration also shorten
the expiration and the entire period of respiration.
A strong signal increases the rate of breathing 30 –
40 breaths /min and weak signals reduce the rate to
any few breaths /min
FUNCTION OF PONTINE CENTERS
 Input to medulla from Pons
 Apneustic Center
prevents inspiratory neurons from being switched off,
thus lengthening inspiration
 Pneumotaxic Center
switches off inspiratory neurons, thus shortening
inspiration
OTHER RECEPTORS
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Several other receptors are involved in the control of
breathing including:
Lung stretch receptors
Joint and muscle receptors
Irritant receptors
Juxtacapillary receptors
OTHER FACTORS MODIFYING RESPIRATION
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Nerve impulses from the hypothalamus and limbic
system allow pain and emotions to affect respiration
.e.g. in gasping, laughing and crying.
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Anxiety often triggers an uncontrollable bout of the
hyperventilation
CHEMICAL REGULATION RESPIRATION
The respiratory system functions to maintain proper levels of
co2 and o2 and is very responsive to changes in the levels of
these gases in body fluids
CHEMORECEPTOR
Sensory neurons responsive to chemicals,.
monitor levels of CO2, H+ and O2 and provide input to the respiratory
center.
adjust pulmonary ventilation to keep these variables within homeostatic
limit
Present In 2 locations:
1 - Central chemoreceptor
2 - Peripheral chemoreceptor
TYPES OF CHEMORECEPTORS
Central chemoreceptor
 In central nervous system, located on the ventrolateral medullary
surface are sensitive to the pH of their environment.
Peripheral chemoreceptor
In carotid artery and aorta act most importantly to detect variation of the
oxygen in the arterial blood in addition monitor PCO2 and pH
CENTRAL CHEMORECEPTORS
Located in the brain stem
On the ventral surface of medulla
Near the point of exit of the glossopharyngeal and vagus nerves and only
a short distance from the medullary inspiratory center.
Communicate directly with the inspiratory center.
Respond to changes in h+ conc. Or PCO2 , or both in CSF.
Exquisitely sensitive to changes in the ph of CSF .
STIMULI FOR CENTRAL CHEMORECEPTORS
Goal of central chemoreceptors is to keep arterial PCO2 normal
Blood CO2 has little direct effect on central chemoreceptors
Main stimulus is H+ ions in CSF not blood H ion
Blood brain barrier does not permit blood H ion
CO2 in blood passes through blood brain barrier combines with water of
CSF to form H2CO3
Carbonic acid in CSF disassociate into H ion and HCO3
This H ion stimulate respiratory center
FUNCTION OF CENTRAL CHEMORECEPTOR
Increases in arterial PCO2
 Produce increase in PCO2 in the brain and the CSF,
 Ph of the CSF decreases
 Stimulate respiratory center, rate of breathing increases, more CO2 is
blown off, PCO2 will return to normal
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Decrease in the pH of CSF produce increase in breathing rate and
increase in pH produce decrease in breathing rate
PERIPHERAL CHEMORECEPTORS
Located outside the brain
Senses changes in O2, CO2 and pH
Location
Carotid bifurcation carotid body
Arch of aorta aortic bodies
May be present in thoracic and abdominal region along arteries
PERIPHERAL CHEMORECEPTORS
Mainly detect changes in arterial O2
Also respond to a lesser extent to changes in CO2 and pH
Information perceived by chemoreceptors in turn transmitted to respiratory
centers to regulate respiratory activity
STRUCTUREOF PERIPHERAL CHEMO RECEPTORS
Carotid bodies
Located bilaterally in bifurcation of common carotid arteries
Sensations pass through HERING’S nerve to glossopharyngeal nerve then
to DRG of medulla
Aortic bodies
Located along arch of aorta
Information travel to medullary DRG through VAGUS nerve
Each chemoreceptor receives special blood supply.
STIMULUS FOR PERIPHERAL CHEMORECEPTORS
Decrease in arterial PO2: the most common responsibility of peripheral
chemoreceptors is to detect changes in arterial PO2.
However PC are relatively insensitive to changes in PO2.
They respond when PO2 decreases to less than 60mmHg
DECREASE IN ARTERIAL PO2
If arterial PO2 is b/w 100 and 60mmhg, the breathing rate is relatively
constant.
However, if arterial po2 is less than 60mmhg, the breathing rate increases
in a very steep and linear fashion.
In this range of po2 pc are very sensitive to o2 and they respond so rapidly
that the firing rate of the sensory neurons may change during a single
breathing cycle
INCREASE IN ARTERIAL PCO2
The peripheral chemoreceptor also detect increases in PCO2 but the effect
is less important than their response to decrease in PO2.
Detection of changes in PCO2 by PC also is less important than detection
of changes in PCO2 by central chemoreceptors
DECREASE IN ARTERIAL PH
Decrease in arterial pH cause an increase in ventilation, mediated by
peripheral chemoreceptor for H+.
effect is independent of changes in the arterial PCO2
is mediated only by chemoreceptor in the carotid bodies (not by those in
aortic bodies).
in metabolic acidosis, there is decreased arterial pH, the peripheral
chemoreceptor are stimulated directly to increase the ventilation rate (the
respiratory compensation for metabolic acidosis).
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