Ventilation- Neural Control of Breathing Flashcards
Describe the neuronal firing during breathing?
The activity of inspiratory neurons increases steadily, apparently through a positive feedback mechanism
At the end of inspiration, the activity shuts off abruptly and expiration takes place through recoil of elastic lung tissue
Where is respiratory rhythm generated?
Medulla
What happens if the brainstem is cut above the level of the pons?
Basic rhythm continues
What happens id all the afferent nerves to the brainstem are cut?
Basic rhythm continues
What happens if you section the spinal cord below C3-C5?
Intercostal muscles are paralysed
What is respiration regulated by?
Nervous or neural
Chemical
Where does breathing originate?
Brainstem
Medulla
Pons
Where are respiratory centres located?
Medulla oblongata
Pons
What is the function of respiratory centres?
Collect sensory information about the levels of oxygen and carbon dioxide in blood
What Is the overall arching function of respiratory centres collectively?
Matching the changes in physical properties of the lungs with the correct breathing pattern
What are the two medullary centres?
Inspiratory
Expiratory
Where is the inspiratory centre and what is its function?
Located in upper part of medulla oblongata
Function is concerned with inspiration- exclusively inspiratory neurons
Control external intercostal muscles and diaphragm
Where is the expiratory centre?
Situated in medulla oblongata, anterior and lateral to the inspiratory centre
Also called ventral respiratory group
Controls internal intercostal muscles
What is the function of the expiratory centre?
Centre is inactive during quiet breathing and when inspiratory centre is active, but during forced breathing or when inspiratory centre is inhibited it becomes active
What are the two pontine centres?
Pneumotaxic centre
Apnuestic
Where is the pneumotaxic centre?
Situated in upper pons
What is the function of pneumotaxic centre?
Controls medullary respiratory centres, especially the inspiratory centre through the apneustic centre
It influences duration of inspiration
What is the function of the apnuestic centre?
Increases depth of inspiration by acting on inspiratory centre
Where is the apnuestic centre?
Lower pons
What do afferent pathways deliver?
Impulses via vagus and glossopharyngeal nerves
What do efferent pathways deliver?
Signals that drive inspiration and expiration
Where do afferent pathways get their signals from?
Thoracic region and lungs
Also chemoreceptors
Describe the flow of signals in the efferent pathway
Nerves from respiratory centre leave brain in anterior part of lateral column in spinal cord
Terminate in motor neurons in cervical and thoracic segments of spinal cord
Supply phrenic nerve that controls diaphragm
Supply fibres for intercostal muscles
What ten things can affect respiratory centres?
Impulses from higher centres
Stretch receptors of lung slowly adapting pulmonary receptors- Hering-Breur Reflex
“J” Receptors- pulmonary C-fibres
Irritant receptors of lungs
Proprioceptors
Thermoreceptors
Pain receptors
Cough reflex
Sneezing reflex
Deglutition relfex
What higher centres can override respiratory centres?
Limbic system
Hypothalamus
Describe the Hering-Breur Reflex
Smooth muscle of upper airways has slowly adap1ng stretch receptors. When lung is inflated these neurones send impulses to DRG via the vagus nerve. This input is inhibitory, limi1ng inspira1on, prevents overinfla1on of lungs
How does “J” Receptors- pulmonary C-fibres affect respiratory centres?
These are juxtacapiilary receptors present in the all of alveoli
Once stimulated by conditions such as pulmonary oedema, congestion pneumonia and histamine, then induce apnea
Temporary suspension of breathing- followed by rapid shallow breathing
How do irritant receptors of lungs override the respiratory centres?
Powerfully simulated by inhalation of irritants
Found on walls of bronchi and bronchioles
Induces rapid shallow breathing from shortening of expiration
Then long deep augmented breaths, which are taken every 5-20 mins to reverse slow collapse of lungs
How do proprioceptors change breathing?
Aim is for reflexes form muscles and joints to stabilize ventilation in the face of changing mechanical conditions
Where are proprioceptors found in regards to the respiratory system?
In joints to measure velocity of ribs
Tendons to detect strength of muscle contraction
Muscle spindles to monitor length of fibres
What are the function of thermoreceptors?
Cutaneous
Supply signals to cerebral cortex, stimulates respiratory centre, hyperventilation
What is the function of pain receptors?
Supply signals to cerebral cortex, stimulating respiratory centres and inducing hyperventilation
How does the cough reflex change ventilation?
Irritation of parts beyond the nose stimulates vagus nerve and cough is induced
Deep inspiration followed by forceful expiration with closed glottis
Glottis opens and explosive outflow of air at high velocity
How does the sneezing reflex change ventilation?
Irritation of nasal mucous membranes causes a deep inspiration followed by forceful expiration with opened glottis
How does the deglutition reflex change breathing?
Respiration arrested during swallowing of food
Describe how you can voluntarily control your breathing
The cortex and by pass the medulla and affect lower motor neurones directly
During this voluntary control there are signals coming from the cerebral cortex to medulla, influencing basic pattern generation by the DRG
What do chemoreceptors respond to?
Changes in chemical constituents of blood or CSF
What chemical changes can chemoreceptors respond to?
Hypoxia
Hypercapnia
Increased hydrogen concentration
What are the two types of chemoreceptors?
Central chemoreceptors
Peripheral chemoreceptors
What is hypercapnia?
Elevated carbon dioxide in blood
Where are the central chemoreceptors located?
Medulla oblongata close to DRG
What is the action of central chemoreceptors?
Sensitive to increase in hydrogen concentration in CSF
Sensitive to increase in PaCO2 in blood
How does central chemoreceptors detect hydrogen ions in the CSF when hydrogen can’t pass across the BBB barrier?
C02 crosses into CSF and forms carbonic acid
This is unstable and dissociates into hydrogen and bicarbonate
Hydrogen then stimulates central chemoreceptors
Where are peripheral chemoreceptors?
Close in location but distinct from baroreceptors
What are peripheral chemoreceptors?
Specialised receptor cells that are stimulated primary by a decrease in PO2 and an increase in hydrogen ions
What do peripheral chemoreceptors synapse on?
With afferent nerves which run to the brainstem
Sensory portion of cranial nerve 10 from aortic bodies
Cranial nerve 9 from carotid bodies
Which peripheral chemoreceptor is more important in respiration?
Carotid
What are peripheral chemoreceptors sensitive to?
PaO2
PaCO2
pH
Blood flow
Temperature
10x less sensitive than central receptors
Where are peripheral chemoreceptors?
Carotid sinus and aortic arch
What are peripheral chemoreceptors also known as?
Glomus Cells
Describe the pathway in chemoreceptors?
When partial oxygen is high the potassium channel is open
When partial oxygen falls the potassium channel closes and causes depolarization
The calcium channels open and stimulates a response in sensory afferents
What nerve innervates the carotid bodies?
Vagus nerve
What nerve innervates the carotid bodies?
Glossopharyngeal nerve
What gas measurement is the most important in respiration?
PCO2
What is normal PCO2?
40mmHg or 5.3 kPa
What happens if there is an increase in PCO2?
CO2 crosses the BBB so as it rises the pH of the CSF decreases
That has excitory input to DRG in medulla and resulting in increased ventilation
As PCO2 returns to normal, pH returns to normal and stimulus for respiration is reduced
How can decreased CO2 during hyperventilation signal a decrease in breathing rate?
At normal PCO2 there is a resting discharge action potential from chemoreceptors
If PCO2 falls the firing rate also falls so there is a corresponding decrease in excitatory input to DRG, with the result of respiration being inhibited
How much must oxygen pressure levels drop in the blood for peripheral chemoreceptors to stimulate ventilation?
From 100mmHg to 60mmHg or 8kPA
Why can’t you put COPD patients on 100% oxygen masks?
Over a long time the central and peripheral chemoreceptors become insensitive to PCO2
As a result the patient relies on a decline in PO2 to stimulate breathing- hypoxic drive
If you put a patient on 100% oxygen the breathing will stop as the peripheral chemoreceptors will fall and depress ventilatory drive
How does pH affect ventilation?
If the PO2 and PCO2 are normal peripheral chemoreceptors will detect a lower pH
This increases the breathing rate to drive CO2 out of the body
How does damage to the CNS affect breathing?
Cause partial or total loss of respiratory function
Vasoconstriction, hypertension, mucus secretion and oedema and result from uncontrolled activity
Strokes can also interfere with the dorsal medullary centres and cause fatal apnoea
How does poliomyelitis affect breathing?
25% of patients require mechanical ventilation during acute phase
Reinnervation of fibres can recover respiratory strength
How does diptheria affect breathing?
Demyelinating neuropathy that can lead to respiratory failure
How does botulism affect breathing?
Innervation of respiratory muscles seems particularly vulnerable, ventilation may be required for extended period
How does DMD affect breathing?
From age 10 vital capacities declines
Nocturnal hypoxaemia develops first
Common cause of death is respiratory failure secondary to pulmonary infection
How does the CSF differ as a buffer to plasma (blood)
Plasma is a better buffer meaning rises in CO2 will have a greater impact on pH in the CSF
what is central chemoreceptors not sensitive to?
PaO2
