Control of Ventilation Flashcards
Where does ventilation control reside?
Within ill-defined centres located in the pons and medulla (respiratory centres)
What muscles are stimulated on inspiration?
Diaphragm and intercostal muscles
What stimulates the diaphragm?
Phrenic nerve
What stimulates the intercostal muscles?
Intercostal nerves
What can you say about the conscious level required to breath?
In subconscious but can be voluntaraly modulated
What levels of the spinal cord does the phrenic nerve originate?
C3 - C5
Where must the spinal cord be severed for breathing to stop?
Above the origin of the phrenic nerve (C3 - C5)
What are the functions of the respiratory centres?
Set an automatic rhythm of breathing through co-ordinating the firing of smooth and repetative bursts of action potentials in dorsal respiratory groups (DRG) which travel to inspiratory muscles
Adjust the rhythm in response to stimuli
What do respiratory centres have their rhythm modulated by?
Emotion (via limbic system of the brain)
Volunrary over ride (via higher centres in the brain)
Mechani-sensory input from thorax (such as stretch reflex)
Chemical composition of the blood detect by chemoreceptors (PCO2, PO2 and ph)
What input has the most inpact on modulation of the respiratory centres rhythm?
Chemoreceptors
What does DRG stand for?
Dorsal respiratory group of neurons
What is VRG stand for?
Ventral respiratory group of neurons
What do the dorsal respiratory group of neurons innervate?
Inspiratory muscles via the phrenic and intercostal nerves
What does the ventral respiratory group of neurons innervate?
Tongue
Pharnx
Larynx
Expiratory muscles
What are the 2 kinds of chemoreceptors?
Central
Peripheral
Where are central chemoreceptors?
Medulla
What do central chemoreceptors respond to?
Directly to H+ (directly reflects PCO2)
Where are peripheral chemoreceptors?
Carotid and aortic bodies
What do peripheral chemoreceptors respond to?
Plasma [H+] (less so to PCO2)
What is the primary ventilation drive?
Central chemoreceptors
What is the secondary ventilation drive?
Peripheral chemoreceptors
What do central chemoreceptors detect?
Changes in [H+] in the CSF around the brain
How do central chemoreceptors respond to changes in [H+] in the CSF around the brain?
Reflex stimulation of ventilation is increase in [H+)
When do central chemoreceptors cause ventilation as a reflex?
When they detect an increase in [H+]
When do central chemoreceptors inhibit ventilation?
When [H+] drops
What is hypercapnia?
Abnormally elevated levels of carbon dioxide in the blood
What is abnormally elevated levels of carbon dioxide in the blood called?
Hypercapnia
What is the process of central chemoreceptor altering ventilation?
1) Arterial PCO2 increases and carbon dioxide crosses the blood brain barrier (not H+)
2) Bicarbonate and H+ are formed and receptors respond to H+
3) Feedback via the respiratory centres increases ventilation in response to increased arterial PCO2
4) Decreased arterial PCO2 slows ventilation rate
5) This means that the central chemoreceptors monitor the PCO2 indirectly in the cerebrospinal fluid
What cross the blood brain barrier before acting of central receptors?
Carbon dioxide, not H+, which is formed once carbon dioxide is broken down in the CSF
What do peripheral chemoreceptors detect changes in?
PO2 and [H+]
What is the process of peripheral chemoreceptors altering ventilation?
1) Arterial PO2 is low so no oxygen combines with oxygen sensor
2) K+ channel closes so the cell depolarises
3) Exocytosis of dopamine containing vesicles
4) Act on dopamine receptors on sensory neuron
5) Action potential generated that signals medullary centres to increase ventilation
When do peripheral chemoreceptors cause a reflex stimulation of ventilation?
Fall in arterial PO2 or rise in [H+]
What causes an increase of [H+] in the blood?
Rise in arterial PCO2
CO2 + H2O ↔ H2CO3 ↔ HCO3 + H+
What do peripheral chemoreceptors not respond to?
Oxygen content, they respond to PO2
When would peripheral chemoreceptor stimulate ventilation?
Plasma pH falls ([H+] increases)
During acidosis
When would peripheral chemoreceptor inhibit ventilation?
When pH rises ([H+] decreases)
During alkalosis or vomiting
What can voluntary descending neural pathways from the cerebral cortex not overide?
Involuntary stimuli such as arterial PCO2 or [H+]
Why is respiration inhibited during swollowing?
Avoid aspiration of foods or fluids into the airways, followed by an expiration in order than any particles are dislodged outwards from the region of the glottis
What are common drugs that affect respiratory centres?
Barbiturates and opioids
Gaseous anaesthetic agents
Nitrous oxide
How do barbiturates and opioids affect respiratory centres?
Depress respiratory centres, overose often results in death as a result of respiratory failure
How do gaseous anaesthetic agents affect respiratory rate?
Increase respiratory rate, but decreases tidal volume
Hows does nitrous oxide affect respiratory centres?
Blunts peripheral chemoreceptor response to falling PaO2
How safe is using nitrous oxide on respiratory centres?
Very safe in most people
Problematic in chronic lung disease cases where individual often on hypoxic drive and administering O2 to these patients aggravates situation
What drugs depress respiratory centres?
Barbiturates and opioids
