Control of Ventilation - Chemoreceptors Flashcards
What does stimulation ventilation require?
It requires the stimulation of the skeletal muscles of inspiration via the phrenic and intercostal nerves.
Where does ventilation control reside?
They reside in ill defined centres located in the pons and the medulla.
Is breathing subconscious?
Yes
What is breathing subject to?
Voluntary modulation
What happens when the spinal cord is severed above the phrenic nerve level (C3-5)?
Breathing ceases
What stimuli modulate the system of breathing in the respiratory system?
- Emotion (via limbic system in the brain)
- Voluntary overides (via higher centres of the brain)
- Mechano-receptors input from the thorax (stretch receptors)
- Chemical composition of the blood detected by chemoreceptors (PCO2, PO2 and pH)
What are the two groups of neurons that are in the respiratory centre?
Dorsal respiratory group of neurons - setting up stimulation of inspiratory muscles (diaphragm and external intercostal muscles)
Ventral Respiratory group - stimulates the muscles of the pharynx, larynx, tongue and the muscles of expiration.
Why is it really important for the ventral respiratory group to be active in stimulating a basal muscular tone during inspiration?
It helps to maintain a patent airways (an airway that is open)
What are the two types of chemoreceptors?
Central and peripheral
What are central chemoreceptors?
They are found in the central nervous system, in the medulla, they respond to hydrogen ions (directly reflects CO2), primary ventilatory drive.
- They detect changes in [H+] in the cerebospinal fluid around the brain
What are the peripheral chemoreceptors?
They are found in the carotid and aortic bodies, they respond to the change in partial pressure of oxygen and plasma [H+]
What happens when we get a rise in the [H+] of the cerebrospinal fluid?
Our rate and depth of breathing is increased.
What is hypercapnea?
An increase in the partial pressure of carbon dioxide in the systemic circulation.
Where are the H+ ions that the chemoreceptors are responding to derived from?
CO2 + H20 = H2CO3 = H+ and HCO3-
The H+ ions are derived from here.
When is ventilation inhibited?
When there is a decrease in the partial pressure of carbon dioxide in the blood which reduces the [H+] in the cerebrospinal fluid.
Does the central chemoreceptors respond to changes in plasma [H+]
No
What happens when the partial pressure of carbon dioxide increases?
Carbon dioxide crosses the blood-brain barrier not H+, increase in [H+] in the cerebrospinal fluid, which stimulates the central chemoreceptors which feeds to the respiratory centres and leads to an increase in cerebrospinal fluid.
What do most people rely on for stimulation of ventilation?
CO2 levels
What happens to the partial pressure of carbon dioxide in chronic lung disease and what happens as a result of it.
It increases and so the central chemoreceptors stop responding to increased levels of carbon dioxide and so they need to use their peripheral receptors.
What do peripheral chemoreceptors respond to?
Change in the partial pressure of oxygen and the changes in hydrogen ion concentration.
What is hypoxic drive?
They have their breathing pattern controlled by hypoxia.
What does a peripheral chemoreceptor cause?
They cause a reflex stimulation of ventilation following a significant fall in arterial PO2.
When do the peripheral chemoreceptors kick in?
When the partial pressure of oxygen falls below 60mmHg
What will happen to respiration rate of an anaemic patient with normal lung function, who has blood oxygen content half the normal value?
It will stay the same.
What do gaseous agents do?
They increase respiratory rate and decrease Tidal volume and thus alveolar ventilation.
What do opioids and barbiturates do?
They depress the respiratory centres, overdoses result in death as a rsult of respiratory failure, decrease sensitivity to ph, reduces response to PCO2, also reduce peripheral chemoreceptors response to Partial pressure of oxygen.
What does nitrous oxide do?
it is used as a common sedative, blunts the peripheral receptors response to falling alveolar partial pressure of Oxygen, very safe for most individuals, problematic in chronic lung disease where individual is on hypoxic drive. Administering this aggravates the situation.
What is the activity of the respiratory centres?
Fire repetitive short bursts of action potentials in DRG which travel to the inspiratory muscles. This sets an autonomic rhythm of breathing. They adjust their rhythm according to the stimuli
Which respiratory centre is located in the pons?
Pontine respiratory group - pneumotaxic area
What change in blood PCO2 does hyperventilation cause?
Reduces PCO2 - reduces CSF H+, Inhibits ventilation
Does the medulla respond to direct changes in the plasma H+?
No
What crosses the blood brain barrier, PCO2 or H+?
PCO2
What do the somatic motor neurones for inspiration and expiration innervate?
Inspiration - Scalene and sternocleidomastoid muscles, external intercostals, diaphragm. Expiration - Internal intercostals, Abdominal muscles
What allows us to have a large degree of voluntary control over breathing?
Descending neural pathways from cerebral cortex to respiratory motor neurones
Can cerebral coretex neural pathways override involuntary stimuli such as PCO2 or H+?
No
What are examples of voluntary control of ventilation?
Breath holding and hyper ventilation
What is the neurotransmitter that responds to low PO2 in the peripheral chemoreceptors?
DOPAMINE
What is the effect of anaesthetic agents?
Increases respiratory rate but decrease Tidal volume so decrease AV.
Describe respiration when swallowing?
Inhibited to avoid aspiration of food or fluids into the airways
Why is swallowing followed by an expiration?
In order that any particles are dislodged outwards from the region of the glottis.
