Flashcards in Acid-Base in the Control of Ventilation Deck (36)
What are the three components that are involved in the control of ventilation?
A central controller, sensors and effeects
What three types of receptors does the respiratory control system have?
Central and peripheral chemoreceptors and peripheral mechanoreceptors
What do central chemoreceptors detect?
CO2, specifically hypercapnia in the CSF they bathe in (within the medulla oblongata) as a result of increased acidity of the CSF
Where are the central chemoreceptors, involved in control of ventilation, found?
Specialised neurones located neurones in the ventral surface of the medulla oblongata
What is the normal pH of CSF?
Why does the CSF have a far weaker buffering capacity than the blood?
Because no plasma proteins are filtered through the choroid plexus, the CSF contains almost no protein
What stimulates most of the urge to breathe?
Drive from the central chemoreceptors in the medulla oblongata
Describe the role of the central chemoreceptors in the control of ventilation
Specialised neurones in medulla oblongata, which detect hypercapnia (as a result of increased acidity in the CSF) --> activates chemoreceptors --> stimulate neurones in the respiratory centres in the medulla --> increased ventilation rate
How can central chemoreceptors detect hypercapnia due to an increased acidity?
Carbon dioxide diffuses freely through the BBB into the CSF where it reacts with water to form carbonic acid which is then converted to protons and bicarbonate via carbonic anhydrase.
Why do central chemoreceptors become desensitised in chronic pulmonary disease?
There are chronically high CO2 levels in the blood, and therefore the central chemoreceptors gradually become less sensitive to this stimuli and therefore the drive for ventilation from CO2 (hypercapnia) is diminished
Where are the peripheral chemoreceptors, involved in control of ventilation, found?
In the carotid bodies at the bifurcation of the common carotid arteries and in the aortic bodies above and below the aortic arch.
What does the carotid body contain?
Most heavily vascularised tissue in the human body
What do peripheral chemoreceptors detect?
Mainly to reduced pO2 and plasma pH (acidity caused by hypercapnia) and less so to raised pCO2 directly
Describe the afferents from the peripheral chemoreceptors
Afferents from the chemoreceptors travel mainly in the glossopharyngeal and vagus nerves to the brainstem
What cells make up the carotid body?
What are the two types of cell that make up the carotid body?
Glomus type I (chief) cells, and glomus type II (sustentacular) cells
Describe glomus type I (chief) cells of the carotid body
These cells are derived from the neuroectoderm like nerve cells and they release neurotransmitters that stimulate the sensory endings of the vagus and glossopharyngeal afferent nerves that project to the medullary respiratory centres
Describe glomus type II (sustentacular) cells of the carotid body
Resemble ganglia and act as supporting cells
How do type I glomus (chief) cells of the carotid body detect hypoxia?
Decrease in pO2 or arterial pH--> depolarisation of cell membrane --> opens voltage-gated Ca2+ channels --> rise in intracellular Ca2+ concentration --> release of variety of neurotransmitters --> act on receptors on afferent nerve fibres adjacent to glomus cell --> cause action potential which travels in vagus and glossopharyngeal fibres to the respiratory centres in the medulla
What are the medullary respiratory centres?
Group of neurones in the reticular formation of the medulla that run from the top of the spinal cord to the thalamus
What are the two parts of the medullary respiratory centre?
Has two parts: nucleus of the solitary tract and nucleus ambiguus.
What are the respiratory centres of the brainstem?
Respiration is controlled by groups of neurones in the pons and medulla and there are three main groups; pontine respiratory centre, ventral and dorsal medullary respiratory centres
What is the efferent action of the respiratory centres?
Axons arise from the cells in all parts of the reticular formation and descend down the spinal cord in the reticulospinal tracts (down spinal cord, around margin of ventral horn and activates the phrenic nerve)
What is the function of the reticulospinal tract?
The reticulospinal tract arises in the pons and medulla and descends diffusely down the spinal cord around the margin of the ventral horn (it also carries the drive to the ANS) and activates the phrenic nerve
Describe the nucleus of the solitary tract in the medullary respiratory centre (dorsal respiratory group)
Sensory integrating centre of the respiratory centre which receives input from pH detectors, pulmonary stretch, cough and J receptors. This centre is found on the dorsal surface of the medulla.
Describe the nucleus ambiguous in the medullary respiratory centre (ventral respiratory group)
Sends axons into the more ventrally positioned motor output centre which sends impulses down to the spinal cord and to the phrenic nerve/
What is Cheyne-Stokes breathing?
A gradual increase in depth and rate of breathing followed by gradual decrease that results in temporary stop of breathing (apnoea)
What is the cause of Cheyne-Stokes breathing?
Heart failure or strokes that affect the blood supply to the medulla or individuals with traumatic injuries or tumours of the brainstem; indicates damage to the brainstem or to the inputs from the respiratory receptors to the brainstem
Describe the pontine respiratory centres
These regulate and modulate the medullary centres during speech when you want to hold your breath voluntarily, for example. These centres are controlled from the cerebral cortex which can modulate but not overcome the actions of the medullary centres.
What is the role of the pulmonary stretch receptors at the end of inspiration?
They send action potentials through afferents of the vagus nerve to the respiratory centres in the pons and medulla to inhibit further respiration (Hering-Breuer reflex)
Describe pulmonary stretch receptors
Depolarise in response to distension in the lung (activity sustained with lung inflation), found in the bronchioles and small bronchi and act as cut-off switches to inhibit inspiration when the lungs are fully inflated (prevents damage due to over-inflation)
Describe irritant receptors of the lungs
Mechanoreceptors that lie between airway epithelial cells and are stimulated by noxious gases, smoke, inhaled dusts and cold air and are found in the lining of the trachea and the large bronchi. Stimulates the cough reflex to clear the airway of danger.
Describe J receptors in the lungs
Chemoreceptors that respond to pulmonary oedema, pulmonary embolus, pneumonia and congestive heart failure --> stimulate increase in ventilation by detecting fluid and sending afferent signals to vagus nerve to increase respiratory rate
Describe how the higher centres of the brain can control respiration
Higher centres of the brain are involved in voluntary control of ventilation rate and this is affected by pain, emotion and temperature, and the brain works to modulate the impulses from the pontine respiratory centre to the medullary respiratory centre before action is sent to the lungs
How do irritant receptors in the lung modulate ventilation when stimulated?
Supresses inhalation by acting at the medullary respiratory centre in order to allow the release of substance inhaled (helps to blow off irritant)