Neural Control of Respiration Flashcards
What is the FRC
- Functional residual capacity
The amount air left at the end of a normal breath
What happens to nerve activity during the respiratory cycle?
It increases during inspiration and reaches its peak just before inspiration shuts off, it then then remains at a low level during passive expiration. Activity of neurons increases steadily, aparently as a result of a positive feedback mechanism
Where does the rhythmic pattern of breathing originate?
The medulla (and the pons)
Do the respiratory muscles have an intrinsic rhythmicity?
No
What part of the brain contains all the components to generate a rhythmic pattern of respiration?
The brainstem
What are respiratory centres?
- Diffuse networks which act together ro bring about the respiratory effect.
- Collect sensory information about the levels of oxygen and CO2 in blood that determines the signal sent to respiratory muscles
- Stimulation leads to respiratory movements which produce alveolar contraction
Where are the respiratory centres located?
The medulla oblongata and the pons
What does the inspiratory centre control?
Diaphragm and external intercostals
What does the expiratory centre control?
Internal intercostals but also partially expiratory neurons .
Inactive during quiet breathing
Where is the inspiratory centre located?
Upper part of medulla oblongata (dorsal part)
Where is the expiratory centre located?
Medulla oblongata, anterior and lateral to the inspiratory centre
What can the inspiratory centre also be called?
The dorsal Respiratory GroupGroup (DRG)
What can the expiratory centre also be called?
Ventral Respiratory Group (VRG)
When is the expiratory centre active?
During forced expiration or when inspiratory centre is inhibited
Where is the pneumotaxic centre located?
The upper pons
What is the function of the pneumotaxic centre?
Controls medullary respiratory centres, especially the inspiratory centre through the apneustic centre. It influences inspiratory centre so that duration of inspiration is under control
Where is the apnuestic centre located?
The Lower pons
What is the function of the apneustic centre?
Increases depth of inspiration by acting on inspiratory centre
What are the afferent connections of the respiratory centre?
- Impulses according to movement of thoracic region and lungs
- Also from chemoreceptors
What are the efferent connections of the respiratory centre?
- Nerves from the respiratory centre leave in anterior part of the 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
Where are the impulses coming from in the respiratory centres?
- Higher centres
- Stretch receptors (Hering-Breur Reflex)
- J receptors, or pulmonary C-fibres
- Irritant receptors of lungs
- Proprioceptors
- Thermoreceptors
- Pain receptors
- Cough reflex
- Sneezing reflex
- Deglutition reflex
What areas which are considered ‘higher centres’ contribute towards stimulating or inhibiting the respiratory centre directly?
- Cerebral cortex
- Limbic system
- Hypothalamus
Describe the Hering-Breur Reflex?
- Smooth muscle receptors of the upper airways have slowly adapting stretch receptors.
- When lung is inflated these neurons send impulses to the DRG via the vagus
- This input is inhibitory limiting inspiration, prevents overinflation of lungs.
When is the Hering-Breur reflex more active?
- In 1st year of life
- During strenous exercise when TV is > 1 L
Where are J receptors (or pulmonary C-fibres) present?
- Juxtacapillary receptors present in wall of alveoli, in close contact with pulmonary capillaries
What are J receptors (or pulmonary C-fibres) stimulated by?
- Stimulated during conditions like pulmonary oedema, congestion, pneumonia, alsos from endogenous chemicals such as histamine
What do ‘J’ receptors induce when stimulated?
Apnea - temporary suspension of breathing - followed by rapid shallow breathing
What are irritant receptors of the lungs stimulated by?
- Inhalation of irritants
- Harmful chemicals such as amonnia and cigarette smoke
Where are irritant receptors of the lung situated?
On the walls of bronchi and bronchioles
What do the irrritant receptors of the lung induce?
- Rapid shallow breathing, mainly from shortening of expiration
- But also, long deep augmented breaths, which are taken by mammals every 5-20 mins on average to reverse slow collapse of lungs that occurs during quiet breathing
What do proprioceptors do?
- Measure how fast you are breathing and the rate of expansion
- Aim is for reflexes from muscles and joints to stabilise ventilation in the face of changing mechanical conditions
Where are proprioceptors located?
- Receptors in chest wall
- Situated in joints - measure velocity of rib movement
- Also found in tendons and muscles
What are thermoreceptors?
- Cutaneous
- Supply signals to cerebral cortex
- Stimulates respiratory centre
- Hyperventilation
What are pain receptors?
- Supply signals to cerebral cortex,
- Stimulates respiratory centres
- Induces hyperventilation
What nerve does the cough reflex stimulate?
Vagus
Describe the cough reflex
- Protective reflex caused by irritation of parts of respiratory tract beyond nose, eg larynx, trachea and bronchi
- Deep inspiration followed by forceful expiration with closed glottis
- Glottis opens and explosive outflow or air at high velocity
Describe the sneezing reflex
- Irritation of nasal mucous membranes
- Deep inspiration followed by forceful expiration with opened glottis
Describe the deglutition reflex?
- Respiration arrested during swallowing of food
- Swallowing apnea or deglutition apnea
What can chemoreceptors change in response to?
- Hypoxia
- Hypercapnea - elevated CO2 in blood
- Increased H+ concentration (decreased pH)
What are the two main groups of chemoreceptors?
- Central chemoreceptors
- Peripheral chemoreceptors
What is a better buffer plasma or CSF?
Blood plasma
How is an increase in H+ concentration detected by the medulla oblongata?
- H+ cannot cross the BBB or CSF barrier
- CO2 can cross into CSF, and forms carbonic acid, which is unstable and rapidly dissociates to H+ and bicarbonate
- H+ then stimulates central chemoreceptors
Why a rise in PaCO2 cause a larger effect on pH in the CSF than the blood?
There is less protein in the CSF than the blood
What are the central chemoreceptors sensitive to?
- Arterial PaCO2
Not arterial H+ or PaO2
Where are central chemoreceptors located?
Medulla oblongata close to DRG
What type of cells are peripheral chemoreceptors?
- Specialised receptor cells (glomus type 1) that are stimulated primarily by a decrease in PO2 and an increase in H+ (the latter occuring as a result of increased CO2)
What nerve do the sensory fibres of the aortic bodies follow?
CN X - Vagus
What nerve do the sensory fibres of the carotid bodies follow?
CN IX - glossopharyngeal
What are the peripheral chemoreceptors stimulated by?
A decrease in PO2
Where are the peripheral chemoreceptors situated?
Around carotid sinus and aortic arch
What peripheral chemoreceptors have the greatest effect on respiration?
Carotid bodies
What are the peripheral chemoreceptors sensitive to?
- PaO2
- PaCO2
- pH
- Blood flow
- Temperature
How much more sensitive are central chemoreceptors to PaCO2 compared with peripheral chemoreceptors ?
10 times
How do Oxygen levels relate to chemical changes within glomus cells in chemoreceptors?
- When PO2 is high, the K+ channel is open, and the glomus cell is quiescent
- When PO2 falls, and the K+ channel closes. Vm depolarises
- Ca2+ channels open
- Ca2+ influx triggers NT release
- Sensory afferents signal to the CNS
What is normal PCO2?
40 mmHg or 5.3 kPa
How is ventilation depressed when CO2 is decreased?
- 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
What must PO2 drop below before PO2 becomes a major stimulus for ventilation?
Must drop below 60 mmHg or 8 kPa from around 100 mmHg (13.3kPa)
Why is not recommended to put a patient with COPD on 100% oxygen?
- Patient’s chemoreceptors become insensitive to PCO2 or have become adjusted to lower than normal O2 and higher CO2
- Patient relies on deline in PO2 to stimulate breathing - HYPOXIC DRIVE
- Risk is that on 100% oxygen patient will cease breathing, because firing rate of peripheral chemoreceptors will fall and depress ventilatory drive
What neuromuscular disorders affect the respiratory system?
- CNS
- Hemispheric strokes
- Poliomyelits
- Diptheria
- Botulism
- DMD
