Ventilation: Control of Breathing Flashcards
What are the 4 sites responsible for the control of PO2 and PCO2 in alveolar ventilation?
- respiratory control centre
- central chemoreceptors
- peripheral chemoreceptors
- pulmonary mechanoreceptors
Describe the experimental evidence there is for the location of respiratory neurons
- section cut between the pons and medulla: breathing and associated activities of breathing maintained
- section cut between medulla and spinal cord: breathing not occuring (no phrenic nerve activity) but associated activities still occured
- therefore neurons controlling breathing must be in the medulla
Where are the respiratory centres located and what does it do?
- medulla oblongata and pons
- collect sensory information about O2 and CO2 levels in blood
- determines signal sent to respiratory muscles which lead to alveolar ventilation
Describe the different neurons in the respiratory groups
- VRG: inpiratory and expiratory neurons
- DRG: just inspiratory neurons
Describe the output of the VRG and DRG
DRG (inspiratory):
- diaphragm and external intercostals
VRG (inspiratory):
- laryngeal and pharyngeal muscles
- diaphragm and internal intercostals
VRG (expiratory):
- abdominal muscles
- internal intercostals
Describe the action of the DRG and where they are found
- found in nucleus tractus solitarius
- receive sensory input from thorax and abdomen muscles
- emit repetitive bursts of inspiratory neuronal APs
- respiratory ramp for 2 seconds and cessation for 3 seconds (when expiration occurs)
What can alter the respiratory ramp in DRG neurons?
- controlling rate of increase of ramp (heavy breathing causes ramp to increase to fill lungs rapidly)
- controlling limiting point at which ramp suddenly stops (controlling rate of respiration)
What are the modulating centres of respiratory output?
- pneumotaxic found dorsally in nucleus parabrachialis medialis of upper pons
- apneustic centre
What are the roles of the pneumotaxic and apneustic centres?
- primary effect is to control switch-off point of respiratory ramp (controls filling phase of lung cycle)
- provide strong pneumotaxic signal to set the duration of inspiration each lung cycle`
Describe the chemical control of ventilation
- maintain proper levels of PO2, PCO2 and pH
- hypercapnia and acidosis detected by central respiratory centre
- hypoxia, hypercapnia and acidosis detected by peripheral chemoreceptors in carotid and aortic bodies
Where is the supposed location of the central chemoreceptors?
- bilaterally beneath ventral surface of medulla
- medullary raphe
Describe the mechanism of action of central chemoreceptors
- responds to changes in pH in the CSF
- H+ ions do not cross blood brain barrier very well, CO2 does though
- increases in PCO2 above normal causes PCO2 to increase in interstitial fluid of medulla and CSF
- CO2 combines with H2O to form H+ ions by carbonic anhydrase
- neurons of central chemoreceptor very sensitive to H+ ions (acidification)
- so sends signals to CNS to restore homeostasis
Describe the mechanism of action of the carotid body
- senses decreased arterial PO2 (low PO2 but normal PCO2 and pH): causes increase in firing rate of carotid sinus nerve
- senses increased arterial PCO2 (fixed PO2/pH): causes increase in firing rate of carotid sinus
- senses decreased arterial pH (fixed PO2): firing rate of carotid sinus is greater over all PCO2 values
Describe how conditions transmit a signal from the glomus cell in the carotid body/sinus
- signal transduction
- hypoxia/hypercapnia/acidosis all inhibit K+ channels
- depolarises the cell opening voltage gated Ca2+ channels
- increased entry of Ca2+
- triggers release of neurotransmitter
- neurotransmitter binds to post-synaptic membrane of afferent nerve fibre to generate AP to be conducted along nerve to medulla
Other than the respiratory neurons, what other inputs contribute to the respiratory system?
- stretch and chemical/irritant receptors
- higher CNS centres that control non-respiratory activity
