lecture 18 - respiratory system 5 Flashcards
regulation of respiration
what is the pons?
site of the pontine respiratory group (PRG/pneumotic centre)
neuroanl input into DRG and VRG to coordinate respiratory rhythm
what is the medulla?
site of the respiratory rhythmicity centre
generates automatic rhythmic breathing pattern
what 2 groups of neurones does the medulla contain?
the dorsal respiratory group - DRG
the ventral respiratory group - VRG
dorsal respiratory group (DRG)
mainly generates inspiratory drive
output via phrenic nerve activates respiratory muscles and generation inspiration
ventral respiratory group (VRG)
mainly expiratory neurones
inactive during quiet breathing
active during forced breathing
pre-botzinger complex - also known as respiratory pattern generator (RPG)
what does activation of the DRG do?
generates automatic rhythmic breathing
during inspiration activity of inspiratory neurones increases
after 2 seconds, neuronal activity abruptly shuts off and inspiration ends
expiration doesn’t usually involve activation of expiratory neurone - lungs deflate passively
what are the inspiratory neurones?
phrenic and intercostal nerves
how long does inspiration and expiration take?
inspiration = 2 seconds expiration = 3 seconds
5 seconds per cycle
12 breaths per minute
role of the pons
activity of the medullary rhythmicity centre is influenced by respiratory centres in the pons
what does the apneustic centre of the pons do?
located dorsally in the pons
promotes inspiration by stimulating the inspiratory neurones in the medulla - DRG
what does the pontine respiratory group do?
located in the lower part of the pons
antagonises and dominates the apneustic centre
inhibits inspiration and allows for smooth breathing
Reciprocal pattern of inhibition of expiratory & inspiratory motor neurones
driven by DRG and VRG not a local spine reflex
inspiration
• gradual increase in activity of nerves supplying diaphragm and external intercostals
expiration
• activity of phrenic nerve decreases gradually
• external intercostal activity stops
• muscles relax
• internal intercostal activity increases
how does a decrease in PO2 modify respiratory rate?
respiratory drive is not very sensitive to changes in PO2
PO2 has to be bowl 60 mmHg before drive increases significantly
hypoxia is sensed by peripheral chemoreceptors
what is hypoxia?
being deprived of oxygen
location of peripheral chemoreceptors
found in 2 places
• aortic bodies - on aortic arch
• carotid bodies - in the bifurcation of the internal and external carotid artery
distinct from the aortic and carotid baroreceptors
what do the peripheral chemoreceptors do?
sample the O2, CO2 and H+ content of passing blood
most sensitive to O2
aortic bodies transmit information to the medullary respiratory centre via vagus nerves
carotid bodies via glossopharyngeal nerve
don’t shoe adaptation to prolonged stimulation
signalling pathway within carotid body
1) low PO2 detected by carotid body
2) K+ channels close
3) cell depolarises
4) causes vg Ca++ to open
5) Ca++ moves in
6) causes exocytosis of DA contains vesicles into extracellar space
7) sensed by DA receptors in sensory neurone
8) creates AP - signals to medullary centre to increase ventilation
how does an increase in CO2 modify respiratory rate
respiration rate sensitive to changes in PCO2
above 40mmHg PCO2 - small rise geernates large changes in rate
below 30mmHg - changes have little effect in rate
ventialtion responds primarily to changes in PCO2 rather than O2
what is PCO2 sensed by?
central chemoreceptors
where are central chemoreceptors?
located on ventral surface of the medulla
how do central chemoreceptors sense PCO2?
sense CO2 levels in CSF directly and indirectly
• increase PCO2 in CSF generates H+
• increase in H+ stimulates chemoreceptor
• increases respiration rate
show adaptation - after several days at high PCO2 respiratory rate returns to normal
what does an increase in blood CO2 trigger?
hyperventilation
what does an decrease in blood CO2 trigger?
hypoventilation
hyperventilation
- more CO2 expired
- blood CO2 decreases
- carbonic acid releases fewer H+
- H+ conc decreases
- blood pH increases
RESPIRATORY ALKALOSIS
hypoventilation
- less CO2 expired
- blood CO2 increases
- carbonic acid releases more H+
- H+ conc increases
- blood pH decreases
RESPIRATORY ACIDOSIS
what is metabolic acidosis?
acidosis resulting from an increase in non-CO2 derived acid (eg. lactic acid)
what happens when you get metabolic acidosis?
H+ cannot enter CSF by brain barrier so can’t stimulate central chemoreceptors
results in increased ventilation - respiratory compensation
reduces volatile acid load
how do stretch receptors modify respiration?
Hering Breuer Reflex
• triggered to prevent over-inflation of lungs
• lung stretch sensed by stretch receptors in lung smooth muscle
• activation leads to stopping of inspiration and onset of expiration
• may play important role in determining respiration rate & depth in newborn
how do irritant receptors modify respiration?
stimulated my mechanical stimuli
• inhaled dust
• cold air
• noxious gases
receptors how rapid adaptation when continuously stimulates
impulses travel via myelinated fibres in vagus
generate a pattern of rapid shallow breaths and the cough reflex
how does a cough work?
1) irritant receptors triggered
2) impulse travels via vagus and superior laryngeal nerves to medulla
3) activates DRG and VRG
4) diaphragm and external intercostal muscles contract
5) pressure in pleural cavity decreases
6) air enters lung and glottis and larynx close
7) abdominal and other expiratory muscles contract - pressure in lungs increases
8) glottis and larynx open - air released at high speed
9) bronchi collapse and air is forced through narrow opening clearing any irritants
how does proprioception modify respiration?
passive movement of limbs stimulates respiration
mediated by proprioception in muscles in joints - stimulate DRG and VRG
anticipates increased requirement for O2 / removal of CO2
recognition by the body that exercise is starting so adaptation are needed
the diving response
stimulated by immersion of face in water • afferent supply - trigerminal nerve • receptors located in nostril • heart rate falls • respiration inhibited
how does HR fall in the diving response?
peripheral vessel constriction
increase in BP - directs blood to brain
spleen contracts increasing number of circulating erythrocytes
how is respiration inhibited in the diving response?
voluntary component
reflex inhibition following trigerminal activation
respiratory drive from peripheral chemoreceptors inhibited
what is the ‘break point’ in diving?
when level of arterial CO2 eventually stimulates breathing and limits the extent of the dive
why do divers hyperventilate before diving?
for preparation
reduces alveolar PCO2 and respiratory drive from central chemoreceptors - increases time to breakpoint
can be fatal if cereal hypoxia occurs before breakpoint
