lecture 18 - respiratory system 5

Question 1

what is the pons?

Answer 1

site of the pontine respiratory group (PRG/pneumotic centre)

neuroanl input into DRG and VRG to coordinate respiratory rhythm

Question 2

what is the medulla?

Answer 2

site of the respiratory rhythmicity centre

generates automatic rhythmic breathing pattern

Question 3

what 2 groups of neurones does the medulla contain?

Answer 3

the dorsal respiratory group - DRG

the ventral respiratory group - VRG

Question 4

dorsal respiratory group (DRG)

Answer 4

mainly generates inspiratory drive

output via phrenic nerve activates respiratory muscles and generation inspiration

Question 5

ventral respiratory group (VRG)

Answer 5

mainly expiratory neurones

inactive during quiet breathing

active during forced breathing

pre-botzinger complex - also known as respiratory pattern generator (RPG)

Question 6

what does activation of the DRG do?

Answer 6

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

Question 7

what are the inspiratory neurones?

Answer 7

phrenic and intercostal nerves

Question 8

how long does inspiration and expiration take?

Answer 8

inspiration = 2 seconds 
expiration = 3 seconds 

5 seconds per cycle

12 breaths per minute

Question 9

role of the pons

Answer 9

activity of the medullary rhythmicity centre is influenced by respiratory centres in the pons

Question 10

what does the apneustic centre of the pons do?

Answer 10

located dorsally in the pons

promotes inspiration by stimulating the inspiratory neurones in the medulla - DRG

Question 11

what does the pontine respiratory group do?

Answer 11

located in the lower part of the pons

antagonises and dominates the apneustic centre

inhibits inspiration and allows for smooth breathing

Question 12

Reciprocal pattern of inhibition of expiratory & inspiratory motor neurones

Answer 12

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

Question 13

how does a decrease in PO2 modify respiratory rate?

Answer 13

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

Question 14

what is hypoxia?

Answer 14

being deprived of oxygen

Question 15

location of peripheral chemoreceptors

Answer 15

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

Question 16

what do the peripheral chemoreceptors do?

Answer 16

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

Question 17

signalling pathway within carotid body

Answer 17

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

Question 18

how does an increase in CO2 modify respiratory rate

Answer 18

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

Question 19

what is PCO2 sensed by?

Answer 19

central chemoreceptors

Question 20

where are central chemoreceptors?

Answer 20

located on ventral surface of the medulla

Question 21

how do central chemoreceptors sense PCO2?

Answer 21

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

Question 22

what does an increase in blood CO2 trigger?

Answer 22

hyperventilation

Question 23

what does an decrease in blood CO2 trigger?

Answer 23

hypoventilation

Question 24

hyperventilation

Answer 24

• more CO2 expired
• blood CO2 decreases
• carbonic acid releases fewer H+
• H+ conc decreases
• blood pH increases

RESPIRATORY ALKALOSIS

Question 25

hypoventilation

Answer 25

• less CO2 expired
• blood CO2 increases
• carbonic acid releases more H+
• H+ conc increases
• blood pH decreases

RESPIRATORY ACIDOSIS

Question 26

what is metabolic acidosis?

Answer 26

acidosis resulting from an increase in non-CO2 derived acid (eg. lactic acid)

Question 27

what happens when you get metabolic acidosis?

Answer 27

H+ cannot enter CSF by brain barrier so can’t stimulate central chemoreceptors

results in increased ventilation - respiratory compensation

reduces volatile acid load

Question 28

how do stretch receptors modify respiration?

Answer 28

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

Question 29

how do irritant receptors modify respiration?

Answer 29

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

Question 30

how does a cough work?

Answer 30

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

Question 31

how does proprioception modify respiration?

Answer 31

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

Question 32

the diving response

Answer 32

stimulated by immersion of face in water 
• afferent supply - trigerminal nerve 
• receptors located in nostril
• heart rate falls 
• respiration inhibited

Question 33

how does HR fall in the diving response?

Answer 33

peripheral vessel constriction

increase in BP - directs blood to brain

spleen contracts increasing number of circulating erythrocytes

Question 34

how is respiration inhibited in the diving response?

Answer 34

voluntary component

reflex inhibition following trigerminal activation

respiratory drive from peripheral chemoreceptors inhibited

Question 35

what is the ‘break point’ in diving?

Answer 35

when level of arterial CO2 eventually stimulates breathing and limits the extent of the dive

Question 36

why do divers hyperventilate before diving?

Answer 36

for preparation

reduces alveolar PCO2 and respiratory drive from central chemoreceptors - increases time to breakpoint

can be fatal if cereal hypoxia occurs before breakpoint