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Lecture 7

PHSI 208 Lecture Notes - Lecture 7: Peripheral Chemoreceptors, Central Chemoreceptors, Carotid Body


Department
Physiology
Course Code
PHSI 208
Professor
Baillie Landon
Lecture
7

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!PHSI%208.6%–%University%of%Saskatchewan%
%Prof:&Landon&Baillie!
January'25,'2016'–'Respiratory'7&
&
During!inspiration,!the!activity!of!inspiratory!neurons!increases!steadily,!apparently!through!a!
positive!feedback!mechanism.!At!the!end!of!inspiration,!the!activity!shuts!off!abruptly!and!
expiration!takes!place!through!the!recoil!of!elastic!lung!tissue.!
!
This!is!believed!to!be!initiated!by!a!pacemaker!cell!from!the!pre-Botzinger!complex.!The!positive!
feedback!loop!recruits!more!neurons,!(“ramping”)!and!more!inspiratory!muscles.!
!
CO2,!O2!and!pH!influence!ventilation.!Peripheral!chemoreceptors!on!the!aortic!and!carotid!
bodies!sense!changes!in!arterial!PO2,!PCO2!and!pH!and!adjust!ventilation!accordingly.!!
!
Glomus!cells!are!the!carotid!body!oxygen!sensors,!and!they!release!a!neurotransmitter!when!
PO2!decreases.!
Low!PO2!causes!O2!sensitive!K+!channels!to!close.!
K+!then!accumulates!in!the!cell,!causing!the!cell!to!depolarize.!
Voltage-gated!Ca2+!channels!then!open,!and!Ca2+!enters!the!cells.!
Exocytosis!of!neurotransmitters,!sending!them!to!a!receptor!on!the!sensory!neuron.!
An!action!potential!is!fired!and!a!signal!is!sent!to!the!medullary!centers!to!increase!
ventilation.!
!
It!takes!a!large!drop!in!arterial!PO2!to!trigger!peripheral!chemoreceptors.!Glomus!cells!can!also!
respond!to!increases!in!H+!(decreases!in!pH).!They!primarily!respond,!however,!to!increases!in!
CO2.!
!
Central!chemoreceptors!are!located!in!the!medulla.!They!provide!continuous!input!to!the!
respiratory!control!centre.!From!what!we!know,!they!respond!mainly!to!changes!in!PCO2.!They!
do,!in!fact!respond!to!pH!changes!in!the!cerebrospinal!fluid!(CSF)!caused!by!CO2,!but!not!to!
changes!in!plasma!pH!(H+!cannot!cross!the!blood!brain!barrier).!
!
Neurons!in!these!regions!contain!H+!sensitive!channels!(ASIC)!that!become!activated!and!
transmit!AP’s!to!the!respiratory!control!center,!but!the!DRG!primarily.!
!
Peripheral!chemoreceptors!–!carotid!bodies!and!aortic!bodies!–!respond!to!changes!in!the!
arterial!blood.!They!are!stimulated!by:!
Significantly!decreased!PO2!(hypoxia).!
Increased!H1!concentration!(metabolic!acidosis).!
Increased!PCO2!(respiratory!acidosis).!
!
Central!chemoreceptors!–!located!in!the!medulla!oblongata!–!respond!to!changes!in!the!brain!
extracellular!fluid.!They!are!stimulated!by!increased!PCO2!via!associated!changes!in!H1!
concentration.!Under!conditions!of!chronic!increases!in!CO2!level,!central!chemoreceptors!
become!desensitized,!whereas!peripheral!chemoreceptors!do!not.!
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