Receptors
- Central chemoreceptors
- Peripherial chemoreceptors
- Lung receptors
- Other receptors
1. Central chemoreceptors
Detects only changes in PaCO2.
--> Thus responsible for hypercapnic drive
NB:
- Peripheral chemoreceptor also contribute somewhat to hypercapnic drive
- Normal pH in CSF = 7.32
Location
- Located in ventral surface of medulla
* Near exit of CN9 and CN10
- Surrounded by CSF and local blood flow
?????? In direct contact with CSF?
Effect of PaCO2 on central chemoreceptors
Changes in PaCO2
--> Changes in PCO2 in CSF
Since there are less protein and haemoglobin in CSF (i.e. less buffer)
--> changes in PCO2 in CSF leads to great changes in pH
When PaCO2 increase
--> Cerebral vessels dilate
--> Faster diffusion of CO2
--> Faster changes in pH in CSF
Prolonged change in PaCO2
When changes are prolonged
--> HCO3 moves across the blood-brain barrier to buffer the pH change in CSF (unknown if transport is active or passive)
--> CSF pH change is buffered BEFORE renal compensation of blood pH changes
--> Respiratory change is reduced
Summary - central chemoreceptor
- Detects changes in PaCO2
- NOT PaO2
2. Peripheral chemoreceptors
Rapid response
- Stimulation of the carotid bodies has predominantly respiratory effect.
- Stimulation of the aortic bodies have a greater cardiovascular effect.
Location
- Carotid bodies (important)
@ Common carotid bifurcation
* Innervated by carotid sinus nerve, then glossopharyngeal
* Connect to medulla [WG22:p674]
- Aoric bodies
@ Above and below aortic arch
* Innervated by vagus nerve [WG22:p674]
Carotid bodies
Contains glomus cells (type I and type II)
- Type I - rich in dopamine, in close apposition to afferent carotid sinus nerve endings
* Dopamine released in response to hypoxia, which act on the carotid sinus nerve
- Very rich capillary supply - respond to arterial changes (not venous)
Factors stimulating peripheral chemoreceptors
Responds to:
- Decreased PaO2 (hypoxemia)
--> Main function
- Increased PaCO2 (hypercapnoea)
--> Not as significant as central chemoreceptor
- Increased [H+] (acidosis)
--> Carotid bodies only
- Hypoperfusion
* e.g. by severe hypotension
- Hyperthermia
- Chemical stimulation
- Nicotine, acetylcholine
--> stimulates sympathetic ganglia
- Cyanide, carbon monoxide => blocks cytochrome oxidase (histotoxic hypoxia)
- Other drugs doxapram, almitrine
Additional effects of peripheral chemoreceptors
In addition to increased ventilation, peripheral chemoreceptor stimulation also causes
- Bradycardia
- Hypertension
- Bronchiolar tone
- Adrenal secretion
Others
- Hyperthermia in itself also enhances ventilatory response to hypoxia and CO2.
Summary - peripheral chemoreceptors
Stimulated by:
- Hypoxemia
- Hypercapnia
- Acidosis
- Hypoperfusion
- Hyperthermia
- Chemical stimulation
3. Lung receptors
Pulmonary stretch receptor
(slow adapting)
1. Hering-Breuer reflex
--> Distension of lung
--> Stimulation
--> Increase in expiration time and decrease in RR
--> Acts primarily to limit or prevent hyperinflation, and are less important for controlling respiratory rate.
2. Deflation reflex
--> deflation of lung
--> tends to initiate inspiratory effort
@@Please WG22:p678-679 for more info
Irritant receptors
Irritation (noxious gas, cigarette, dusts, cold air)
--> bronchocontriction and hypercapnoea
Others
- J receptors
- Bronchial receptors
4. Other receptors
Arterial baroreceptors
- Decreased BP
--> hyperventilation
Pain and temperature
- Pain
--> Apnoea, followed by hyperventilation
- Heating of skin
--> Hyperventilation
Joint/muscle receptor
- Stimulates ventilation at early stages of exercise
Other notes
- [WG22:p674] Blood flow in cartoid body is about 0.04mL/min or 2000mL/100g/min (i.e. very huge)