Receptors

1. Central chemoreceptors
2. Peripherial chemoreceptors
3. Lung receptors
4. 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

1. Carotid bodies (important)
   @ Common carotid bifurcation
   * Innervated by carotid sinus nerve, then glossopharyngeal
   * Connect to medulla [WG22:p674]
2. 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
  1. Nicotine, acetylcholine
     --> stimulates sympathetic ganglia
  2. Cyanide, carbon monoxide => blocks cytochrome oxidase (histotoxic hypoxia)
  3. 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)