Central and Peripheral Chemoreceptors

Definition

I. Central Chemoreceptors

Location

• Ventral surface of the medulla oblongata, near the exit points of CN IX and X
• Additional sites: nucleus tractus solitarius (NTS), locus coeruleus, hypothalamus (recent evidence)
• Closely communicate with the dorsal respiratory group (DRG/inspiratory center)

Primary Stimulus

• H+ concentration in CSF (not arterial PO2 or PCO2 directly)
• CO2 freely crosses the blood-brain barrier (BBB); H+ and HCO3- do NOT

Mechanism of CO2 Sensing

1. Arterial PCO2 rises → CO2 freely diffuses across BBB into CSF and brain interstitial fluid
2. In CSF: CO2 + H2O → H2CO3 → H+ + HCO3-
3. CSF pH falls (increased H+)
4. Central chemoreceptors detect the H+ rise → signal DRG → increase ventilation rate (hyperventilation)
5. Increased ventilation expels CO2 → arterial PCO2 returns to normal

Key Features

• Primary minute-to-minute drive for breathing under normal conditions
• Exquisitely sensitive: a rise in PCO2 from 40 to ~45 mmHg (only 12.5%) can double ventilation
• NOT sensitive to hypoxia (O2 does not penetrate BBB meaningfully to act here)
• In metabolic acidosis, the low H+ signal is buffered by plasma proteins and does not cross the BBB well → slower and weaker central response compared to respiratory acidosis
• Chronic hypercapnia: choroid plexus compensates by transporting HCO3- into CSF, gradually restoring pH → blunted response (CO2 narcosis in COPD)

II. Peripheral Chemoreceptors

Location

• Carotid bodies - at the bifurcation of the common carotid arteries; afferents via CN IX (carotid sinus nerve → glossopharyngeal)
• Aortic bodies - along the underside of the aortic arch; afferents via CN X (vagus)
• Both relay information to the DRG in the medulla

Note: Carotid bodies are the clinically dominant peripheral chemoreceptors

Chemosensor Cell

• Glomus (Type I) cells - the actual chemosensors; they synapse on sensory nerve endings
• Type II (sustentacular) cells - supporting cells

Stimuli and Responses

Mechanism of Hypoxia Sensing (Glomus Cell)

• Hypoxia inhibits O2-sensitive K+ channels on glomus cells → membrane depolarization
• Voltage-gated Ca2+ channels open → Ca2+ influx
• Release of neurotransmitters (dopamine, ATP, substance P) → activate afferent nerve → stimulate DRG → increase ventilation

Interaction of Stimuli

• Hypercapnia and acidosis potentiate the response to hypoxia (synergistic effect)
• Respiratory alkalosis blunts the response to hypoxia

III. Comparison Table

IV. Clinical Correlates

• COPD/Chronic hypercapnia: Blunted central response → peripheral chemoreceptors (hypoxic drive) become dominant. Giving high-flow O2 may abolish this drive → respiratory depression
• High altitude: Hypoxia stimulates peripheral chemoreceptors → hyperventilation → respiratory alkalosis → blunts central drive; acclimatization restores balance
• Metabolic acidosis: Peripheral carotid bodies stimulated directly by low pH → Kussmaul breathing (respiratory compensation)
• Cushing response: Brain ischemia → local PCO2 rise → medullary chemoreceptors → intense sympathetic activation → severe hypertension + bradycardia (raised ICP)