how central chemoreceptor respond to increase in co2

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How Central Chemoreceptors Respond to Increased CO2

Location

Central chemoreceptors sit within the brain parenchyma, predominantly on the anterolateral (ventrolateral) surface of the medulla oblongata - including the retrotrapezoid nucleus (RTN), medullary raphe nuclei, nucleus ambiguus, nucleus tractus solitarius (NTS), and the locus coeruleus. They are bathed in brain extracellular fluid (BECF) and CSF.

The Core Mechanism (Step by Step)

Step 1 - CO2 crosses the blood-brain barrier (BBB)

The BBB is highly permeable to small neutral molecules like CO2, but nearly impermeable to ions (H+, HCO3-, Na+, Cl-). So when arterial PCO2 rises, CO2 freely diffuses across the BBB into the BECF and CSF. Importantly, H+ and HCO3- cannot cross as readily.

Step 2 - CO2 is hydrated to H+ in the CSF/BECF

CO2 + H2O ↔ H+ + HCO3-

This reaction (catalyzed by carbonic anhydrase) lowers the pH of the CSF and BECF. Because the CSF/BECF has far less protein than blood plasma, its non-bicarbonate buffering power is much lower - so for a given rise in PCO2, the pH drop in the CSF is larger and faster than in blood. This amplifies the stimulus.

Step 3 - Chemoreceptors detect the H+ rise (not CO2 directly)

The actual stimulus to central chemoreceptors is the fall in pH (rise in [H+]) in the BECF/CSF - not CO2 per se. Evidence: perfusing the cerebral ventricles with an acidic solution (even at normal PCO2) increases ventilation; conversely, metabolic acidosis (low pH at fixed PCO2) has a much weaker ventilatory effect because H+ cannot cross the BBB as readily.

Step 4 - Neuronal activation and output

Chemosensitive neurons (many are serotonergic; others are glutamatergic) are excited by the local acidosis. GABAergic neurons are inhibited. These neurons project to the medullary respiratory centers (central pattern generator), which then increase the rate and depth of breathing (alveolar ventilation) to blow off excess CO2 and restore pH.

Ventilatory Response Curve

This is the normal CO2-ventilation response curve:

Key features:

The relationship between PaCO2 and minute ventilation is nearly linear over the physiological range (40-70 mmHg)
A rise in PaCO2 from 40 to ~45 mmHg (just ~12.5% increase) doubles ventilation - showing extreme sensitivity
The full response takes 5-10 minutes to develop (due to delay in pH change at the chemosensory site, chemotransduction time, and CPG response lag)
At very high PaCO2 (>80-100 mmHg), the response reverses - CO2 narcosis causes CNS depression and respiratory failure

Why Central > Peripheral for CO2

Feature	Central Chemoreceptors	Peripheral Chemoreceptors
Location	Ventrolateral medulla	Carotid & aortic bodies
Primary stimulus	CSF/BECF [H+] (from CO2)	PaO2, PaCO2, arterial pH
Response to hypercapnia	Dominant (~70-80% of response)	Contributes ~20-30%
Response to hypoxia	Depressed by hypoxia	Stimulated by hypoxia
Speed	Slower (5-10 min full response)	Faster

Why Metabolic Acidosis Has a Weaker Effect

Even though metabolic acidosis lowers blood pH, the H+ ions cannot freely cross the BBB. The choroid plexus actively maintains CSF pH. So metabolic acid-base disturbances alter steady-state brain pH only 10-35% as much as equivalent respiratory disturbances. This is why ventilation correlates with BECF pH, not arterial blood pH per se.

Compensation Over Time

With chronic hypercapnia, the choroid plexus and BBB actively transport HCO3- into the CSF, partially restoring CSF pH. This is the metabolic compensation to respiratory acidosis - it explains why patients with chronic CO2 retention have a blunted ventilatory drive over days.

Sources:

Medical Physiology (Boron & Boulpaep), pp. 1065-1066 - detailed mechanism of BBB permeability and BECF acidosis
Morgan and Mikhail's Clinical Anesthesiology, p. 972-973 - clinical overview of central vs. peripheral sensors and CO2-narcosis threshold
Medical Physiology, N32-19/N32-22 - RTN neurons, serotonergic chemoreceptors, and SIDS implications

why central chemoreceptors are depressed by hypoxia

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why central chemoreceptors depressed by hypoxia mechanism adenosine neuronal metabolism

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https://pmc.ncbi.nlm.nih.gov/articles/PMC4802370

I now have enough from the textbooks and supplementary sources to give a thorough, accurate answer. Here it is:

Why Central Chemoreceptors Are Depressed by Hypoxia

This is one of the most important contrasts in respiratory physiology - and the answer operates at several levels.

1. The Key Experimental Proof

When peripheral chemoreceptors are denervated in animals and the animal is then made hypoxic, hypoxia actually depresses respiratory output rather than stimulating it. This shows that the net central effect of hypoxia is inhibitory - the only reason hypoxia normally increases breathing is because peripheral chemoreceptors (carotid bodies) overcome and dominate this central depression.

"In an animal with denervated peripheral chemoreceptors, hypoxia actually depresses respiratory output."

Medical Physiology (Boron & Boulpaep), p. 1070

2. Mechanisms of Central Depression by Hypoxia

There are three overlapping mechanisms:

A. Direct Neuronal Metabolic Failure (Primary mechanism)

The medullary neurons that act as central chemoreceptors (serotonergic neurons in raphe, RTN neurons, NTS neurons) are highly aerobic and oxygen-dependent. When O2 drops:

Mitochondrial oxidative phosphorylation is impaired
ATP production falls
Neurons cannot maintain their membrane potential or firing rates
The result is generalised CNS depression, just as hypoxia causes confusion and loss of consciousness at the cortical level - the brainstem is not immune

This is the same mechanism by which severe hypoxia causes loss of consciousness - neurons simply fail when deprived of oxygen.

B. Adenosine Release - The "Sleep Chemical" Effect

During hypoxia, ATP breakdown accelerates and adenosine accumulates in brain tissue. Adenosine is a potent neuroinhibitor - it acts via A1 receptors to:

Hyperpolarise neurons
Reduce neurotransmitter release
Suppress firing of medullary chemosensitive neurons

This is why caffeine (an adenosine receptor antagonist) is used clinically in neonatal apnea - it blocks this adenosine-mediated central respiratory depression.

C. Hypoxia Does Not Change CSF [H+] the Way CO2 Does

Central chemoreceptors detect H+ in the BECF/CSF, not O2 directly. Hypoxia alone:

Does not appreciably raise PCO2 (in fact, hyperventilation from peripheral receptors lowers PCO2)
If anything, the hypocapnia induced by peripheral-chemoreceptor-driven hyperventilation raises CSF pH (less H+)
Less H+ = less stimulus to the central chemoreceptors

So even if the neurons were fully functional, falling CO2 (from reflex hyperventilation) removes the acid stimulus that central chemoreceptors depend on.

3. The Biphasic Hypoxic Ventilatory Response

This explains the classic two-phase response to sustained hypoxia:

Phase	Time	Driver	What happens
Early stimulation	0-2 min	Peripheral carotid bodies (O2-sensitive)	Rapid increase in ventilation
Late depression	2-20 min	Central depression overrides	Ventilation falls back partway toward baseline

The late "roll-off" (hypoxic ventilatory decline) reflects the gradual dominance of central neuronal depression overcoming the peripheral drive.

4. Contrast with Peripheral Chemoreceptors

Feature	Central Chemoreceptors	Peripheral (Carotid Bodies)
Response to hypoxia	Depressed	Strongly stimulated
Primary stimulus	CSF/BECF H+ (from CO2)	PaO2, pH, PCO2
Why different?	Metabolically vulnerable neurons; no O2 sensor	Glomus (type I) cells have specialized O2-sensing via TASK channels and HIF pathways
Hypoxia + CO2 interaction	Already depressed; CO2 signal attenuated	Hypoxia potentiates CO2 response (synergistic)

5. Clinical Relevance

Chronic CO2 retainers (COPD): When central chemoreceptors adapt to chronic hypercapnia (CSF pH normalises), hypoxic drive from peripheral receptors becomes the primary breathing stimulus. Giving high-flow O2 removes this drive → hypoventilation → CO2 narcosis → death. This is why careful O2 titration (target SpO2 88-92%) is used in these patients.
Neonatal apnea: Infants have an exaggerated central depression to hypoxia (immature serotonergic neurons) - this is why caffeine/theophylline are used to block adenosine and stimulate the respiratory center.
SIDS: Abnormalities of serotonin neurons in the medullary raphe (the same neurons that are central chemoreceptors) may cause an exaggerated inability to respond to hypoxia during sleep.

Sources:

Medical Physiology (Boron & Boulpaep), pp. 1069-1070 - denervation experiments proving central depression
Morgan & Mikhail's Clinical Anesthesiology, p. 972-973 - clinical overview; "central chemoreceptor activity is depressed by hypoxia"
Cummings Otolaryngology, p. 3498 - biphasic response and neonatal hypoxic depression
Central Chemoreceptors: Locations and Functions, PMC4802370 - adenosine/ATP and glial mechanisms

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