Regulation of Breathing

Overview of Respiratory Regulation

Respiration is a highly regulated process that involves the coordination of ventilation, gas exchange, and gas transport.

• Efficiency of the System: The regulatory mechanisms are extremely effective. There is virtually no change in arterial partial pressure of oxygen ($P_{O2}$) or carbon dioxide ($P_{CO2}$) across a wide range of activity levels, from sleep to strenuous exercise.

• Mechanism of Control: Regulation is achieved through sensory inputs provided to central regulatory centers in the brain, which in turn modulate the output to the muscles of respiration.

The Respiratory Center Architecture

The "Respiratory Center" is not a single anatomical cluster of neurons but a distributed collection of neuronal groups located bilaterally within the brainstem, specifically in the medulla oblongata and the pons.

There are three major functional groupings of these neurons:

1. Dorsal Respiratory Group (DRG): Located in the medulla.

2. Ventral Respiratory Group (VRG): Located in the medulla.

3. Pneumotaxic Center: Located in the pons.

4. Apneustic Center: Located in the lower pons.

The Dorsal Respiratory Group (DRG)

The DRG is considered the most critical component of the respiratory center.

• Anatomical Location: These neurons are found in the dorsal region of the medulla within the Nucleus Tractus Solitarius (NTS).

• Connection to Neurobiology: There is a significant link between the nucleus of the solitary tract and the overall control of respiration.

• Sensory Input Integration: The NTS is a primary terminus for sensory information related to breathing:

  • Vagal Afferents (Cranial Nerve X): Carry information from lung stretch receptors, baroreceptors, and peripheral chemoreceptors.

  • Glossopharyngeal Afferents (Cranial Nerve IX): Carry information from peripheral chemoreceptors and baroreceptors.

• Intrinsic Rhythm Generation: The DRG is the source of the basic rhythm of breathing. This rhythmic discharge occurs even if the area is completely denervated, suggesting an internal medullary network that provides excitatory input followed by sudden cessation.

• The Inspiratory Ramp Signal: The rhythmic activity takes the form of a "ramp signal."

  • Inhalation Phase: A gradual increase in excitatory input to the inspiratory muscles over a period of approximately $2\,seconds$.

  • Cessation: After the ramp reaches its limit, the excitation stops abruptly.

  • Exhalation Phase: Efferent activity falls to zero, skeletal muscles relax, and passive exhalation occurs over approximately $3\,seconds$.

Regulation of the DRG Ramp Signal

There are two primary methods for adjusting resting respiration via the ramp signal:

1. Regulating the Slope: Increasing or decreasing the rate at which the efferent activity rises. A steeper slope results in faster inhalation, which is utilized during active respiration.

2. Regulating the Upper Limit: Cutting off the inspiratory activity at a lower maximal level. This results in shorter duration and shallower depth of breathing, which typically leads to an increased respiratory rate.

The Ventral Respiratory Group (VRG)

• Anatomical Location: These neurons are located anterior and ventral to the DRG in the Nucleus Ambiguus.

• Function at Rest: During normal resting respiration, VRG neurons remain quiet and uninvolved.

• Function during Heavy Breathing: When respiratory drive is significantly increased, the DRG activates the VRG.

• Role in Exhalation: While some VRG neurons assist with inhalation, their most vital role is the strong stimulation of muscles used for forced exhalation. The VRG serves to reinforce and assist the DRG during heavy metabolic demand.

Pontine Respiratory Centers: Pneumotaxic and Apneustic

The Pneumotaxic Center

• Location: Located in the upper pons within the Nucleus Parabrachialis.

• Function: This center constantly transmits action potentials to the DRG to "switch off" the inspiratory ramp signal.

• Effect on Breathing: By limiting the duration of the filling cycle (inhalation), it limits tidal volume. Because it shortens the overall respiratory cycle, it effectively increases the rate of respiration.

• Analogy: The function is similar to a dog panting (fast, shallow breaths).

The Apneustic Center

• Location: Located in the lower pons.

• Clinical Significance: its activity is usually masked and only becomes apparent after a vagal transection (cutting the vagus nerve). It does not play a major role in normal healthy breathing.

• Effect: When activated (often in certain disease states), it causes sustained deep inhalation followed by gasping or panting.

The Hering-Breuer Reflex

The Hering-Breuer reflex is a protective mechanism designed to prevent the over-inflation of the lungs.

• Mechanism: Stretch receptors are located in the walls of the bronchi and bronchioles. As the lungs inflate, these receptors increase the frequency of action potential generation.

• Pathway: Signals are sent via Vagal (X) afferents to the DRG.

• Result: The input inhibits further DRG activity, stopping inhalation.

• Human Threshold: In humans, this reflex is only triggered when tidal volume exceeds $1.5\,liters$ (roughly three times the normal resting tidal volume).

Central Chemoreceptors

Central chemoreceptors monitor the chemical composition of the blood to ensure homeostatic levels of gases and pH.

• Sensitivity: These receptors are highly sensitive to changes in arterial $P_{CO2}$ and hydrogen ion concentration ($H^+$). They do NOT monitor $P_{O2}$.

• Mechanism of Stimulation: When chemosensitive neurons detect hypercapnia (high $CO_2$) or acidosis, they provide strong excitatory input to the DRG to increase ventilation.

• Acute vs. Chronic Response:

  • Acute: A very strong stimulus. Arterial $P_{CO2}$ rising from $40\,mmHg$ to $50\,mmHg$ can increase ventilation by approximately $4\text{-fold}$.

  • Chronic: The effect gradually decreases over $1\text{-}2\,days$, dropping to roughly $20\%$ of the initial response. This is due to the kidneys clearing hydrogen ions to compensate for the pH shift.

Peripheral Chemoreceptors

• Location: Found in the Carotid Bodies (at the bifurcation of the common carotid arteries) and Aortic Bodies (within the aortic arch).

• Sensitivities: They respond to a drop in arterial $P_{O2}$, a rise in $P_{CO2}$, or a drop in pH.

• Primary Stimulus: They are primarily responsive to low $P_{O2}$, particularly in the range of $60\text{-}30\,mmHg$.

• Significance in Hypercapnia: Although less sensitive to $CO_2$ than central chemoreceptors, the peripheral response is much faster. They provide the initial rapid response to high $CO_2$ before the central chemoreceptors produce a more profound response.

• Pathways:

  • Carotid Bodies: Signal via the Glossopharyngeal nerve (Cranial Nerve IX).

  • Aortic Bodies: Signal via the Vagus nerve (Cranial Nerve X).

Respiratory Regulation During Exercise

During exercise, oxygen consumption and carbon dioxide production can increase up to $20\text{-fold}$. Paradoxically, arterial blood gases ($P_{O2}$, $P_{CO2}$, and pH) remain at resting levels.

Because chemoreceptors are located on the systemic arterial circulation, they do not sense changes that have not yet occurred in the arterial blood. The increase in ventilation is driven by other mechanisms:

1. Collateral Motor Impulses: The primary and secondary motor cortices sending signals to muscles also send collateral branches to the DRG, stimulating respiration in anticipation of increased demand.

2. Joint Receptors: Body movement stimulates joint receptor afferents, which send collaterals to the respiratory center to increase ventilation.

3. Local Muscle Hypoxia: It is hypothesized that hypoxia in contracting muscles may send sensory impulses back to the respiratory center.

4. Learned Cortical Response: The subconscious mind (especially in trained athletes) increases respiration just by the thought of exercise. This reflects the direct influence the cerebral cortex has over the brainstem respiratory centers.

Clinical Correlation: CNS and Respiratory Depressants

Many substances act as CNS depressants, effectively inhibiting the respiratory control centers in the brainstem. If the depression is severe enough, it can lead to total cessation of breathing and death.

• Examples of Respiratory Depressants:

  • Anesthetics: Early anesthetics like pentobarbital (no longer commonly used for this reason).

  • Opioids: Morphine and Heroin. Overdose of Heroin is a leading cause of death due to total respiratory inhibition.

  • Alcohol: Excessive consumption acts as a potent CNS inhibitor.

• Anesthesia Precaution: Modern anesthetics are carefully chosen to avoid the potent inhibition of respiratory control seen in historical compounds.