Neural Regulation of Respiration: Role of Respiratory Centers

1. Overview of the Respiratory Center

"The respiratory center is composed of several groups of neurons located bilaterally in the medulla oblongata and pons of the brain stem... It is divided into three major collections of neurons: (1) a dorsal respiratory group... which mainly causes inspiration; (2) a ventral respiratory group... which contains inspiratory and expiratory neurons; and (3) the pneumotaxic center... which mainly controls rate and depth of breathing."

• Guyton and Hall Textbook of Medical Physiology

Fig. 5.32 - Brain stem control of breathing. Afferent inputs from peripheral/central chemoreceptors, lung stretch receptors, and muscle/joint receptors reach the DRG (Inspiratory Center). The apneustic center excites (+) and the pneumotaxic center inhibits (-) the inspiratory center. Efferent output travels via the phrenic nerve to the diaphragm. (Costanzo Physiology, 7th Ed.)

2. Medullary Respiratory Centers

A. Dorsal Respiratory Group (DRG) - The "Inspiratory Center"

• Location: Nucleus tractus solitarius (NTS) in the dorsal medulla
• Primary role: Generates the basic rhythm of breathing (inspiratory drive) and sets the frequency of inspiration
• Motor output: Sends signals via the phrenic nerve to the diaphragm
• Sensory inputs it receives:
  • Peripheral chemoreceptors (O₂, CO₂, H⁺) via CN IX (glossopharyngeal) and CN X (vagus)
  • Baroreceptors, gastrointestinal and hepatic receptors via the vagus
  • Lung mechanoreceptors (stretch receptors) via the vagus

1. A period of quiescence (expiratory pause)
2. A gradually increasing burst of action potentials over ~2 seconds (inspiratory ramp) - this progressively increases diaphragm contraction until tidal volume is reached
3. Sudden cessation of the ramp (abrupt switch-off), allowing passive expiration to begin

B. Ventral Respiratory Group (VRG) - Expiratory and Accessory Inspiratory Center

• Location: Ventrolateral medulla (nucleus ambiguus rostrally and nucleus retroambiguus caudally)
• Role: Contains both inspiratory and expiratory neurons; largely silent during quiet breathing (eupnea) since normal expiration is passive
• Activated during: Exercise, forceful breathing, high ventilation demands

1. Pre-Bötzinger complex (rostral VRG): Rhythm generator, projects to DRG and drives the inspiratory ramp
2. Bötzinger complex (rostral to pre-Bötzinger): Contains inhibitory neurons that switch off inspiration and contribute to the transition from inspiration to expiration
3. Caudal VRG: Sends powerful expiratory signals to intercostal and abdominal muscles during active (forced) expiration - acts as an "overdrive" mechanism during heavy exercise

3. Pontine Respiratory Centers

A. Pneumotaxic Center (Pontine Respiratory Group)

• Location: Nucleus parabrachialis in the upper pons
• Role: Turns off (inhibits) inspiration by cutting off the inspiratory ramp signal
  • Limits tidal volume (prevents over-inflation)
  • Secondarily regulates respiratory rate: more frequent switch-off = faster rate
  • A strong pneumotaxic signal produces rapid, shallow breathing
  • A weak pneumotaxic signal allows slower, deeper breaths

B. Apneustic Center

• Location: Lower pons
• Role: Excites the inspiratory center - prolongs the inspiratory ramp by stimulating the DRG, delaying switch-off
• When the apneustic center is stimulated experimentally, the result is apneusis: prolonged inspiratory gasps interrupted by brief expirations
• Under normal conditions, the pneumotaxic center overrides the apneustic center, preventing apneusis

• Apneustic center tries to keep inspiration ON
• Pneumotaxic center turns inspiration OFF
• The balance between them determines respiratory rhythm depth and rate

4. The Inspiratory-Expiratory Switching Mechanism

Quiescence (expiration)
      ↓
DRG fires → Inspiratory ramp begins → Diaphragm contracts → Lungs inflate
      ↓
Pneumotaxic center fires → DRG inhibited → Ramp terminates
      ↓
Expiration (passive elastic recoil) → quiescence restores DRG
      ↓
Cycle repeats (~12-20 times/min at rest)

5. Higher Center Influences

Cerebral Cortex (Voluntary Control)

• The cerebral cortex can temporarily override the automatic brainstem centers
• Allows voluntary hyperventilation (raising RR and TV, reducing PaCO₂, raising pH) - self-limiting because the resulting hypocapnia causes unconsciousness, reverting to automatic control
• Allows voluntary breath-holding (hypoventilation) - limited by rising PaCO₂ and falling PaO₂ creating irresistible drive to breathe

Hypothalamus and Limbic System

• The hypothalamus and limbic system modify breathing in response to emotional states, fever, and thermoregulatory demands
• Anxiety or fear can trigger hyperventilation; deep sleep reduces respiratory drive

6. Chemoreceptor Integration with Respiratory Centers

Central Chemoreceptors

• Located on the ventral surface of the medulla, near CN IX/X exit points, close to the DRG
• Stimulus: Decreased pH of cerebrospinal fluid (CSF)
• Mechanism: CO₂ crosses the blood-brain barrier (BBB) freely; H⁺ and HCO₃⁻ do not. CO₂ enters CSF and is hydrated to H⁺ + HCO₃⁻. The resulting fall in CSF pH is detected by central chemoreceptors, which signal the DRG to increase respiratory rate (hyperventilation)
• Goal: Maintain PaCO₂ within the normal range (35-45 mmHg)
• CO₂/H⁺ is the dominant minute-to-minute controller of breathing

Peripheral Chemoreceptors

• Carotid bodies: Located at bifurcation of common carotid arteries; afferent signals via Hering's nerve → CN IX → DRG
• Aortic bodies: Along aortic arch; afferents via CN X → DRG
• Stimuli: Primarily low PaO₂ (<70 mmHg); also elevated PCO₂ and H⁺
• Role: Respond when O₂ falls dangerously low - serve as the "backup" system for hypoxic drive to breathe
• O₂ has virtually no direct effect on the medullary respiratory center itself - it acts exclusively through peripheral chemoreceptors

7. Reflex Inputs to Respiratory Centers

8. Clinical Significance

• Cheyne-Stokes breathing: Alternating hyperpnea and apnea, seen in heart failure/brain injury; results from instability in the CO₂ feedback loop to medullary centers
• Central apnea: Loss of automatic respiratory drive from medullary dysfunction (stroke, opioids, severe acidosis)
• Ondine's curse (congenital central hypoventilation): Failure of automatic breathing during sleep due to brainstem respiratory center dysfunction
• Apneusis: Sustained inspiratory spasm from pontine lesions (removing pneumotaxic inhibition)
• Opioid-induced respiratory depression: Mu-receptor agonists suppress DRG activity, reducing respiratory rate and tidal volume

Summary Diagram of Regulatory Hierarchy

CEREBRAL CORTEX
    ↓ (voluntary override)
PONS:  Pneumotaxic Center (inhibits DRG) ←→ Apneustic Center (excites DRG)
         ↓
MEDULLA: DRG (Inspiratory Center / rhythm generator)
              ↕ pre-Bötzinger complex
         VRG (Expiratory center - active during exercise)
              ↓
         Phrenic nerve → Diaphragm
         Intercostal nerves → Intercostal muscles

SENSORY INPUTS TO DRG:
- Central chemoreceptors (CO₂/pH via CSF)
- Peripheral chemoreceptors (O₂, CO₂, pH via CN IX/X)
- Lung stretch receptors (Hering-Breuer, via CN X)
- Muscle/joint receptors (exercise drive)

• Guyton and Hall Textbook of Medical Physiology - Chapter 42: Regulation of Respiration
• Costanzo Physiology, 7th Edition - Chapter 5: Respiratory Physiology (pp. 239-241)