Control of Breathing — Ventilatory Control

Respiration

• Normal RR ~ 10 breaths per minute
• Normal tidal volume ~ 500ml/kg (in 70kg male)
• Therefore normal minute ventilation approx 5l/min

Afferents / Sensors

Chemoreceptors

• Central
  Located in retrotrapezoidal nucleus
  • Sensitive to changes in CSF [H⁺]
    • CO₂ readily diffuses across BBB and converted to H⁺ and HCO₃^– via carbonic anhydrase
    • Increased [H⁺] (and therefore CO₂) stimulated respiration
• Peripheral
  Located in aortic bodies (innervated by vagus nerve) and carotid bodies (innervated by glossopharyngeal nerve)
  • Sensitive to PO₂, [H⁺], PCO₂ and blood flow
    • O₂ dependent K⁺ channels
      • Respiration increases as O₂ drops below 50mmHg
  • CO₂
    • Linear increase in respiration as CO₂ increases

Baroreceptors:

• Located in aortic arch and carotid sinuses
  • Responds to stretch
    • As MAP drops → less stretch on vessel walls → increased sympathetic outflow from medullary vasomotor centre → triggers increase in respiration

Pulmonary receptors

• Stretch receptors
  • Increased stretch of pulmonary parenchyma triggers inflation reflex → inhibits inspiration to prevent overdistention
  • Collapse of pulmonary parenchyma triggers deflation reflex → inhibits expiration to prevent atelectasis and loss of FRC
• J fibres
  • Nociceptive mechano-chemoreceptors
    • On stimulation → Bronchospasm, apnoea, bradycardia and hypotension

Others

• Joint and muscle receptors stimulate ventilation
• Pain and temperature sensation can alter ventilation via the cortex and limbic system

Central control of breathing

Central input

• There is input from the hypothalamus and cortex, with the ability of the cortex to override the medulla and bring ventilation under voluntary control

Controller

• Medullary Respiratory Centre
  • Dorsal respiratory group (DRG)
    • Located in and adjacent to Nucleus Tractus Solitarus
    • Associated with inspiration and timing
    • Works as an ‘integrating centre’ with VRG
  • Ventral respiratory group (VRG)
    • Including Pre-Botzinger and Botzinger complex → Central Pattern Generator
    • Associated with control of expiration, airway dilation and central pattern generation
    • sends inhibitory impulses to Apneustic centre
• Pontine respiratory group (PRG)
  • Pneumotaxic centre
    • controls both the rate and the pattern of breathing
    • Sends inhibitory impulses to the inspiratory area
    • Antagonist to apneustic center
    • decreases tidal volume
  • Apneustic centre:
    • sends signals for inspiration for long and deep breaths
    • controls the intensity of breathing and is inhibited by the stretch receptors of the pulmonary muscles at maximum depth of inspiration, or by signals from the pnuemotaxic center
    • increases tidal volume.

• Inspiratory phase:
  • Gradual ramping up of nerve activity – ↑muscle contraction
• Expiratory phase I:
  • Gradual reduction of nerve activity – ↓muscle contraction
• Expiratory phase II:
  • Inspiratory muscles inactive
  • If increased respiratory drive, expiratory muscles are activated

Efferents

• Phrenic nerve (C3, 4, 5) → Innervates diaphragm – Main inspiratory muscle
• Spinal nerves to intercostal muscles (external intercostal → inspiration, internal intercostal → expiration)
• Brachial plexus → Pectoralis major (forced breathing inspiration)
• Accessory muscle → Sternocleidomastoid (forced breathing inspiration)
• Spinal nerves to abdominal muscles (forced expiration)

Mooney / Sakurai / JC 2020

Answer by JC, Mooney, Sakurai