Neural Control of Breathing

Question 1

Sympathetic Innervation of Airways

Answer 1

• Smooth m. of conducting airways (up & incl. bronchi) is innervated by the ANS
• Sympathetic adrenergic neurons activate:
  
  • Beta2-adrenergic receptors → relaxation and dilation of the airways
    
    • these receptors can also be activated by:
      
      • epinephrine from the adrenal medulla
      • Beta2-adrenergic agonists like albuterol and isoproterenol, which are used in the tx of asthma

Question 2

Parasympathetic Innervation of Airways

Answer 2

• Parasympathetic cholinergic neurons activate muscarinic receptors → contraction and constriction of airways
• Change in the diameter of airways → change in resistance → change in airflow
• Can block this effect using muscarinic antagonists (e.g., atropine, pralidoxime)

Question 3

Control of Normal, Involuntary Breathing

Answer 3

• Medullary Respiratory Center
  
  • Dorsal Respiratory Group (DRG) responsible for inspiration and basic rhythm of breathing
  • Afferents: carotid & aortic bodies, afferents from lung stretch receptors
  • Efferents: CN XI, cervical spinal nn. C5-C8, phrenic n., intercostal nn.,
• Apneuristic Center
  
  • Located in the lower pons
  • Stimulates inspiration
• Pneumotaxic Center
  
  • Located in the upper pons
  • Inhibits inspiration

Question 4

Afferents

Answer 4

• Central Control
  
  • Chemoreceptors/Medullary receptors stimulated by ⇡PaCO2 sensed by a decrease in CSF pH [CO2 freely crosses blood-brain barrier and combines with H2O to form carbonic acid (H2CO3), which dissociates into H+ and HCO3- → ⇡[H+] and ⇣pH]
• Peripheral Control
  
  • Carotid body and aortic body chemoreceptors are stimulated by ⇣PaO2 and ⇣arterial pH (acidosis)
  • The only sites that detect changes in PaO2
  • Carotid body chemoreceptors
    
    • located at common carotid bifurcation
    • send signal to medulla via CN IX
  • Aortic body chemoreceptors
    
    • Located in the aortic arch
    • send signal to medulla via CN X
• Lung Control
  
  • Stretch receptors are stimulated by distension
    
    • Carried by CN X
    • Responsible for Herring-Breurer reflex (apnea after large lung inflations)
  • Receptors in the wall of the bronchi respond to irritants → coughing, sneezing, and breath holding

Question 5

Efferents

Answer 5

• Diaphragm innervated by phrenic n. (C3, C4, C5)
• Internal and external intercostal mm. innervated by their respective intercostal nn. from T1-T12
• SCM m. innervated by CN XI (spinal accessory n.)
• Scaleni m. innervated by C1-C4
• Serratus anterior: C5-C7
• Serratus posterior: T9-T12
• Rectus abdominus: T7-T12
• Transverse abdominus: T6-L1
• Internal oblique: T6-T11
• External oblique: T7-T12