Regulation of Respiration - Chemoreceptors

Recap: Respiratory Control

• Respiratory control: Both voluntary and involuntary
  • Voluntary control: By the cortex
  • Involuntary control: By the brain stem (medulla and pons)
• Medulla
  • Dorsal group: Controls inspiration
  • Ventral group: Controls inspiratory and expiratory neurons (mainly forced expiration)
• Pons
  • Apneustic center: Stimulates the inspiratory center
  • Pneumotaxic center: Inhibits the apneustic center and controls the rate and depth of breathing

Receptors

• Receptors gather information from stimuli (chemical and non-chemical)
  • Non-chemical stimuli: Lung stretch (stretch receptors), proprioception (proprioceptors in muscles/joints)
  • Chemical stimuli: Detected by chemoreceptors

Chemoreceptors

• Types: Central and Peripheral
• Goal: Maintain constant blood oxygen, carbon dioxide, and hydrogen ion concentration

Central Chemoreceptors

• Location: Central nervous system (ventral medulla near the inspiratory center)
• Detect changes in carbon dioxide and hydrogen ion concentration (not oxygen)
• Mechanism
  • CO2 crosses the blood-brain barrier, combines with water to form carbonic acid
  • Carbonic acid dissociates into hydrogen ions and bicarbonate
  • Increased hydrogen ions stimulate the chemosensitive area
  • Increases rate and depth of breathing to normalize CO2 levels
  • Kidneys eventually take over to maintain pH (secrete excess hydrogen ions & reabsorb bicarbonate)

Peripheral Chemoreceptors

• Location: Carotid bodies (at the bifurcation of the carotid artery) and aortic bodies (arch of aorta)
• Afferent Neurons
  • Carotid body: Glossopharyngeal nerve (9th nerve)
  • Aortic body: Vagus nerve (10th nerve)
• Structure: Each body is called a glomus, with type 1 (glomus cells) and type 2 cells (support cells)
• Detection
  • Respond to changes in blood oxygen levels
  • Glomus cells sensitive to low arterial oxygen (below 60 mmHg)
  • Mechanism
    • Low oxygen closes potassium channels in glomus cells
    • Voltage-gated calcium channels open, Ca2+ enters cell
    • Neurotransmitter release stimulates afferent neurons
    • Impulses travel to the dorsal respiratory group, increasing respiration

Other Functions

• Peripheral chemoreceptors also detect changes in arterial CO2 and pH
  • Detection of pH independent of CO2 (important in metabolic acidosis/alkalosis)
  • Carotid bodies play a key role in maintaining acid-base balance

Conclusion

• Central and peripheral chemoreceptors work together to maintain blood gas and pH levels.

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