control of breathing

• TWO KEY TASKS OF BREATHING

  1. establish automatic rhythm
  2. adjust the rhythm to accommodate: metabolic (arterial blood gases + pH), mechanical (postural change) and episodic non-ventilatory behaviours (e.g. speaking sniffing, eating)
• • Under normal conditions:

  1. O2- rate of absorption is matched to delivery
  2. CO2- rate of generation is matched to removal

  • Balance achieved by:

  1. changes in blood flow and oxygen delivery --> local
  2. changes in depth and rate of respiration --> central

  • Complexities:

  1. no single pacemaker generating basic rhythm of breathing
  2. no single muscle devoted to the pumping of air
• LOCAL CONTROL OF GAS TRANSPORT

  • PO2 and PCO2 in active tissue

  1. decreased PO2 --> increased O2 delivery
  2. increased PO2 --> vasodilation --> increased blood flow --> increased O2 delivery and CO2 removal

  • LUNG PERFUSION

  1. decreased PO2 --> vasoconstriction --> decreased blood flow
  2. direct blood to areas of higher PO2

  • ALVEOLAR VENTILATION

  1. increased PCO2 --> broncoconstriction --> air flow
  2. direct airflow to area of higher PCO2
• CENTRAL CONTROL OF VENTILATION

  • SENSORS- central + peripheral chemoreceptors and mechanoreceptors
  • CENTRAL CONTROLLER- respiratory centres In the pons and medulla
  • EFFECTORS- muscles of ventilation
  • AIM is to keep arterial PCO2 and PO2 as constant as possible
• SENSORS

  • central chemoreceptors- medulla

  1. change in pH
  2. hypercapnia
  3. no effect of hypoxia

  • peripheral chemoreceptors- aortic and carotid body

  1. hypoxia
  2. hypercapnia
  3. change in pH

  • mechanoreceptors- lung receptors

  1. respond to stretch
  2. rapidly + slowly adapting receptors
  3. c-fibres receptos
• FACTORS INFLUENCING RATE AND DEPTH OF BREATHING

  • changing body demands (e.g exercise)
  • altitude- acute mountain sickness
  • disease
  • changing levels of CO2, H+, O2, in the arterial blood
• CENTRAL CHEMORECEPTROS

  • just beneath the ventral surface of the medulla
  • close to the entry of VIII and XI cranial nerves
  • stimulated by acidic or high PCO2 in the CSF
  • increased PCO2 --> decreased ph --> increased ventilation --> decreased PCO2
  • CO2 crosses the blood-brain barrier as its lipid soluble (CSF is only weakly buffered)
• PERIPHERAL CHEMORECEPTORS

  • detect changes in PO2, PCO2 and pH
  • outside the brain
  • carotid body at bifurcation of carotid arteries
  • innervated by carotid sinus nerve
  • aortic bodies above and below aortic arch
  • innervated bt vagus
• • if PO2 increases, (e.g. breathing oxygen-rich gas mixtures) -->. generates free radicals leading to coma and death
  • if PO2 decreased:

  1. arterial PO2 must drop below 60 mmHg before ventilation is increased
  2. central chemoreceptors switch off
  3. peripheral chemoreceptors increase breathing rate
• MECHANORECPTORS: SLOW ADAPTING- STRETCH RECEPTORS

  • located in visceral pleura, bronchioles and alveoli
  • innervated by fibres of vagus nerve
  • over inflation --> increased discharge --> inhibition of respirate centres
  • response = hiring Breuer reflex
• MECHANORECEPTOR- RAPID ADAPTING- IRRITANT RECEPTOR

  • located in airway epithelia (close to mucosa)
  • noxious gases (smoke/ dust/ cold air) --> increased discharge --> bronchoconstriction
  • shape the ventilatory pattern and protecting the airway
  • initially fire rapidly but then soon decreases their fire rate
  • response to coughing reflex
• MECHANORECEPTOR- C-FIBRES

  • located in alveoli wall (close to capillaries) and conducting airways (bronchial mucosa)
  • chemical/ mechanical stimuli --> increased discharge --> bronchoconstriction + rapid shallow breathing + mucus secretion
  • response is defence mechanism
• RHYTHMICITY CENTRE- medulla

  • controls automatic breathing
  • interacting neurons that fire during inspiration (I neurons) and expiration (E neurons)

  PNEUOTAXIC AND APNEUSTIC CENTRES- pons

  • modifies firing pattern of medullary centres

  
  

  • I neurons in DRG --> regulate activity of phrenic nerve --> sets rhythm and stimulates muscles of quiet inspiration
  • E neruons in VRG --> passive process
  • activation of E neruons inhibit I neurons