Respiratory Physiology: Control of Breathing

Question 1

Basic Elements of a Control System

Answer 1

Question 2

Name the sensors

Answer 2

• Central chemoreceptors
  
  • Rostral and caudal
  • ventral surface of the medulla
• Peripheral chemoreceptors
  
  • Carotid Bodies – at the bifurcation of the common carotid arteries
  • Aortic Bodies – located above and below the aortic arch
• Lung reflexes
  
  • Pulmonary stretch receptors
  • Irritant receptors
  • Juxta -capillary receptors
  • Nose and upper airway receptors
  • Joint and muscle proprio receptors
  • Muscle spindles
  • Arterial baroreceptors
  • Pain and temperature receptors

Question 3

Respiratory centre - transections

Answer 3

• Transection 1
  
  • breathing pattern is essentially normal
  • Vagotomy removes afferent input from stretch receptors
  • inspiration is enhanced because the Hering-Breuer response is abolished
• Transection II
  
  • Above apneustic centre
  • depth of breathing increases
  • no upper pons to send signals to terminate inspiration
  • Vagi cut -> remove peripheral signals to inhibit insiration -> apneustic breathing
• Transection III
  
  • above medullary centre
  • breathing is maintained (irregular)
  • Vagotomy has little effect (CPG below)
• Transection IV
  
  • below medullary centre
  • Breathing stops

Question 4

The Dorsal Respiratory Group (DRG)

Answer 4

• located in the floor of the 4th ventricle
• near the tractus solitarius where sensory afferents from cranial nerves IX and X terminate
• inspiratory neurones (phrenic and intercostal motor neurones)
• UMN to the anterior horn cells on the opposite side of the spinal cord
• timing of the respiratory cycle

Question 5

Ventral Respiratory Group (VRG)

Answer 5

• four nuclei

1. Botzinger’s Complex: nucleus retrofacialis (pons); expiration
2. Nucleus para-ambigualis: controls inspiratoy muscles on opposite side of body
3. Nucleus ambiguus: controls the dilator function of the larynx, pharynx and tongue on the same side of the body
4. Nucleus retro-ambigualis: most caudal nucleus, predominantly expiratory; UMN pass to expiratory muscles on opposite side of body

Question 6

Pneumoaxic Centre (PRG)

Apneustic Centre

Answer 6

• PRG:
  
  • acts as an ‘off-switch’ for inspiration
  • Stimulation -> earlier termination of inspiration -> higher respiratory frequency reduced tidal volumes
• Apneustic centre - poorly defined area

Question 7

Functional Intergration of DRG & VRG

Answer 7

• Areas with predominantly expiratory activity are shaded in a darker colour
• Fibres that decussate are shown crossing the midline
• Broken lines indicate expiratory pathways that inhibit inspiratory neurones

Question 8

Cellular mechanism of central pattern generation

Answer 8

• combination of potassium and calcium channels are involved (like other pacemaker cells)
• slow membrane depolarization occurs producing a spontaneous discharge
• generates excitatory postsynaptic potentials (EPSPs) in other I.Aug neurones, and a burst of I.Aug neurones depolarize
• intracellular calcium concentrations increase, which in turn activates calcium-dependent potassium channels -> repolarize the membrane and switch the I.Aug respiratory group off

Question 9

Other Connections to the CNS

Answer 9

• Voluntary control (cortical) -> resp MN
• Posture (eg exercise) - cerebellum -> resp MN
• Cough/sneeze (medulla) -> resp MN
• PRG (pontine respiratory group)
  
  • influences medullary respiratory neurones via a multi-synaptic pathway contributing to fine control of rspiratory rhythm
  • ->central pattern generator -> respiratory MN
• Emotion (forebrain) ->limbic system ->reticular foreamen -> CPG and resp MN

Question 10

Chemoreceptors

what types of chemoreceptors are there

what is the most important driving factor

Answer 10

• Central Chemoreceptors: Receptors responsive to P_aO₂ by way of hydrogen ion concentration in CSF
• Peripheral Chemoreceptors: Receptors responsive to P_aO₂, PCO₂ and H⁺ concentrations in the periphery
• most important single driver of ventilation is PaCO2, acting on the central chemoreceptors by altering CSF pH

Question 11

Central chemoreceptors

Location

Answer 11

• antero-lateral (ventral) surface of the medulla
• separate from the VRG neurones
• 200-400 μm below surface of the medulla (vagus and glossopharyngeal nerves also have their origin)
• crossed by the anterior inferior cerebellar artery (AICA)
• two areas (a rostral and a caudal area) on each side of the brainstem
• intermediate zone where the two connect

Question 12

Central Chemoreceptors

Mechanism of Action

Answer 12

• respond to brain extracellular pH
• BBB being permeable to CO2, but not H+ or bicarbonate ions
• CO2 freely diffuses into the brain extracellular fluid
• generates H+ and HCO3- ions
• pH-sensitive enzyme
• Chemoreceptors generate 80% of the minute ventilation response to CO2 in air, and respond to changes in arterial PCO2 relatively slowly (time constant 1-3 min)