Breathing

Question 1

what are the respiratory control functions

Answer 1

• the respiratory control system is remarkably complex and incompletely understood
• the respiratory control system must be able to regulate:
  
  • blood-gas tensions and acid-base balance (alveolar ventilation)
  • speech and breath-holding
  • airway defence (cough, swallow)

Question 2

what are the basic elements of the respiratory control system

Answer 2

• central controller
  
  • pons, medulla, other parts of brain
• effectors
  
  • respiratory muscles
• sensors
  
  • chemoreceptors, lung and other receptors

Question 3

what is Gordon’s favourite equation

Answer 3

• to understand the function and operation of a biological neural control system, it is important to define the primary regulated variable
  
  • in the case of breathing, the primary regulated variable is arterial CO2 pressure (PaCO2)
  • the CO2 mass balance equation explains how the respiratory control system operates with regard to PaCO2 homeostasis
    -PaCO2 = (RxTxVCO2)/(Va)
  • R = gas constant 2.785
  • T = temperature 273+C
  • R and T are essentially a constant of 863

Question 4

what is PaCO2 regulation in terms of feedforward and feedback control

Answer 4

• feedback is PaCO2 regulation by closed-loop negative chemo feedback
• feedforward is PaCO2 regulation by commands that translate goals, targets and information about potential disturbances, independent of chemoreception
• however, the respiratory control system is not a fixed system

Question 5

what is adaptive control in PaCO2 regulation

Answer 5

• unlike the moment to moment regulation offered by feedforward and feedback control, adaptive control is PaCO2 regulation by long-lasting modifications to the control system (e.g. neuroplasticity)
  
  • occurs at multiple levels of respiratory control

Question 6

what is respiratory neuroplasticity

Answer 6

• “a persistent change in the neural control system based on experiences”
• this change in neural control behaviour is essential to ensure PaCO2 homeostasis in the face of recurrent or enduring perturbations (e.g. injury / disease)

Question 7

what is the generation of inspiratory rhythm (pre-Botzinger complex)

Answer 7

• upper pons must include an inspiratory off-switch
• rhythm is independent of the pons
• rhythm must occur within the medulla
• recordings directly from thin sections of the medulla demonstrated the presence of pacemaker cells now known as the pre-botzinger complex

Question 8

what is the ventral respiratory group (VRG)

Answer 8

• the VRG is split into rostral (rVRG) and caudal (cVRG) aspects
  
  • rVRG = inspiratory (diaphragm and external intercostals)
  • cVRG = expiratory (abdominal and internal intercostals)

Question 9

what is the dorsal respiratory group (DRG)

Answer 9

• the DRG is a small collection of inspiratory (predominantly diaphragm) premotor neurons located in the causal NTS
  
  • the NTS integrates sensorimotor information, receiving input from CN IX, X and XII

Question 10

what is the pontine respiratory group (PRG)

Answer 10

• the PRG contains the kolliker-fuse (KF) nucleus and parabrachial (PB) nucleus
  
  • these neurons inhibit inspiration and prolong expiration
  • responsible for the Hering-Brewer lung inflation reflex

Question 11

what is involved in inspiration

Answer 11

• sternocleidomastoids
• scalenes
• external intercostals
• diaphragm

Question 12

what is involved in expiration

Answer 12

• internal intercostals
• external obliques
• rectus abdominis
• transverse abdominis
• internal obliques

Question 13

what is the difference between voluntary and automatic breathing

Answer 13

• breathing is often mistaken as “automatic”
  
  • breathing is more akin to somatic processes and is largely an automatic behaviour
• phrenic motor neurons receive input from the primary motor cortex and from the brainstem
• directs inputs from the cortex to allow for voluntary control of breathing
  
  • direct inputs from the brainstem allow for automatic control of breathing
  • a relay from the cortex to brainstem exists via corticobulbar tracts and allows the cortex to override automatic breathing (e.g. speech)

Question 14

what is cortical / voluntary control

Answer 14

• breathing is under both automatic and voluntary control
  
  • the cortex can override automatic processes
• examples of voluntary behaviours include:
  
  • speech / swallow, singing, sniffing, coughing, spirometry tests, breath-holding
• deep sea divers lower PaCO2 via hyperventilation prior to breath-holding
  
  • hyperventilation halves PaCO2 resulting in alkalosis
  • urge to breathe during a breath-hold occurs around 50mmttg PaCO2

Question 15

what are the central chemoreceptors (Pco2 sensors)

Answer 15

• central chemoreceptors are located primarily in the ventral surface of the medulla, known as the retro trapezoid nucleus
• H+ does not cross the blood-brain-barrier (BBB) - CO2 diffuses through into the cerebral spinal fluid (CSF)
• CO2 binds with H2) to form carbonic acid
• carbonic acid dissociates into bicarbonate and H+
• changes in CSF CO2 / pH activate central chemoreceptors

Question 16

what are the peripheral chemoreceptors (Po2 sensors)

Answer 16

• low PaO2 (<60Torr) is detected by an unknown receptor on glamus (type 1) cells
• K+ channels close
• cell is depolarised, which open Ca2+ channels
• Ca2+ enteres cell, causing release of ATP and acetylcholine (ACh)
• afferent neuron (carotid sinus nerve) is activated
• activates respiratory control centers to increase breathing

Question 17

how are the central and peripheral chemoreceptors interdependent

Answer 17

• the central and peripheral chemoreceptors do not work independently
• isolating the carotid body from the systemic and cerebral circulation
• CB stimulation (hypoxia and hypercapnia) = increased controller gain
• CB inhibition (hyperoxia and hypercapnia) = decreased controller gain