Respiration 3

• Control of respiration


  Respiration is regulated to meet the metabolic demands for delivery of O2 and removal of CO2

  
• Components of the respiratory center in the CNS

  • Dorsal respiratory group (DRG) of neurons
  • Ventral respiratory group (VRG) of neurons
  • Apneustic centre
  • Pneumotaxic centre

  
• Dorsal Respiratory Group


  • Site: Dorsal region of medulla, Nucleus tractus solitarius
  • Role: Inspiration; inspiratory neurons, Set rhythm of breathing
  • Mechanism: Contains motor neurons which supply the diaphragm and neurons which inputs to VRG, Inspiratory ramp signals
  • Inhibited by pneumotaxic centre

  
• Ventral Respiratory Group


  • Site: Ventral region of medulla, Rostral nucleus retrofacialis, caudal nucleus retroambiguus and nucleus para-ambiguus
  • Role: Inspiratory and expiratory neurons, Remains inactive during quite breathing, Active when demand is high, Involved in forced inspiration and expiration
  • Mechanism: Contains motor neurons which supply the accessory inspiratory muscles, Contains motor neurons with inputs to the expiratory muscles

  
• Apneustic Centre


  • Site: Caudal pons
  • Role: Excitation of medullary inspiratory area, Believed to be associated with deep inspirations (apneusis)
  • Mechanism: Prolong inspiratory ramp; sustained discharge of medullary respiratory neurons

  
• Pneumotaxic Centre


  • Site: Rostral pons
  • Role: Inhibits inspiration; initiates the termination of inspiration and hence facilitate expiration
  • Mechanism: "Switching off" inspiratory ramp; acts as an off switch to terminate inhalation
• Other respiratory centres


  • Cerebral cortex: Conscious override of autonomous control of breathing
  • Hypothalamus and limbic system: Override the breathing cycle during fear or excitement

  
• Pulmonary and airway receptors

  • Pulmonary stretch receptors
  • Irritant receptors
  • Juxtacapillary receptors
  • Muscle spindle
• Pulmonary stretch receptors


  • Nerve endings associated with smooth muscle in the trachea and bronchi stimulated by stretch e.g. during lung inflation
  • Firing rates from these receptors increase progressively as the lung inflates; are thought to be responsible for the inhibition of breathing caused by lung inflation (Hering-Breuer reflex)
• Irritant receptors (rapidly adapting stretch receptors)


  • Myelinated nerve endings branching among epithelial cells in the larynx, trachea, large bronchi and intrapulmonary airways
  • Stimulated by mechanical deformation of the airways, such as during irritation of the airway surface
  • Stimuli: Irritant gases, dusts, mucus accumulations, histamine release
  • Stimulation of these receptors leads to cough, bronchoconstriction, mucus secretion, and rapid, shallow breathing (hyperpnea) - protective responses

  
• C fibers (juxtacapillary receptors)

  • Located in the pulmonary interstitium close to pulmonary capillaries
  • Monitor blood composition or the degree of distention of the interstitium
  • C-fiber activation may be responsible for the increase in respiratory rate (tachypnea) that accompanies allergic, infectious or vascular diseases

  
• Muscle receptors


  • Density varies in different respiratory muscles
  • Diaphragm - few muscle receptors
  • Intercostal muscles - well supplied with tendon organs and muscle spindles
  • Control (reflexively) the strength of respiratory muscle contraction and adjust when ventilation is impeded, e.g. airway obstruction

  
• Chemoreceptors


  • Peripheral chemoreceptors: Carotid and aortic bodies
  • Central chemoreceptors: Found in the medulla
• Peripheral chemoreceptors


  • Respond to decreased PO2, increased PCO2 (mainly due to H+ ions) and increased H+ ions

  
• Central chemoreceptors


  • Respond to increased H+ concentrations (due to increased PCO2) in the interstitial fluid of the brain
  • H+ ions do not cross the blood brain barrier

  
• Ascent to high altitude

  • Accompanied by a decrease in inspired oxygen tension and consequently by hypoxemia, which leads to an increase in ventilation
  • Acute hypoxia causes hyperventilation mediated through activation of the peripheral chemoreceptors
  • Longer-term adjustment to hypoxia involves production of more erythrocytes (under the influence of erythropoietin), decreased affinity of hemoglobin for oxygen (because of increased concentrations of 2,3-DPG) and increased capillary density in muscle

  
• Respiratory cycle


  • Single cycle of inhalation and exhalation
  • Amount of air moved in one cycle (tidal volume)

  
• Neural mechanisms underlying the events of the respiratory cycle during eupnea

  Involves neurons in the medulla and pons

  
• Inspiratory phase of cycle

  Activation of the inspiratory neurons of the DRG and the VRG
• Expiratory phase of cycle

  1. Lung inflation causes activation of pulmonary stretch receptors

  2. Activation of the pneumotaxic centre and inhibition of inspiratory neurons

  3. Relaxation of inspiratory muscles

  4. Elastic recoil results in exhalation

  
• Hypoxia


  Low oxygen levels in tissues
• Body's response to hypoxia

  1. Systemic vasodilation

  2. Increased perfusion

  3. Lungs – hypoxic pulmonary vasoconstriction, Protective reflex to redirect blood flow from poorly-ventilated lung regions to well-ventilated lung regions

  4. Increased ventilation

  
• Acidosis


  Acidosis is caused by an overproduction of acid that builds up in the blood or an excessive loss of bicarbonate from the blood (metabolic acidosis) or by a buildup of carbon dioxide in the blood that results from poor lung function or depressed breathing (respiratory acidosis)

  
• Body's response to acidosis

  1. Central chemoreceptors detect the drop in blood pH, stimulating faster and deeper breathing (respiratory compensation)

  2. Increases the amount of carbon dioxide exhaled, which raises the blood pH back toward normal

  
• Hypercapnia


  A buildup of carbon dioxide in the bloodstream
• Body's response to hypercapnia

  1. Central chemoreceptors sense hypercapnia and signal the respiratory centre to increase breathing depth

  2. Respiratory pattern: Deep, slow breaths, with a relatively more significant increase in tidal volume than respiratory rate

  3. This pattern aims to optimize carbon dioxide elimination