Explain the role of the baroreceptors in the control of blood pressure

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College Answer

Good answers included the following         

  • description of, and types of, baroreceptors (e.g. stretch-receptors),     
  • their locations (e.g. walls of the aorta, carotid sinuses, the atria, etc),  
  • the stimulus they respond to (e.g. pressure, volume),          
  • short term and long term responses,
  • alteration to set points,
  • impulse frequency / pressure curve
  • A brief description of the afferent and efferent pathways and the resultant efferent effects (e.g. alterations to heart rate, blood pressure, etc).

Discussion

  • Description:
    • Baroreceptors are mechanoreceptors which respond to stretch stimuli.
    • This strecth deforms  mechanically sensitive sodium channels (DEG/ENaC, degenerin/epithelial sodium channels)
    • With sufficient stimulus, sodium current increases to the point where the membrane potential reaches the threshold of local voltage-gated sodium channels, and generates a propagating action potential 
  • Location:
    • Arterial baroreceptors ("high pressure baroreceptors") are located at the junction of the intima and media of the aortic arch and carotid sinuses
    • Similar "low pressure" mechanoreceptors are present in the atria, and they mediate the Bainbridge reflex 
  • Stimulus:
    • Increased blood pressure (increased stretch, increased receptor firing rate)
    • Decreased blood pressure (decreased receptor firing rate)
  • Afferent pathway:
    • From the carotid sinus: carotid sinus nerve, a branch of the glossopharyngeal nerve
    • From the aortic arch: aortic nerve, a branch of the vagus nerve
    • Both of these nerves travel through the jugular foramen to enter the medulla 
  • Processor: 
    • Nucleus of the solitary tract receives afferent fibres and redistributes the signal into several efferent regulatory systems:
      • Excitatory glutamate-mediated neurotransmission to the nucleus ambiguus translates the afferent signal into increased vagal activity
      • GABA-ergic inhibitory neurons of the caudal ventral medulla translate the afferent signal into the inhibition of the rostral ventrolateral medulla, which coordinates sympathetic tone
      • Effrent fibres to the hypothalamus help coordinate the humoural response to changes in blood pressure.
  • Efferent nerves: 
    • Sympathetic fibres to the heart and peripheral resistance vessels
    • Vagal efferents to the cardiac ganglion (heart rate)
  • Effector:  Myocardium, SA and AV nodes, vascular smooth muscle
  • Effect:
    • In response to arterial hypotension:
      • Decreased receptor discharge rate
      • Thus, decreased vagal and disinhibited sympathetic efferents
      • Thus, systemic vasoconstriction and tachycardia
    • In response to arterial hypertension:
      • Increased receptor discharge rate
      • Thus, increased vagal and inhibited sympathetic efferents
      • Thus, systemic vasodilation and bradycardia

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