Question 8

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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