Question 16

College Answer

This is a core topic and a detailed knowledge was expected. Baroreceptors are stretch receptors 
located in the walls of the heart and blood vessels and are important in the short term control of 
blood pressure. Those in the carotid sinus and aortic arch monitor the arterial circulation. Others, 
the cardiopulmonary baroreceptors, are located in the walls of the right and left atria, the 
pulmonary veins and the pulmonary circulation. They are all stimulated by distention and 
discharge at an increased rate when the pressure in these structures rises. Better answers 
provided some detail on the innervation for these receptors. It was expected candidates would 
describe that increased baroreceptor discharge inhibits the tonic discharge of sympathetic 
nerves and excites the vagal innervation of the heart. This results in vasodilation, venodilation, a 
drop in blood pressure, bradycardia and a decreased cardiac output. 
Some candidates had a major misunderstanding around the purpose of "low pressure 
baroreceptors" with many believing that these are the ones that respond to lower blood 
pressures, while the "high pressure baroreceptors" respond to higher blood pressures

Discussion

The precise focus of the quetion is somewhat unclear here, as the college clearly mention the atrial stretch receptors (suaully associated with the Bainbridge reflex), but spend most of their time discussing the conventional arterial baroreceptor function. Echoing this confusion, the following suggested answer mentions these "low pressure" receptors once, and never returns to them. 

• 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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