Question 17

Outline the various cardiac reflexes and the mechanisms by which they maintain physiological homeostasis.

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College Answer

This question required candidates to provide an answer that integrates their knowledge of various
aspects of cardiovascular physiology. Cardiac reflexes are fast-acting reflex loops between the
heart and central nervous system that contribute to regulation of cardiac function and maintenance
of physiologic homeostasis. This was often overlooked by many candidates. For a good answer it
was expected that at least the chemo and baroreceptor, Bainbridge (elicited by stretch receptors
located in the right atrial wall and the cavoatrial junction), Cushing (result of cerebral medullary
vasomotor centre ischemia), oculocardic (provoked by pressure applied to the globe of the eye or
traction on the surrounding structures), Bezold-Jarisch (responds to noxious ventricular stimuli
sensed by chemoreceptors and mechanoreceptors within the LV wall) reflexes be mentioned and
described.
Syllabus: C1e, C1c,2g, C1g
References: Millers Anaesthesia Chp 16

Discussion

In this answer, the college examiners again confused the connection between the question they asked and the answer they wanted. For example, there is no way one could possibly describe the oculocardiac reflex as something that "maintains physiological homeostasis", as it is a reflex which can drop you into asystole for 28 seconds. In short, many of the phenomena mentioned by the examiners are reflexes, but only the baroreceptor reflex and Bainbridge reflexes are concerned with the maintenance of homeostasis. Clearly, the real SAQ asked "Describe the various rapidly acting cardiac reflexes that influence cardiac function and the mechanisms by which they act". Or at least, that is what the college asked in the very similar Question 2 from the second paper of 2013, after they had three years to reconsider the stem phrasing. It did not help: in 2013, the pass rate was actually lower (22%).

  • Definition of cardiac reflexes
    • "Reflex loops between the heart and central nervous system" which regulate heart rate and peripheral vascular resistance 
  • Baroreceptor reflex
    • Sensors: pressure (carotid sinus and aortic arch)
    • Afferent: vagus and glossopharyngeal nerves
    • Processor: nucleus of the solitary tract and nucleus ambiguus
    • Efferent: vagus nerve and sympathetic chain
    • Effect: increased HR and BP in response to a fall in BP
  • Bainbridge reflex
    • Afferent: vagus (atrial stretch) 
    • Processor: nucleus of the solitary tract and the caudal ventral medulla
    • Efferent: vagus nerve and sympathetic chain
    • Effect: increased RA pressure produces an increased heart rate;
  • Chemoreceptor reflex
    • Afferent: carotid / aortic chemoreceptors (low PaO2 and/or high PaCO2)
    • Processor: nucleus of the solitary tract and nucleus ambiguus
    • Efferent: vagus nerve and sympathetic chain
    • Effect: bradycardia and hypertension in response to hypoxia
      (also secondary tachycardia from Bainbridge and Hering-Breuer reflexes)
  • Cushing reflex
    • Afferent: mechanosensors in the rostral medulla?
    • Processor: rostral ventrolateral medulla
    • Efferent: sympathetic fibres to the heart and peripheral smooth muscle
    • Effect: hypertension and baroreflex-mediated bradycardia
  • Bezold-Jarisch reflex
    • Afferent: vagus (mechanical/chemical sttimuli to the cardiac chambers)
    • Processor: nucleus of the solitary tract
    • Efferent: vagus nerve and sympathetic chain
    • Effect: hypotension and bradycardia in response to atrial stimulation
  • Oculocardiac reflex
    • ​​​​​​​Afferent: trigeminal nerve (pressure to the globe of the eye) 
    • Processor: sensory nucleus of CN V; nucleus of the solitary tract
    • Efferent: vagus nerve and sympathetic chain
    • Effect: vagal bradycardia, systemic vasoconstriction, cerebral vasodilation
  • Diving reflex
    • ​​​​​​​Afferent: trigeminal nerve (cold temperature; pressure of immersion)
    • Processor: sensory nucleus of CN V; nucleus of the solitary tract
    • Efferent: vagus nerve and sympathetic chain
    • Effect: vagal bradycardia, systemic vasoconstriction, cerebral vasodilation
  • Barcroft-Edholm (vasovagal) reflex
    • ​​​​​​​Afferent: emotional distress, hypovolaemia
    • Processor: unknown
    • Efferent: vagus nerve and sympathetic chain
    • Effect: bradycardia, systemic vasodilation, hypotension
  • Respiratory sinus arrhythmia
    • Afferent: central respiratory pacemaker
    • Processor: nucleus ambiguus
    • Efferent: vagus nerve, via the cardiac ganglion
    • Effect: cyclical increase of heart rate during inspiration

References

Zucker, Irving H., and Joseph P. Gilmore. Reflex control of the circulationCRC Press, 2020.

Hainsworth, Roger. "The physiological approach to cardiovascular reflexes." Clinical Science 91.s1 (1996): 43-49.

Sleight, Peter. "Reflex control of the heart." The American journal of cardiology 44.5 (1979): 889-894.

Mancia, G., et al. "Reflex cardiovascular regulation in humans." Journal of cardiovascular pharmacology 7 (1985): S152-S159.