Describe the various rapidly acting cardiac reflexes that influence cardiac function and the mechanisms by which they act.
College Answer
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. It was
expected candidates would include within their answer a mention of the stimulus and how it is sensed,
the reflex arc and the resultant effect. Thus candidates could have mentioned the Baroreceptor
Reflex/Carotid Sinus Reflex, Chemoreceptor, Bainbridge, Cushing, Oculocardiac and Bezold-Jarisch
(involves response to ventricular stimuli, sensed by receptors within the LV wall that trigger vagal
afferent type C fibers and the resultant triad of hypotension, bradycardia, and coronary artery
dilatation) reflexes.
Discussion
- 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