Describe the Various Rapidly Acting Cardiac Reflexes That Influence Cardiac Function and the Mechanisms by Which They Act

2013 Second Sitting Paper 1 Question 2

2013-2-2 Describe the various rapidly acting cardiac reflexes that influence cardiac function and the mechanisms by which they act. (22% Pass) As Per Richard (2010-2-17)

Cardiac reflex: “a fast-acting reflex loop between the heart and central nervous system that contributes to regulation of cardiac function and maintenance of physiologic homeostasis”

1. Chemo receptor reflex ! Sensor ! Carotid body, central chemoreceptors ! Stimulus ! ↓in PaO2, ↑ in PaCO2 ! Response ! Increased SNS stimulation ! Increased HR, SV (via increased contractility), PVR 2. Baroreceptor response (high pressure) ! Sensor ! Arterial baroreceptors ! Aortic arch (CN X) ! Carotid sinus (CN IX) ! Stimulus ! Increased MAP ! Continuous resting tone when MAP >60mmHg ! Response ! Increase SNS stimulation ! Increased HR, SV (via increased contractility), PVR 3. Baroreceptor response (low pressure) ! Sensor ! Atrial pressure sensors ! Stimulus ! Atrial filling ! Atrial contraction ! Response ! Peripheral vasodilation ! Increased HR ! Also renal and hormonal effects to decrease circulating volume 4. Bainbridge reflex ! Sensor ! Atrial stretch receptors – right atrial wall, cavoatrial jucntion ! Stimulus ! Increased atrial blood volume ! Response ! Increased HR 5. Cushing reflex ! Sensor ! Medullary sympathetic cell bodies ! Stimulus ! Compression of cell bodies with increased intracranial pressure ! Response ! Increased SNS stimulation -> increased SVR

Gladwin 2016 2013 Second Sitting Paper 1 Question 2

! Reflex bradycardia ! Via reflex #3 6. Oculocardiac ! Sensor ! Via CN III ! Stimulus ! Ocular compression ! Response ! Vagal stimulation -> decreased HR 7. Bezold-Jarisch reflex ! Sensor ! Chemoreceptors and mechanoreceptors in LV wall ! Stimulus ! Noxious chemical stimuli ! Response ! Increase in parasympathetic outflow ! Bradycardia ! Peripheral vasodilation

Examiner Comments

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.

Gladwin 2016