Baroreceptor Reflex
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Direct answer
The baroreceptor reflex is the moment-to-moment negative feedback control of arterial pressure by stretch receptors in the carotid sinus and the aortic arch. A rise in pressure increases receptor firing, which travels through the glossopharyngeal and vagus nerves to the nucleus tractus solitarius in the medulla, where it activates vagal outflow and suppresses sympathetic outflow, producing bradycardia and vasodilatation; a fall in pressure produces the opposite response.
What you must remember
- Receptors: bare nerve endings in the walls of the carotid sinus, at the bifurcation of the common carotid artery, and in the wall of the aortic arch; they respond to stretch, not to pressure itself.
- Afferents: from the carotid sinus through the nerve of Hering, a branch of the glossopharyngeal nerve, and from the aortic arch through the vagus nerve; both converge on the nucleus tractus solitarius.
- Central connections: the nucleus tractus solitarius excites the vagal centres (nucleus ambiguus and dorsal motor nucleus) and inhibits the vasomotor centre of the rostral ventrolateral medulla.
- Response to a pressure rise: increased firing, vagal stimulation and sympathetic inhibition give bradycardia, reduced contractility, arteriolar and venous dilatation — lowering cardiac output and total peripheral resistance.
- Response to a pressure fall: reduced firing releases the vasomotor centre, causing tachycardia and vasoconstriction — the basis of the compensatory response to standing and to haemorrhage.
- Properties: the receptors are called buffer nerves, respond more to phasic than to steady stretch, and reset to a new operating pressure within one to two days, so they cannot set the long-term pressure level.
- Clinical: orthostatic intolerance when the reflex fails, as in autonomic neuropathy; carotid sinus hypersensitivity causing syncope, sometimes treated by carotid sinus massage or pacing.
Common confusion
Students often claim the reflex causes hypertension or controls it long term. It only buffers fluctuations around whatever set point exists, and because it resets within days, sustained hypertension proceeds unopposed by it — long-term control belongs to the kidney. The nerve of each receptor is also swapped: the carotid sinus afferents travel in the glossopharyngeal nerve, the aortic arch afferents in the vagus. Finally, the receptors sense wall stretch, so a rigid arteriosclerotic wall dampens the reflex in the elderly.
Exam-focused takeaway
In theory, draw the reflex arc from receptor to effector with neurotransmitters at each level, describe the response to a sudden rise and a sudden fall, and explain resetting and the buffer nerve concept. In viva, expect what happens on standing up, why the reflex fails in autonomic neuropathy, and the difference between the baroreflex and the chemoreceptor response. In practicals and OSCE, demonstrate a standing test or head-up tilt with heart rate and blood pressure, and explain carotid sinus massage — never performing it on a volunteer with a bruit.
Frequently asked questions
Where are the baroreceptors located and which nerves carry their signals?
In the carotid sinus at the common carotid bifurcation and in the aortic arch. Carotid fibres reach the brainstem through the nerve of Hering in the glossopharyngeal nerve, aortic fibres through the vagus.
What happens in the baroreceptor reflex when blood pressure rises?
Stretch increases firing to the nucleus tractus solitarius, which boosts vagal activity and cuts sympathetic drive. The result is bradycardia, reduced contractility and vasodilatation, bringing the pressure down.
What is resetting of baroreceptors?
Within one to two days of a sustained change, the receptors shift their curve and fire as if the new pressure were normal, so chronic hypertension goes uncorrected.
How does the reflex respond to standing up?
Pooling in the legs drops venous return and pressure, baroreceptor firing falls, and sympathetic activation produces tachycardia and vasoconstriction. Failure of this response causes postural hypotension and syncope.