The baroreceptor reflex is a rapid negative feedback loop that helps maintain blood pressure homeostasis on a moment-to-moment basis by adjusting heart rate, contractility, and vascular tone.
Baroreceptors Location and Function
| Location | Description | Cranial Nerve Involved |
|---|---|---|
| Carotid sinus | At the bifurcation of the common carotid artery (near C3–C4 vertebral level) | Glossopharyngeal nerve (CN IX) |
| Aortic arch | Within the wall of the aortic arch | Vagus nerve (CN X) |
High-Yield :
The carotid sinus is more sensitive to pressure changes and plays a more dominant role in minute-to-minute blood pressure control .
Mechanism of Action
When Mean Arterial Pressure (MAP) Increases:
- Increased arterial stretch → more firing of baroreceptors.
- Afferent signals :
- From carotid sinus via CN IX
- From aortic arch via CN X
- Signal reaches the nucleus tractus solitarius (NTS) in the medulla oblongata .
- The NTS:
- Inhibits sympathetic outflow (↓ HR, ↓ contractility, vasodilation)
- Activates parasympathetic (vagal) output (↑ vagal tone → ↓ HR)
When MAP Decreases:
- Reduced stretch → decreased baroreceptor firing.
- Leads to:
- Disinhibition of sympathetic outflow
- Reduced vagal tone
- Result: ↑ HR, ↑ contractility, vasoconstriction , ↑ TPR → restores BP.
Clinical Correlation
- Carotid massage : Increases stretch → mimics ↑ BP → enhances vagal tone → can treat supraventricular tachycardia (SVT)
- Cushing’s triad : Hypertension, bradycardia, and irregular respirations due to increased ICP → baroreceptor-mediated response to cerebral ischemia
- Orthostatic hypotension : Impaired baroreceptor reflex in elderly or autonomic dysfunction (e.g., diabetes)
High yield Notes
- Baroreceptors are mechanoreceptors (stretch receptors) .
- Reflex operates on a second-to-second basis acute compensation , not long-term regulation (the kidneys handle that via RAAS).
- Cranial Nerve IX (glossopharyngeal) is the afferent pathway from the carotid sinus .
- Cranial Nerve X (vagus) is the afferent pathway from the aortic arch .
