## Baroreceptor Reflex Mechanism **Key Point:** The baroreceptor reflex is the primary short-term (seconds to minutes) blood pressure regulation mechanism. When baroreceptors in the carotid sinus and aortic arch detect increased pressure, they increase their firing rate. ### Neural Pathway 1. Increased baroreceptor firing → increased afferent input to the nucleus tractus solitarius (NTS) in the medulla 2. NTS activates the dorsal motor nucleus of the vagus nerve 3. Result: **increased parasympathetic (vagal) outflow** to the heart and **decreased sympathetic outflow** to blood vessels and heart ### Hemodynamic Effects | Parameter | Change | Mechanism | |-----------|--------|----------| | Heart rate | ↓ | Increased vagal tone | | Cardiac contractility | ↓ | Decreased sympathetic tone | | Peripheral resistance | ↓ | Decreased sympathetic vasoconstriction | | Blood pressure | ↓ | Combined effect of ↓ CO and ↓ SVR | **High-Yield:** The baroreceptor reflex is **parasympathetic-dominant** in its response to hypertension — increased vagal tone is the hallmark. **Clinical Pearl:** This reflex is why carotid sinus massage (or pressure) can acutely lower blood pressure and heart rate in hypertensive patients. It is also why chronic hypertension causes **baroreceptor resetting** — the set point shifts upward, reducing the reflex's effectiveness over time. **Mnemonic:** **PACED** — **P**ressure ↑ → **A**ctivate baroreceptors → **C**ranial nerves IX & X → **E**nhance parasympathetic tone → **D**ecrease BP.
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