The arterial baroreceptor reflex responds to acute hypertension. The afferent limb of this reflex carries signals via cranial nerves IX and X to the nucleus tractus solitarius (NTS). From NTS, the efferent limb ultimately achieves vasodilation via inhibition of which rostral medullary nucleus?

• A Nucleus ambiguus (NA) — increasing cardiac vagal tone only
• B Rostral ventrolateral medulla (RVLM) — the primary source of tonic sympathetic vasoconstrictor drive ✓
• C Caudal ventrolateral medulla (CVLM) — which normally inhibits RVLM
• D Parabrachial nucleus — which relays to the hypothalamus for hormonal vasodilation

Explanation

The RVLM (rostral ventrolateral medulla) is the primary vasomotor center maintaining tonic sympathetic outflow to vascular smooth muscle. NTS activation during baroreceptor stimulation excites the CVLM via GABAergic neurons, but the net effect is inhibition of RVLM. Reduced RVLM activity decreases sympathetic vasoconstrictor tone → vasodilation and reduced cardiac output → BP lowering. The NTS also directly activates the dorsal motor nucleus of vagus/nucleus ambiguus for increased cardiac vagal tone (bradycardia). The RVLM is a key target for many antihypertensive drugs acting centrally (e.g., rilmenidine via I1-imidazoline receptors).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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