baroreflexes

Baroreflexes are rapid, short-term reflexes that help maintain arterial blood pressure within a normal range. They rely on specialized pressure sensors, the baroreceptors, located mainly in the carotid sinus and the aortic arch. These receptors respond to stretch of the arterial wall, increasing their firing rate when blood pressure rises and reducing firing when pressure falls.

Afferent pathways transmit signals from the carotid sinus via the glossopharyngeal nerve (cranial nerve IX) and from the aortic arch via the vagus nerve (cranial nerve X) to the nucleus tractus solitarius in the medulla oblongata. The nucleus tractus solitarius integrates this information with other autonomic inputs and modulates autonomic output through pathways to the rostral ventrolateral medulla (which drives sympathetic activity) and to parasympathetic nuclei such as the dorsal motor nucleus of the vagus and the nucleus ambiguus. Increases in baroreceptor firing typically produce increased parasympathetic activity and decreased sympathetic outflow, leading to lower heart rate, reduced contractility, and vasodilation; decreases in firing have the opposite effects, tending to raise blood pressure.

Baroreflexes operate on a beat-to-beat basis and can reset with changes in posture or in chronic conditions. They are strongest for short-term regulation and interact with slower mechanisms such as the renin–angiotensin system. Baroreflex sensitivity declines with age and in various diseases, and baroreflex failure can contribute to orthostatic hypotension or labile blood pressure. Clinically, baroreflex function is assessed using methods that quantify the relationship between blood pressure changes and heart rate, including sequence and spectral analyses, and pharmacological or mechanical provocations.