The arterial baroreflex

Abstract

The arterial baroreceptor reflex, or baroreflex, is the most important mechanism for moment-to-moment control of arterial blood pressure (ABP). The baroreflex buffers acute fluctuations of ABP that occur during changes in posture, exercise, emotion, and other conditions. Neurologic disorders affecting central or peripheral baroreflex mechanisms may manifest with paroxysmal hypertension, reflex syncope, or neurogenic orthostatic hypotension (OH). Recent insights into the transduction mechanisms, central pathways, and effectors of the baroreflex provide the basis for new pharmacologic approaches to these disorders. FUNCTIONAL ANATOMY AND NEUROCHEM-ISTRY Regulation of ABP and tissue blood flow, both at rest and in response to changes in metabolic activity, posture, exercise, emotional stress, or hypoxia, depends on two groups of mechanisms. Feedforward or "central command" mechanisms evoke cardiovascular changes that are part of generalized adaptive physiologic responses in situations such as stress and exercise. Feedback or reflex mechanisms allow a moment-to-moment control of ABP in response to inputs from peripheral cardiovascular, respiratory, vestibular, and other receptors. The arterial baroreflex is a critical cardiovascular reflex that provides a continuous buffering of acute fluctuations of ABP in situations such as changes in posture, exercise, and emotion.The baroreflex controls the two variables that determine ABP, cardiac output and total peripheral resistance. Activation of the baroreceptors in response to an increase in ABP results in a decrease in the activity of sympathetic nerves innervating the heart, arterioles, and veins and in an increase of the vagal output to the heart. This results a decrease in total peripheral resistance, venous return, heart rate, and cardiac output. In contrast, in response to a reduction of ABP, reduced baroreceptor activity results in reflex sympathoexcitation and inhibition of cardiovagal output, leading to an increase in total peripheral resistance and tachycardia.Central baroreflex arc. The essential pathways and neurochemistry of the arterial baroreflex are well established 1,2 (figure). The arterial baroreceptors are mechanosensitive afferent nerve terminals located in the adventitia of the carotid sinuses (innervated by the glossopharyngeal nerve) and aortic arch (innervated by the vagus nerve). Mechanical deformation of the vessel wall elicited by an increase in ABP leads to the opening of mechanosensitive cation channels resulting in baroreceptor afferent depolarization and increased firing. 3 Mechanotransduction in peripheral baroreceptor afferents involves degenerin/epithelial sodium channels (DegENa), acid-sensing channels (ASIC), and transient receptor potential (TRP) channels and is regulated by several potassium currents and local signals, such as prostacyclin and nitric oxide (NO). 4,5 Primary baroreceptor afferents provide monosynaptic excitatory input to neurons located in the nucleus of the solitary tract (NTS). 6 The primary neurotransmitter is L-glu...

GABAergic Excitation of Vasopressin Neurons

“…Baroreflex is responsible for the overall tone as well as the acute moment-tomoment regulation of blood pressure by modulating cardiac output and total peripheral resistance. 41,42 Under resting conditions, baroreceptors respond to acute increases in arterial pressure by activating the vagal parasympathetic center. They also respond to pressure increases by inhibiting the vasoconstrictive center of the medulla and suppressing the electric activity of AVP neurons through the release of GABA.…”

Baroreflex sensitivity: mechanisms and measurement

“…The inverse response occurs when BP lowers. Moreover, the sympathetic outflow to the whole body is likewise modulated by the baroreflex, thus constantly adjusting vascular resistance [2].…”

Autonomic control of blood circulation in patients undergoing elective carotid endarterectomy

“…When BP lowers, it is the other way around. The baroreflex also constantly adjusts vascular resistance in a similar way: modulating the sympathetic responses in the whole body 2 . This buffering mechanism is one, and maybe the most relevant function of the baroreflex.…”