Baroreflex 1174240 219471529 2008-06-15T11:39:47Z Pedvi 3403244 /* Anatomy of the reflex */ In [[cardiovascular physiology]], the '''baroreflex''' or '''baroreceptor reflex''' is one of the body's [[homeostasis|homeostatic]] mechanisms for maintaining [[blood pressure]]. It provides a [[negative feedback loop]] in which an elevated blood pressure reflexively causes blood pressure to decrease; similarly, decreased blood pressure depresses the baroreflex, causing blood pressure to rise. The system relies on specialized [[neuron]]s ([[baroreceptor]]s) in the [[aortic arch]], [[carotid sinus]]es, and elsewhere to monitor changes in blood pressure and relay them to the [[brainstem]]. Subsequent changes in blood pressure are mediated by the [[autonomic nervous system]]. ==Anatomy of the reflex== Baroreceptors include those in the auricles of the [[heart]] and [[vena cavae]], but the most sensitive baroreceptors are in the [[carotid sinus|carotid sinuses]] and [[aortic arch]]. The [[carotid sinus]] [[baroreceptors]] are innervated by the [[glossopharyngeal nerve]] (CN IX); the aortic arch baroreceptors are innervated by the [[vagus nerve]] (CN X). Baroreceptor activity travels along these nerves, which contact the [[nucleus of the solitary tract]] (NTS) in the brainstem. The NTS sends excitatory fibers ([[glutamate|glutamatergic]]) to the [[caudal ventrolateral medulla]] (CVLM), thus activating the CVLM. The activated CVLM then sends inhibitory fibers (GABAergic) to the [[rostral ventrolateral medulla]] (RVLM), thus inhibiting the RVLM. The RVLM is the primary regulator of [[sympathetic nervous system]], sending excitatory fibers ([[catecholamine|catecholaminergic]]) to the [[sympathetic nervous system|sympathetic]] preganglionic neurons in the spinal cord. Hence, when the baroreceptors are activated (by an increased blood pressure), the NTS activates the CVLM, which in turn inhibits the RVLM, thus inhibiting the [[sympathetic nervous system|sympathetic]] branch of the autonomic nervous system leading to a decrease in blood pressure. Likewise, low blood pressure causes an increase in [[sympathetic nervous system|sympathetic]] tone via "disinhibition" (less inhibition, hence activation) of the RVLM. The NTS also sends excitatory fibers to the [[Nucleus ambiguus]] (vagal nuclei) that regulate the [[parasympathetic nervous system]], aiding in the decrease in sympathetic activity during conditions of elevated blood pressure. ==Baroreceptor activation== The [[baroreceptors]] are [[stretching|stretch]]-sensitive [[mechanoreceptor]]s. When blood pressure rises, the carotid and aortic sinuses are distended, resulting in stretch and therefore activation of the baroreceptors. Active baroreceptors fire [[action potential]]s ("spikes") more frequently than inactive baroreceptors. The greater the stretch, the more rapidly baroreceptors fire action potentials. These action potentials are relayed to the nucleus of the [[Solitary nucleus|tractus solitarius]] (NTS), which uses frequency as a measure of blood pressure. As discussed previously, increased activation of the NTS inhibits the vasomotor center and stimulates the [[vagal]] nuclei. The end result of baroreceptor activation is inhibition of the [[sympathetic nervous system]] and activation of the [[parasympathetic nervous system]]. The [[sympathetic nervous system|sympathetic]] and [[parasympathetic]] branches of the [[autonomic nervous system]] have opposing effects on blood pressure. Sympathetic activation leads to an elevation of [[total peripheral resistance]] and [[cardiac output]] via increased [[contractility]] of the heart, [[heart rate]], and arterial [[vasoconstriction]], which tends to increase blood pressure. Conversely, [[parasympathetic]] activation leads to a decreased [[cardiac output]] via decrease in [[contractility]] and heart rate, resulting in a tendency to decrease blood pressure. By coupling [[sympathetic nervous system|sympathetic]] inhibition and [[parasympathetic]] activation, the baroreflex maximizes blood pressure reduction. [[sympathetic nervous system|Sympathetic]] inhibition leads to a drop in peripheral resistance, while parasympathetic activation leads to a depressed heart rate and contractility. The combined effects will dramatically decrease blood pressure. Similarly, [[sympathetic nervous system|sympathetic]] activation with [[parasympathetic]] inhibition allows the baroreflex to elevate blood pressure. ==Set point and tonic activation== [[Baroreceptors]] are active above the baroreceptor [[set point]] at [[mean arterial pressure]]s (MAP) above approximately 70 mm Hg. When MAP falls below the set point, baroreceptors are almost silent. The [[baroreceptor]] [[set point]] is not fixed; its value may change with changes in blood pressure or physiological situation. For example, in [[hypertension]], the set point will increase; on the other hand, [[hypotension]] will result in a depression of the [[baroreceptor]] [[set point]]. [[Exercise]] also increases the set point, although unlike [[hypertension]] the change is much more [[acute]]. At a MAP below approximately 50 mm Hg, baroreceptors are completely silent. ==Effect on heart rate variability== The baroreflex may be responsible for a part of the low-frequency component of [[heart rate variability]], the so called [[Mayer waves]], at 0.1 Hz [Sleight, 1995]. ==See also== *[[Autonomic nervous system]] *[[Baroreceptor]] *[[Blood pressure]] *[[Heart rate turbulence]] ==References== * {{cite book | author=Berne, Robert M., Levy, Matthew N. | year=2001 | title=Cardiovascular Physiology | publisher=Mosby | location=Philadelphia, PA | ID=ISBN 0-323-01127-6}} * {{cite book | author=Boron, Walter F., Boulpaep, Emile L. | year=2005 | title=Medical Physiology: A Cellular and Molecular Approach | publisher=Elsevier/Saunders | location=Philadelphia, PA | ID=ISBN 1-4160-2328-3}} * {{citation |last=Sleight |first=P. |coauthors=M.T. La Rovere, A. Mortara, G. Pinna, R. Maestri, S. Leuzzi, B. Bianchini, L. Tavazzi, L. Bernardi |title=Physiology and pathophysiology of heart rate and blood pressure variability in humans. Is power spectral analysis largely an index of baroreflex gain? |journal=Clinical Science, |volume=88 |year=1995 |pages=103-109}} {{Cardiovascular physiology}} {{Reflex}} [[Category:Autonomic nervous system]] [[Category:Reflexes]]