Baroreceptors (or archaically,pressoreceptors) arestretch receptors that senseblood vesseldeformation. The term "baroreceptors" is somewhat a misnomer, since they detect stretch rather than pressure directly. Increases in vessel diameter triggers increasedaction potential generation rates and provides information to thecentral nervous system. This sensory information is used primarily inautonomic reflexes that in turn influencecardiac output and vascular smooth muscle to influence vascular resistance.[1] Baroreceptors act immediately as part of anegative feedback system called thebaroreflex[2] as soon as there is a change from the usualmean arterial blood pressure, returning the pressure toward a normal level. These reflexes help regulate short-term blood pressure. Thesolitary nucleus in themedulla oblongata of the brain recognizes changes in the firing rate of action potentials from the baroreceptors, and influences cardiac output and systemic vascular resistance.[3]
Baroreceptors can be divided into two categories based on the type of blood vessel in which they are located: arterial baroreceptors and low-pressure baroreceptors (also known as cardiopulmonary[4] or volume receptors[5]).
Arterial baroreceptors are located in thecarotid sinus (at the bifurcation ofcommon carotid artery intoexternal andinternal carotids) and in theaortic arch.[6] The baroreceptors can identify the changes in both the average blood pressure or the rate of change in pressure with each arterial pulse. Action potentials triggered in the baroreceptor ending are then directly conducted to the brainstem where central terminations (synapses) transmit this information to neurons within thesolitary nucleus[7] which lies in the medulla. Reflex responses from such baroreceptor activity can trigger increases or decreases in the heart rate. Arterial baroreceptor sensory endings are simple, splayed nerve endings that lie in thetunica adventitia of the artery. An increase in the mean arterial pressure increasesdepolarization of these sensory endings, which results inaction potentials. These action potentials are conducted to the solitary nucleus in the central nervous system byaxons and have a reflex effect on the cardiovascular system throughautonomic neurons.[8]Hormone secretions that target the heart and blood vessels are affected by the stimulation of baroreceptors.[citation needed]
At normal resting blood pressures, baroreceptors discharge with each heart beat. If blood pressure falls, such as onorthostatic hypotension or inhypovolaemic shock, baroreceptor firing rate decreases and baroreceptor reflexes act to help restore blood pressure by increasing heart rate. Signals from the carotid baroreceptors are sent via theglossopharyngeal nerve (cranial nerve IX). Signals from the aortic baroreceptors travel through thevagus nerve (cranial nerve X).[9] Carotid sinus baroreceptors are responsive to both increases or decreases in arterial pressure, while aortic arch baroreceptors are only responsive to increases in arterial pressure.[7] Arterial baroreceptors inform reflexes about arterial blood pressure but other stretch receptors in the large veins and right atrium convey information about the low pressure parts of the circulatory system.[citation needed]
Baroreceptors respond very quickly to maintain a stable blood pressure, but their responses diminish with time and thus are most effective for conveying short term changes in blood pressure. In people withessential hypertension the baroreceptors and their reflexes change and function to maintain the elevated blood pressure as if normal. The receptors then become less sensitive to change.[10]
Electrical stimulation of baroreceptors has been found to activate thebaroreflex, reducing sympathetic tone throughout the body and thereby reducing blood pressure in patients with resistant hypertension.[11]
The low-pressure baroreceptors are found in large systemicveins, in pulmonary vessels, and in the walls of the right atrium and ventricles of theheart (theatrial volume receptors).[5] The low-pressure baroreceptors are involved with the regulation of blood volume. The blood volume determines the mean pressure throughout the system, in particular in the venous side where most of the blood is held.[citation needed]
The low-pressure baroreceptors have both circulatory and renal effects; they produce changes inhormone secretion, resulting in profound effects on the retention ofsalt and water; they also influence intake of salt and water. The renal effects allow the receptors to change the long-term mean pressure in the system.[citation needed]
Baroreceptors are integral to the body's function, as pressure changes in the blood vessels would not be detected as quickly in the absence of baroreceptors. When baroreceptors are not working, blood pressure continues to increase, but within an hour, the blood pressure returns to normal as other blood pressure regulatory systems take over.[12]
Baroreceptors can also become oversensitive in some people (usually the carotid baroreceptors in older males). This can lead tobradycardia, dizziness andfainting (syncope) from touching the neck (often whilst shaving). This is an important cause to exclude in men having pre-syncope or syncope symptoms.[citation needed]
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