Baroreceptors (or archaically, pressoreceptors) are sensors located in the

carotid sinus In human anatomy, the carotid sinus is a dilated area at the base of the internal carotid artery just superior to the bifurcation of the internal carotid and external carotid at the level of the superior border of thyroid cartilage. The carotid s ...

(at the bifurcation of external and

internal Internal may refer to: *Internality as a concept in behavioural economics *Neijia, internal styles of Chinese martial arts *Neigong or "internal skills", a type of exercise in meditation associated with Daoism *''Internal (album)'' by Safia, 2016 ...

carotids) and in the

aortic arch The aortic arch, arch of the aorta, or transverse aortic arch () is the part of the aorta between the ascending and descending aorta. The arch travels backward, so that it ultimately runs to the left of the trachea. Structure The aorta begins a ...

. They sense the blood pressure and relay the information to the brain, so that a proper blood pressure can be maintained. Baroreceptors are a type of

mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, ...

sensory neuron that are excited by a stretch of the blood vessel. Thus, increases in the pressure of blood vessel triggers increased

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

generation rates and provides information to the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...

. This sensory information is used primarily in autonomic reflexes that in turn influence the heart

cardiac output In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: t ...

and vascular smooth muscle to influence vascular resistance. Baroreceptors act immediately as part of a

negative feedback Negative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by othe ...

system called the

baroreflex The baroreflex or baroreceptor reflex is one of the body's homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels. The baroreflex provides a rapid negative feedback loop in which an elevated blood pressure causes th ...

, as soon as there is a change from the usual mean arterial blood pressure, returning the pressure toward a normal level. These reflexes help regulate short-term blood pressure. The

solitary nucleus In the human brainstem, the solitary nucleus, also called nucleus of the solitary tract, nucleus solitarius, and nucleus tractus solitarii, (SN or NTS) is a series of purely sensory nuclei (clusters of nerve cell bodies) forming a vertical column ...

in the medulla oblongata of the brain recognizes changes in the firing rate of action potentials from the baroreceptors, and influences cardiac output and systemic vascular resistance. Baroreceptors can be divided into two categories based on the type of blood vessel in which they are located: high-pressure arterial baroreceptors and low-pressure baroreceptors (also known as cardiopulmonary or volume receptorsStanfield, CL; Germann, WJ. (2008) Principles of Human Physiology, Pearson Benjamin Cummings. 3rd edition, pp.430-431.).

Arterial baroreceptors

Arterial baroreceptors are stretch receptors that are stimulated by distortion of the arterial wall when pressure changes. 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 the

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 the

tunica adventitia The tunica externa (New Latin "outer coat"), also known as the tunica adventitia (New Latin "additional coat"), is the outermost tunica (layer) of a blood vessel, surrounding the tunica media. It is mainly composed of collagen and, in arteries, ...

of the artery. An increase in the mean arterial pressure increases depolarization of these sensory endings, which results in

action potentials An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ca ...

. These action potentials are conducted to the solitary nucleus in the central nervous system by

axons An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...

and have a reflex effect on the cardiovascular system through autonomic neurons. Hormone secretions that target the heart and blood vessels are affected by the stimulation of baroreceptors. At normal resting blood pressures, baroreceptors discharge with each heart beat. If blood pressure falls, such as on orthostatic hypotension or in

hypovolaemic shock Hypovolemia, also known as volume depletion or volume contraction, is a state of abnormally low extracellular fluid in the body. This may be due to either a loss of both salt and water or a decrease in blood volume. Hypovolemia refers to the loss ...

, 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 the

glossopharyngeal nerve The glossopharyngeal nerve (), also known as the ninth cranial nerve, cranial nerve IX, or simply CN IX, is a cranial nerve that exits the brainstem from the sides of the upper medulla, just anterior (closer to the nose) to the vagus nerve. Bei ...

(

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

IX). Signals from the aortic baroreceptors travel through the vagus nerve (

cranial nerve X The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right v ...

). 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. 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. 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 with

essential hypertension Essential hypertension (also called primary hypertension, or idiopathic hypertension) is the form of hypertension that by definition has no identifiable secondary cause. It is the most common type affecting 85% of those with high blood pressure. T ...

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. Electrical stimulation of baroreceptors has been found to activate the

, reducing sympathetic tone throughout the body and thereby reducing blood pressure in patients with resistant hypertension.

Low-pressure baroreceptors

The low-pressure baroreceptors, are found in large systemic

vein Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated ...

s, in pulmonary vessels, and in the walls of the right atrium and ventricles of the

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to ...

(the

atrial volume receptors Atrial volume receptors (also known as Veno-atrial stretch receptors) are low pressure baroreceptors that are found in the atria of the heart. They are myelinated vagal fibres in the endocardium found at the junction between atria and the vena ca ...

). 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. The low-pressure baroreceptors have both circulatory and renal effects; they produce changes in hormone secretion, resulting in profound effects on the retention of salt and water; they also influence intake of salt and water. The renal effects allow the receptors to change the mean pressure in the system in the long term. Denervating these receptors 'fools' the body into thinking that it has too low blood volume and initiates mechanisms that retain fluid and so push up the blood pressure to a higher level than it would otherwise have.

Baroreceptor dysfunction

Baroreceptors are integral to the body's function: 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. Baroreceptors can also become oversensitive in some people (usually the carotid baroreceptors in older males). This can lead to

bradycardia Bradycardia (also sinus bradycardia) is a slow resting heart rate, commonly under 60 beats per minute (BPM) as determined by an electrocardiogram. It is considered to be a normal heart rate during sleep, in young and healthy or elderly adults, a ...

, dizziness and fainting (syncope) from touching the neck (often whilst shaving). This is an important cause to exclude in men having pre-syncope or syncope symptoms.

References

External links

* {{Somatosensory system Sensory receptors Homeostasis Receptor cells

Arterial baroreceptors

Low-pressure baroreceptors

Baroreceptor dysfunction

See also

References

External links