Baroreceptors

Original Editor - Lucinda hampton

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Introduction[edit | edit source]

473px-Gray's Anatomy with markup showing carotid artery bifurcation.png

Baroreceptors are a type of mechanoreceptor allowing for the relay of information derived from blood pressure within the autonomic nervous system.

  • They are spray-type nerve endings in the walls of blood vessels and the heart that are stimulated by the absolute level of, and changes in, arterial pressure. They are extremely abundant in the wall of the bifurcation of the internal carotid arteries (carotid sinus) and in the wall of the aortic arch[1].

The primary site of termination of baroreceptor afferent fibers is the nucleus tractus solitarius (NTS).  The NTS has been described by many as the primary visceral sensory relay station within the brain. It receives and responds to stimuli from the respiratory, cardiovascular, and gastrointestinal systems[2].

Information is then passed in rapid sequence to alter the total peripheral resistance and cardiac output maintaining blood pressure within a preset, normalized range.

There are two types of baroreceptors:

  1. High-pressure arterial baroreceptors and low-pressure volume receptors which are both stimulated by stretching of the vessel wall. Arterial baroreceptors are located within the carotid sinuses and the aortic arch.
  2. Low-pressure volume receptors, or cardiopulmonary receptors, are located within the atria, ventricles, and pulmonary vasculature[3]

Function[edit | edit source]

Baroreceptor reflex block diagram.png

The function of the baroreceptors is to maintain systemic blood pressure at a relatively constant level, especially during a change in body position.

High Pressure Baroreceptors

Intact baroreceptors are extremely effective in preventing rapid changes in blood pressure from moment to moment or hour to hour, but because of their adaptability to prolonged changes of blood pressure (> 2 or 3 days), the system is incapable of long-term regulation of arterial pressure.

  • Stretching of the baroreceptors as a result of increased blood pressure causes an increase in the activity of the vagal nerve by projection to the nucleus ambiguus. It also causes inhibition of the sympathetic outflow and ultimately leads to decreased heart rate and blood pressure.
  • Conversely, decreased blood pressure results in decreased signal output from the baroreceptors, leading to disinhibition of the central sympathetic control sites and decreased parasympathetic activity. The final effect is an increase in blood pressure.[1]

Low pressure Baroreceptors

These are found in the large veins and in the walls of the atria of the heart. 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 which have profound effects on the retention of salt and water and 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[4].

  • Denervating these receptors fools the body into thinking that we have too low blood volume and initiates mechanisms which retain fluid and so push up the blood pressure to a higher level than we would otherwise have.

Pathophysiology[edit | edit source]

Carotid-pacemaker.jpg

Baroreceptors respond very quickly to maintain a stable blood pressure, but they only respond to short term changes. Over a period of days or weeks they will reset to a new value. In people with essential hypertension the baroreceptors behave as if the elevated blood pressure is normal and aim to maintain this high blood pressure.

  • Considering new approaches to treating hypertension is crucial. High blood pressure is a very common and important cause of disease and death resulting from problems with the heart, and with the blood vessels in the body and brain. Treatment to lower high blood pressure is supposed to continue for decades. However, even by 12 months after the starting treatment, around 50% of patients are not taking their tablets regularly, if at all[5].
  • Carotid pacemakers, Image at R, also known as implantable carotid sinus stimulators, are devices that deliver activation energy, via carotid leads, to the carotid baroreceptors. This is sometimes offered for drug-resistant hypertension. The baroreceptors send signals to the brain and the signals are interpreted as a rise in blood pressure. The brain sends signals to other parts of the body to reduce blood pressure such as the blood vessels, heart and kidneys[6].

On the opposite end of the spectrum, carotid sinus syndrome is a syndrome in which the carotid sinus is particularly sensitive to external pressure. Increased pressure on the carotid sinus, such as from a particularly tight collar or sustained turn of the head, results in significant hypotension and possibly syncope[3].

References[edit | edit source]

  1. 1.0 1.1 Kass JS, Mizrahi EM. Neurology secrets e-book. Elsevier Health Sciences; 2010 Apr 30.Available from: https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/baroreceptor (accessed 26.2.2021)
  2. Kenhub STN Available from: https://www.kenhub.com/en/library/anatomy/the-solitary-tract-and-nucleus (accessed 26.2.2021)
  3. 3.0 3.1 Armstrong M, Moore RA. Physiology, Baroreceptors. StatPearls [Internet]. 2020 Mar 23.Available from:https://www.statpearls.com/articlelibrary/viewarticle/18138/ (accessed 26.2.2021)
  4. Academic kids Baroreceptors Available from: https://www.academickids.com/encyclopedia/index.php/Baroreceptor(accessed 26.2.2021)
  5. The conversation Simple operation to cure high blood pressure tested in rats Available from: https://theconversation.com/simple-operation-to-cure-high-blood-pressure-tested-in-rats-17793 (accessed 26.2.2021)
  6. Radiopedia Carotid pacemaker Available from: https://radiopaedia.org/articles/carotid-pacemaker?lang=us (accessed 26.2.2021)