Baroreflex

Baroreflex

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Encyclopedia
The baroreflex or baroreceptor reflex is one of the body's homeostatic
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

 mechanisms for maintaining blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

. It provides a negative feedback loop in which an elevated blood pressure reflexively causes heart rate to decrease therefore causing blood pressure to decrease; likewise, decreased blood pressure activates the baroreflex, causing heart rate to increase thus causing an increase in blood pressure.

The system relies on specialized neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s, known as baroreceptor
Baroreceptor
Baroreceptors are sensors located in the blood vessels of several mammals. They are a type of mechanoreceptor that detects the pressure of blood flowing through them, and can send messages to the central nervous system to increase or decrease total peripheral resistance and cardiac output...

s, in the aortic arch
Aortic arch
The arch of the aorta or the transverse aortic arch is the part of the aorta that begins at the level of the upper border of the second sternocostal articulation of the right side, and runs at first upward, backward, and to the left in front of the trachea; it is then directed backward on the left...

, carotid sinus
Carotid sinus
In human anatomy, the carotid sinus is a localized dilation of the internal carotid artery at its origin, the common carotid artery.-Functions:...

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
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

. Atrial natriuretic peptide
Atrial natriuretic peptide
Atrial natriuretic peptide , atrial natriuretic factor , atrial natriuretic hormone , or atriopeptin, is a powerful vasodilator, and a protein hormone secreted by heart muscle cells. It is involved in the homeostatic control of body water, sodium, potassium and fat...

 forms a parallel negative feedback loop in an endocrinological contrast to the renin-angiotensin system
Renin-angiotensin system
The renin-angiotensin system or the renin-angiotensin-aldosterone system is a hormone system that regulates blood pressure and water balance....

.

Anatomy of the reflex


Baroreceptors include those in the auricles of the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 and vena cavae, but the most sensitive baroreceptors are in the carotid sinus
Carotid sinus
In human anatomy, the carotid sinus is a localized dilation of the internal carotid artery at its origin, the common carotid artery.-Functions:...

es and aortic arch
Aortic arch
The arch of the aorta or the transverse aortic arch is the part of the aorta that begins at the level of the upper border of the second sternocostal articulation of the right side, and runs at first upward, backward, and to the left in front of the trachea; it is then directed backward on the left...

. The carotid sinus
Carotid sinus
In human anatomy, the carotid sinus is a localized dilation of the internal carotid artery at its origin, the common carotid artery.-Functions:...

 baroreceptors are innervated by the glossopharyngeal nerve
Glossopharyngeal nerve
The glossopharyngeal nerve is the ninth of twelve pairs of cranial nerves . It exits the brainstem out from the sides of the upper medulla, just rostral to the vagus nerve...

 (CN IX); the aortic arch baroreceptors are innervated by the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...

 (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 (glutamatergic) to the caudal ventrolateral medulla (CVLM), activating the CVLM. The activated CVLM then sends inhibitory fibers (GABAergic) to the rostral ventrolateral medulla
Rostral ventrolateral medulla
Control of blood pressure is crucially dependent on the integrity of a small region of the brainstem called the ventrolateral medulla. Within this part of the brain are the cells that control the heart, blood vessels, swallowing, breathing and many other functions of the body that are not noticed...

 (RVLM), thus inhibiting the RVLM. The RVLM is the primary regulator of the sympathetic nervous system
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

, sending excitatory fibers (glutamatergic) to the sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 preganglionic neurons located in the intermediolateral nucleus of 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
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 branch of the autonomic nervous system, leading to a decrease in blood pressure. Likewise, low blood pressure causes an increase in sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 tone via "disinhibition" (less inhibition, hence activation) of the RVLM.

The NTS also sends excitatory fibers to the Nucleus ambiguus
Nucleus ambiguus
The nucleus ambiguus is a region of histologically disparate cells located just dorsal to the inferior olivary nucleus in the lateral portion of the upper medulla...

 (vagal nuclei) that regulate the parasympathetic nervous system
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...

, aiding in the decrease in sympathetic activity during conditions of elevated blood pressure.

Baroreceptor activation


The baroreceptors are stretch
Stretching
Stretching is a form of physical exercise in which a specific skeletal muscle is deliberately elongated, often by abduction from the torso, in order to improve the muscle's felt elasticity and reaffirm comfortable muscle tone. The result is a feeling of increased muscle control, flexibility and...

-sensitive mechanoreceptor
Mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...

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
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

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 tractus solitarius
Solitary nucleus
The solitary tract and nucleus are structures in the brainstem that carry and receive visceral sensation and taste from the facial , glossopharyngeal and vagus cranial nerves.-Anatomy:...

 (NTS), which uses frequency as a measure of blood pressure. As discussed previously, increased activation of the NTS inhibits the vasomotor center
Vasomotor center
The vasomotor center is a portion of the medulla oblongata that regulates blood pressure and other homeostatic processes. Upon blood increase in carbon dioxide it stimulates the sympathetic system to constrict vessels. This is opposite to carbon dioxide in tissues causing vasoconstriction,...

 and stimulates the vagal nuclei. The end-result of baroreceptor activation is inhibition of the sympathetic nervous system
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 and activation of the parasympathetic nervous system
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...

.

The sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 and parasympathetic branches of the autonomic nervous system
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

 have opposing effects on blood pressure. Sympathetic activation leads to an elevation of total peripheral resistance
Total peripheral resistance
Vasculature throughout the entire body can be thought of as two separate circuits - one is the systemic circulation, while the other is the pulmonary circulation. Total peripheral resistance is the sum of the resistance of all peripheral vasculature in the systemic circulation...

 and cardiac output
Cardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...

 via increased contractility
Contractility
Myocardial contractility is the intrinsic ability of the heart to contract independent of preload and afterload. Changes in the ability to produce force during contraction result from different degrees of binding between myosin and actin filaments...

 of the heart, heart rate
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....

, and arterial vasoconstriction
Vasoconstriction
Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in...

, which tends to increase blood pressure. Conversely, parasympathetic activation leads to decreased cardiac output
Cardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...

 via decrease in heart rate, resulting in a tendency to lower blood pressure.

By coupling sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 inhibition and parasympathetic activation, the baroreflex maximizes blood pressure reduction. Sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 inhibition leads to a drop in peripheral resistance, while parasympathetic activation leads to a depressed heart rate (reflex bradycardia
Reflex bradycardia
Reflex bradycardia is a bradycardia in response to the baroreceptor reflex, one of the body's homeostatic mechanisms for preventing abnormal increases blood pressure...

) and contractility
Contractility
Myocardial contractility is the intrinsic ability of the heart to contract independent of preload and afterload. Changes in the ability to produce force during contraction result from different degrees of binding between myosin and actin filaments...

. The combined effects will dramatically decrease blood pressure.

In a similar manner, sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 activation with parasympathetic inhibition allows the baroreflex to elevate blood pressure.

Set point and tonic activation


Baroreceptor firing has an inhibitory effect on sympathetic outflow. The sympathetic neurones fire at different rates, releasing various amounts of Norepinephrine. Norepinephrine constricts blood vessels to increase blood pressure. When Baroreceptors are stretched (due to an increased blood pressure) their firing rate increases which in turn increases the inhibitory effect on sympathetic outflow resulting in reduced norepinephrine production and a corresponding reduction in blood pressure. When the blood pressure is low, baroreceptor firing is lowered this reduces the inhibitory effect on the sympathetic outflow and consequently an increased amount of norepinephrine is released to receptors- causing blood vessels to constrict thus inreasing blood pressure.

Effect on heart rate variability


The baroreflex may be responsible for a part of the low-frequency component of heart rate variability
Heart rate variability
Heart rate variability is a physiological phenomenon where the time interval between heart beats varies. It is measured by the variation in the beat-to-beat interval....

, the so-called Mayer waves
Mayer waves
Mayer waves are waves in arterial blood pressure brought about by oscillations in baroreceptor and chemoreceptor reflex control systems. The waves are seen both in the ECG and in blood pressure curves and have a frequency about 0.1 Hz...

, at 0.1 Hz [Sleight, 1995].

Baroreflex activation therapy for treatment of resistant hypertension


Published feasibility studies have shown that a pacemaker-like device designed to electrically activate the baroreflex, also known as baroreflex activation therapy, significantly lowers blood pressure in patients with treatment-resistant hypertension. One study published on a group of 16 patients reported an average systolic blood pressure reduction of 34 mmHg after three months of treatment and 35 mmHg after 24 months. A drop in systolic blood pressure of at least 20 mmHg was achieved in 12 of 16 (75%) patients at 2 years, and 5 of 16 (31%) achieved a systolic BP of less than 140 mmHg at 2 years. Results published on a separate group of 10 patients from another feasibility trial reported an average systolic blood pressure reduction of 24 mmHg after three months of treatment. Baroreflex activation therapy devices are not currently available outside of clinical research studies.

See also

  • Autonomic nervous system
    Autonomic nervous system
    The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

  • Baroreceptor
    Baroreceptor
    Baroreceptors are sensors located in the blood vessels of several mammals. They are a type of mechanoreceptor that detects the pressure of blood flowing through them, and can send messages to the central nervous system to increase or decrease total peripheral resistance and cardiac output...

  • Blood pressure
    Blood pressure
    Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

  • Heart rate turbulence
    Heart rate turbulence
    Heart rate turbulence is the return to equilibrium of heart rate after a premature ventricular contraction . It consists of a brief speed-up in heart rate, followed by a slow decrease back to the baseline rate...

  • Reflex bradycardia
    Reflex bradycardia
    Reflex bradycardia is a bradycardia in response to the baroreceptor reflex, one of the body's homeostatic mechanisms for preventing abnormal increases blood pressure...

  • Valsalva Maneuver
    Valsalva maneuver
    The Valsalva maneuver or Valsalva manoeuvre is performed by moderately forceful attempted exhalation against a closed airway, usually done by closing one's mouth and pinching one's nose shut...