human anatomy Human anatomy (gr. ἀνατομία, "dissection", from ἀνά, "up", and τέμνειν, "cut") is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and microscopic anatomy. Gross ...

, the carotid sinus is a dilated area at the base of the

internal carotid artery The internal carotid artery is an artery in the neck which supplies the anterior cerebral artery, anterior and middle cerebral artery, middle cerebral circulation. In human anatomy, the internal and external carotid artery, external carotid ari ...

just superior to the

bifurcation Bifurcation or bifurcated may refer to: Science and technology * Bifurcation theory, the study of sudden changes in dynamical systems ** Bifurcation, of an incompressible flow, modeled by squeeze mapping the fluid flow * River bifurcation, the for ...

of the internal carotid and

external carotid The external carotid artery is the major artery of the head and upper neck. It arises from the common carotid artery. It terminates by splitting into the superficial temporal and maxillary artery within the parotid gland. Structure Origin Th ...

at the level of the superior border of thyroid cartilage. The carotid sinus extends from the bifurcation to the "true" internal carotid artery. The carotid sinuses are sensitive to pressure changes in the

arterial blood Arterial blood is the oxygenated blood in the circulatory system found in the pulmonary vein, the left chambers of the heart, and in the artery, arteries. It is bright red in color, while venous blood is dark red in color (but looks purple through ...

at this level. They are two out of the four baroreception sites in humans and most mammals.

Structure

The carotid sinus is the

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

area of the

carotid artery Carotid artery may refer to: * Common carotid artery, often "carotids" or "carotid", an artery on each side of the neck which divides into the external carotid artery and internal carotid artery * External carotid artery, an artery on each side of ...

, consisting of

baroreceptor Baroreceptors (or archaically, pressoreceptors) are stretch receptors that sense blood pressure. Thus, increases in the pressure of blood vessel triggers increased action potential generation rates and provides information to the central nervous s ...

s which monitor blood pressure.

Function

The carotid sinus contains numerous

s which function as a "sampling area" for many

homeostatic In biology, homeostasis (British also homoeostasis; ) is the state of steady internal physical and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and includes many variables, su ...

mechanisms for maintaining

blood pressure Blood pressure (BP) is the pressure of Circulatory system, circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term ...

. The carotid sinus baroreceptors are innervated by the

carotid sinus nerve The carotid branch of the glossopharyngeal nerve (carotid sinus nerve or Hering's nerve) is a small branch of the glossopharyngeal nerve (cranial nerve IX) that innervates the carotid sinus, and carotid body. Anatomy Course and relations It ...

, which is a branch of 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 oblongata, medulla, just anterior (closer to the nose) to t ...

(CN IX). The neurons which

innervate A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called ...

the carotid sinus centrally project to the

solitary nucleus The solitary nucleus (SN) (nucleus of the solitary tract, nucleus solitarius, or nucleus tractus solitarii) is a series of neurons whose cell bodies form a roughly vertical column of grey matter in the medulla oblongata of the brainstem. Their a ...

in the

medulla Medulla (Latin for "marrow") or medullary may refer to: Science * Medulla oblongata, a part of the brain stem * Renal medulla, a part of the kidney * Adrenal medulla, a part of the adrenal gland * Medulla of ovary, a stroma in the center of the ...

of the brainstem. The solitary nucleus indirectly modulates the activity of sympathetic and

parasympathetic The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system. The autonomic nervous system is responsible for regulat ...

neurons in the medulla and pons through the

hypothalamus The hypothalamus (: hypothalami; ) is a small part of the vertebrate brain that contains a number of nucleus (neuroanatomy), nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrin ...

. These neurons then regulate the autonomic control of the

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

and

blood vessel Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...

s. 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 ...

baroreceptors are innervated by the

aortic nerve The aorta ( ; : aortas or aortae) is the main and largest artery in the human body, originating from the left ventricle of the heart, branching upwards immediately after, and extending down to the abdomen, where it splits at the aortic bifurcat ...

(nerve of Cyon, Ludwig nerve), which combines with CN X (

vagus nerve The vagus nerve, also known as the tenth cranial nerve (CN X), plays a crucial role in the autonomic nervous system, which is responsible for regulating involuntary functions within the human body. This nerve carries both sensory and motor fibe ...

) and travels to the NTS.

Clinical significance

It is a sensitive site of the body because stimulation can drive large-scale reflex effects throughout the body. This can be used therapeutically in treatment of

resistant hypertension Hypertension, also known as high blood pressure, is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms itself. It is, however, a major risk ...

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 cause ...

activation.

Physical assault In the terminology of law, an assault is the act of causing physical harm or unwanted physical contact to another person, or, in some legal definitions, the threat or attempt to do so. It is both a crime and a tort and, therefore, may result i ...

at this point, producing massive baroreflex activation can cause dramatic falls in blood pressure and

cerebral ischemia Brain ischemia is a condition in which there is insufficient bloodflow to the brain to meet metabolic demand. This leads to poor oxygen supply in the brain and may be temporary such as in transient ischemic attack or permanent in which there is ...

. This is the mechanism of baroreflex activation therapy.

Disease

The carotid sinus often has

atherosclerotic plaque An atheroma, or atheromatous plaque, is an abnormal accumulation of material in the inner layer of an arterial wall. The material consists of mostly macrophage cells, or debris, containing lipids, calcium and a variable amount of fibrous conne ...

s because of disturbed

hemodynamics Hemodynamics or haemodynamics are the dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms of autoregulation, just as hydraulic circuits are controlled by control systems. The hemodynamic response continuously ...

(low

wall shear stress Shear stress (often denoted by , Greek: tau) is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. ''Normal stress'', on the o ...

, flow reversal/recirculation). Since these plaques, if large and unstable, predispose to

ischemic strokes Stroke is a medical condition in which poor cerebral circulation, blood flow to a part of the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemor ...

and transient ischemic attacks, carotid endarterectomies are frequently done for

prophylaxis Preventive healthcare, or prophylaxis, is the application of healthcare measures to prevent diseases.Hugh R. Leavell and E. Gurney Clark as "the science and art of preventing disease, prolonging life, and promoting physical and mental health a ...

Carotid sinus hypersensitivity

The carotid sinus baroreceptor can be oversensitive to manual stimulation from the pressure applied at the carotid sinus at the

carotid bifurcation In anatomy, the left and right common carotid arteries (carotids) () are arteries that supply the head and neck with oxygenated blood; they divide in the neck to form the external and internal carotid arteries. Structure The common carotid ar ...

. It is a condition known as 'carotid sinus hypersensitivity' (CSH), 'carotid sinus syndrome' or 'carotid sinus syncope', in which manual stimulation causes large changes in

heart rate Heart rate is the frequency of the cardiac cycle, heartbeat measured by the number of contractions of the heart per minute (''beats per minute'', or bpm). The heart rate varies according to the body's Human body, physical needs, including the nee ...

and

, which can result in

bradycardia Bradycardia, also called bradyarrhythmia, is a resting heart rate under 60 beats per minute (BPM). While bradycardia can result from various pathological processes, it is commonly a physiological response to cardiovascular conditioning or due ...

vasodilation Vasodilation, also known as vasorelaxation, is the widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. Blood vessel wa ...

, and

hypotension Hypotension, also known as low blood pressure, is a cardiovascular condition characterized by abnormally reduced blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood and is ...

. This classically presents as a patient who has "

fainted Syncope , commonly known as fainting or passing out, is a loss of consciousness and muscle strength characterized by a fast onset, short duration, and spontaneous recovery. It is caused by a decrease in blood flow to the brain, typically from l ...

" (actually a

presyncope Lightheadedness is a common and typically unpleasant sensation of dizziness or a feeling that one may faint. The sensation of lightheadedness can be short-lived, prolonged, or, rarely, recurring. In addition to dizziness, the individual may fee ...

) on several occasions while shaving, or in some other way coming into contact with their carotid sinus.

Denervation Denervation is any loss of nerve supply regardless of the cause. If the nerves lost to denervation are part of neural communication to an organ system or for a specific tissue function, alterations to or compromise of physiological functioning ca ...

is performed as treatment for this. Usually older patients with syncope and unexplained falls often have coexisting conditions such as carotid sinus hypersensitivity,

orthostatic hypotension Orthostatic hypotension, also known as postural hypotension, is a medical condition wherein a person's blood pressure drops when they are standing up ( orthostasis) or sitting down. Primary orthostatic hypotension is also often referred to as ne ...

, and

vasovagal syncope Reflex syncope is a brief loss of consciousness due to a nervous system, neurologically induced drop in blood pressure and/or a decrease in heart rate. Before an affected person passes out, there may be sweating, a decreased ability to se ...

Carotid sinus syndrome

Carotid sinus syndrome (CSS) is a temporary loss of consciousness that sometimes accompanies convulsive seizures because of the intensity of the carotid sinus

when pressure builds in one or both carotid sinuses.

Treatment of resistant hypertension

Stimulation of

s at the carotid sinus can be used to treat

via activation of the

. A pacemaker-like device can be implanted to electrically stimulate the receptors chronically, which is found to lower blood pressure by 15–25 mmHg.

Massage

Massage Massage is the rubbing or kneading of the body's soft tissues. Massage techniques are commonly applied with hands, fingers, elbows, knees, forearms, feet, or a device. The purpose of massage is generally for the treatment of body stress or pa ...

of the carotid sinus, carotid sinus massage is used to diagnose carotid sinus syncope and is sometimes useful for differentiating

supraventricular tachycardia Supraventricular tachycardia (SVT) is an umbrella term for fast heart rhythms arising from the upper part of the heart. This is in contrast to the other group of fast heart rhythms – ventricular tachycardia, which start within the lower cham ...

(SVT) from

ventricular tachycardia Ventricular tachycardia (V-tach or VT) is a cardiovascular disorder in which fast heart rate occurs in the ventricles of the heart. Although a few seconds of VT may not result in permanent problems, longer periods are dangerous; and multiple ...

. Like the

valsalva maneuver The Valsalva maneuver is performed by a forceful attempt of exhalation against a closed airway, usually done by closing one's mouth and pinching one's nose shut while expelling air, as if blowing up a balloon. Variations of the maneuver can be ...

, it is a treatment for acute SVT. It is less effective than pharmaceutical management of SVT with

verapamil Verapamil, sold under various trade names, is a calcium channel blocker medication used for the treatment of high blood pressure, angina (chest pain from not enough blood flow to the heart), and supraventricular tachycardia. It may also be use ...

adenosine Adenosine (symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9- glycosidic bond. Adenosine is one of the four nucleoside build ...

, but it is still the preferred first line of treatment in a hemodynamically stable patient.

Carotid sinus reflex death

Carotid sinus reflex death is a potential

etiology Etiology (; alternatively spelled aetiology or ætiology) is the study of causation or origination. The word is derived from the Greek word ''()'', meaning "giving a reason for" (). More completely, etiology is the study of the causes, origins ...

of sudden

death Death is the end of life; the irreversible cessation of all biological functions that sustain a living organism. Death eventually and inevitably occurs in all organisms. The remains of a former organism normally begin to decompose sh ...

in which manual stimulation of the carotid sinus allegedly causes strong

(

Vagus nerve The vagus nerve, also known as the tenth cranial nerve (CN X), plays a crucial role in the autonomic nervous system, which is responsible for regulating involuntary functions within the human body. This nerve carries both sensory and motor fibe ...

is for

s) impulses leading to terminal

cardiac arrest Cardiac arrest (also known as sudden cardiac arrest CA is when the heart suddenly and unexpectedly stops beating. When the heart stops beating, blood cannot properly Circulatory system, circulate around the body and the blood flow to the ...

. Carotid sinus reflex death has been pointed out as a possible cause of death in cases of

strangulation Strangling or strangulation is compression of the neck that may lead to unconsciousness or death by causing an increasingly hypoxic state in the brain by restricting the flow of oxygen through the trachea. Fatal strangulation typically occurs ...

hanging Hanging is killing a person by suspending them from the neck with a noose or ligature strangulation, ligature. Hanging has been a standard method of capital punishment since the Middle Ages, and has been the primary execution method in numerou ...

and autoerotic strangulation, but such deductions remain

controversial Controversy (, ) is a state of prolonged public dispute or debate, usually concerning a matter of conflicting opinion or point of view. The word was coined from the Latin '' controversia'', as a composite of ''controversus'' – "turned in an opp ...

. Medical literature examining the use of carotid sinus massage involving brief gentle pressure of the carotid sinus in therapeutic settings as a diagnostic and therapeutic examination tool have reported few potentially fatal complications. A carotid massage can also possibly dislodge a

thrombus A thrombus ( thrombi) is a solid or semisolid aggregate from constituents of the blood (platelets, fibrin, red blood cells, white blood cells) within the circulatory system during life. A blood clot is the final product of the blood coagulatio ...

, or some

plaque Plaque may refer to: Commemorations or awards * Commemorative plaque, a plate, usually fixed to a wall or other vertical surface, meant to mark an event, person, etc. * Memorial Plaque (medallion), issued to next-of-kin of dead British military p ...

. This could lead to any number of life-threatening effects, including

stroke Stroke is a medical condition in which poor cerebral circulation, blood flow to a part of the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemor ...

In martial arts and self defense

Stimulation of the carotid sinus via a slap or a strike, to induce (usually temporary, but sometimes lethal) loss of consciousness is a

self-defense Self-defense (self-defence primarily in Commonwealth English) is a countermeasure that involves defending the health and well-being of oneself from harm. The use of the right of self-defense as a legal justification for the use of Force (law), ...

technique, and is often taught in

martial arts Martial arts are codified systems and traditions of combat practiced for a number of reasons such as self-defence; military and law enforcement applications; combat sport, competition; physical, mental, and spiritual development; entertainment; ...

such as

karate (; ; Okinawan language, Okinawan pronunciation: ), also , is a martial arts, martial art developed in the Ryukyu Kingdom. It developed from the Okinawan martial arts, indigenous Ryukyuan martial arts (called , "hand"; ''tī'' in Okinawan) un ...