Subfornical organ
Encyclopedia
The subfornical organ, situated on the ventral surface of the fornix, at the interventricular foramina (foramina of Monro), is one of the circumventricular organs
of the brain
.
in the brainstem and the organum vasculosum of the lamina terminalis
(OVLT).
The OVLT and the SFO are both strongly interconnected with the nucleus medianus, and together these three structures comprise the so called "AV3V" region - the region anterior and ventral to the third ventricle
. The AV3V region is very important in the regulation of fluid and electrolyte balance, by controlling thirst
, sodium excretion, blood volume regulation, and vasopressin
secretion.
The SFO is outside the blood-brain barrier
, and so neurons in this region can respond to factors that are present in the systemic circulation.
s, being sensitive to the osmotic pressure
of the blood. These neurons project to the supraoptic nucleus
and paraventricular nucleus
to regulate the activity of vasopressin
-secreting neurons. These neurons also project to the nucleus medianus (also called the median preoptic nucleus) which is involved in controlling thirst
. Thus, The subfornical organ is involved in fluid balance
.
In addition, neurons in the SFO have receptors for many hormones that circulate in the blood but which do not cross the blood-brain barrier, including angiotensin
, atrial natriuretic peptide
, endothelin
and relaxin
.
Circumventricular organs
Circumventricular organs are so named because they are positioned at distinct sites around the margin of the ventricular system of the brain.They are among the few sites in the brain which have an incomplete blood–brain barrier...
of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
.
Relations with other circumventricular organs
Other circumventricular organs are the area postremaArea postrema
The area postrema is a medullary structure in the brain that controls vomiting. Its privileged location in the brain also allows the area postrema to play a vital role in the control of autonomic functions by the central nervous system.-Anatomy:...
in the brainstem and the organum vasculosum of the lamina terminalis
Organum vasculosum of the lamina terminalis
The organum vasculosum of the lamina terminalis is one of the circumventricular organs of the brain. Other circumventricular organs are the subfornical organ and the area postrema in the brainstem.-AV3V region:...
(OVLT).
The OVLT and the SFO are both strongly interconnected with the nucleus medianus, and together these three structures comprise the so called "AV3V" region - the region anterior and ventral to the third ventricle
Third ventricle
The third ventricle is one of four connected fluid-filled cavities comprising the ventricular system within the human brain. It is a median cleft between the two thalami, and is filled with cerebrospinal fluid ....
. The AV3V region is very important in the regulation of fluid and electrolyte balance, by controlling thirst
Thirst
Thirst is the craving for fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids and/or an increase in the concentration of certain osmolites, such as salt...
, sodium excretion, blood volume regulation, and vasopressin
Vasopressin
Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...
secretion.
The SFO is outside the blood-brain barrier
Blood-brain barrier
The blood–brain barrier is a separation of circulating blood and the brain extracellular fluid in the central nervous system . It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation. Endothelial cells restrict the diffusion...
, and so neurons in this region can respond to factors that are present in the systemic circulation.
Receptors
Some neurons in the SFO are osmoreceptorOsmoreceptor
An osmoreceptor is a sensory receptor primarily found in the hypothalamus of most homeothermic organisms that detects changes in osmotic pressure. Osmoreceptors can be found in several structures, including the organum vasculosum of the lamina terminalis and the subfornical organ...
s, being sensitive to the osmotic pressure
Osmotic pressure
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
of the blood. These neurons project to the supraoptic nucleus
Supraoptic nucleus
The supraoptic nucleus is a nucleus of magnocellular neurosecretory cells in the hypothalamus of the mammalian brain. The nucleus is situated at the base of the brain, adjacent to the optic chiasm...
and paraventricular nucleus
Paraventricular nucleus
The paraventricular nucleus is a neuronal nucleus in the hypothalamus. It contains multiple subpopulations of neurons that are activated by a variety of stressful and/or physiological changes. Many PVN neurons project directly to the posterior pituitary where they release oxytocin or vasopressin...
to regulate the activity of vasopressin
Vasopressin
Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...
-secreting neurons. These neurons also project to the nucleus medianus (also called the median preoptic nucleus) which is involved in controlling thirst
Thirst
Thirst is the craving for fluids, resulting in the basic instinct of animals to drink. It is an essential mechanism involved in fluid balance. It arises from a lack of fluids and/or an increase in the concentration of certain osmolites, such as salt...
. Thus, The subfornical organ is involved in fluid balance
Fluid balance
Fluid balance is the concept of human homeostasis that the amount of fluid lost from the body is equal to the amount of fluid taken in. Euvolemia is the state of normal body fluid volume. Water is necessary for all life on Earth...
.
In addition, neurons in the SFO have receptors for many hormones that circulate in the blood but which do not cross the blood-brain barrier, including angiotensin
Angiotensin
Angiotensin, a peptide hormone, causes blood vessels to constrict, and drives blood pressure up. It is part of the renin-angiotensin system, which is a major target for drugs that lower blood pressure. Angiotensin also stimulates the release of aldosterone, another hormone, from the adrenal cortex...
, 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...
, endothelin
Endothelin
Endothelins are proteins that constrict blood vessels and raise blood pressure. They are normally kept in balance by other mechanisms, but when they are over-expressed, they contribute to high blood pressure and heart disease....
and relaxin
Relaxin
Relaxin is a protein hormone first described in 1926 by Frederick Hisaw.The relaxin-like peptide family belongs in the insulin superfamily and consists of 7 peptides of high structural but low sequence similarity; relaxin-1 , 2 and 3 , and the insulin-like peptides, INSL3, INSL4, INSL5 and INSL6...
.