Arcuate nucleus
Encyclopedia
The arcuate nucleus is an aggregation of neuron
s in the mediobasal hypothalamus
, adjacent to the third ventricle
and the median eminence
. The arcuate nucleus includes several important populations of neurons, including: neuroendocrine neurons, centrally-projecting neurons, and others.
The arcuate nucleus also contains a population of specialized ependymal cells, called tanycytes
.
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 in the mediobasal hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
, adjacent 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 ....
and the median eminence
Median eminence
The median eminence is part of the inferior boundary for the hypothalamus part of the human brain. A small swelling on the tuber cinereum posterior to the infundibulum - atop the pituitary stalk - the median eminence lies in the area roughly bounded on its posterolateral region by the cerebral...
. The arcuate nucleus includes several important populations of neurons, including: neuroendocrine neurons, centrally-projecting neurons, and others.
Neuroendocrine neurons
- Neuroendocrine neuronsNeuroendocrinologyNeuroendocrinology is the study of the extensive interactions between the nervous system and the endocrine system, including the biological features of the cells that participate, and how they functionally communicate...
with nerve endings in the median eminence release dopamineDopamineDopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...
into the hypophysial portal blood. These are sometimes called the "tuberoinfundibular dopamine" (TIDA) neurons. In lactating females, TIDA neurons are inhibited by the stimulus of suckling. Dopamine released from their nerve endings at the median eminence is transported to the anterior pituitary gland, where it regulates the secretion of prolactinProlactinProlactin also known as luteotropic hormone is a protein that in humans is encoded by the PRL gene.Prolactin is a peptide hormone discovered by Henry Friesen...
; dopamine inhibits prolactin secretion, so, when the TIDA neurons are inhibited, there is increased secretion of prolactin, which stimulates lactogenesis (milk production). Prolactin acts in a short-loop negative feedback manner to decrease its levels by stimulating the release of dopamine. Dopaminergic neurons of the arcuate also inhibit the release of gonadotropin-releasing hormoneGonadotropin-releasing hormoneGonadotropin-releasing hormone , also known as Luteinizing-hormone-releasing hormone and luliberin, is a tropic peptide hormone responsible for the release of follicle-stimulating hormone and luteinizing hormone from the anterior pituitary. GnRH is synthesized and released from neurons within...
, explaining in part why lactating (or otherwise hyperprolactinemic) women experience oligomenorrhea or amenorrhea (infrequency or absence of menses).
- Neuroendocrine neurons, mainly in the ventrolateral part of the nucleus, make growth hormone-releasing hormone (GHRH). Like the TIDA neurons, these neurons have nerve endings in the median eminence. GHRH released into the hypophysial portal bloodHypophyseal portal systemThe hypophyseal portal system is the system of blood vessels that link the hypothalamus and the anterior pituitary in the brain.It allows endocrine communication between the two structures. It is part of the hypothalamic-pituitary-adrenal axis. The anterior pituitary receives releasing and...
is transported to the anterior pituitary gland, where it regulates the secretion of growth hormoneGrowth hormoneGrowth hormone is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans and other animals. Growth hormone is a 191-amino acid, single-chain polypeptide that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior...
; GHRH stimulates growth hormone secretion. These neurons are inhibited by somatostatinSomatostatinSomatostatin is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G-protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones.Somatostatin...
. The reciprocal relationship between the electrical activity of GHRH neurons and somatostatin neurons leads to pulsatile secretion of growth hormone, a pattern of secretion that is important for its biological effectiveness.
Centrally-projecting neurons
- Centrally-projecting neurons that contain neuropeptide YNeuropeptide YNeuropeptide Y is a 36-amino acid peptide neurotransmitter found in the brain and autonomic nervous system."NPY has been associated with a number of physiologic processes in the brain, including the regulation of energy balance, memory and learning, and epilepsy." The main effect is increased food...
(NPY), agouti-related protein (AGRP), and the inhibitory neurotransmitter GABAGabâGabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...
. These neurons, in the most ventromedial part of the nucleus, project strongly to the lateral hypothalamusLateral hypothalamusThe lateral hypothalamus or lateral hypothalamic area is a part of the hypothalamus.It is concerned with hunger. Damage to this area can cause reduced food intake...
and to the paraventricular nucleusParaventricular nucleusThe 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...
of the hypothalamus, and are important in the regulation of appetiteAppetiteThe appetite is the desire to eat food, felt as hunger. Appetite exists in all higher life-forms, and serves to regulate adequate energy intake to maintain metabolic needs. It is regulated by a close interplay between the digestive tract, adipose tissue and the brain. Decreased desire to eat is...
. When activated, these neurons can produce ravenous eating. These neurons are inhibited by leptinLeptinLeptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones...
, insulinInsulinInsulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
, and peptide YYPeptide YYPeptide YY is a short protein released by cells in the ileum and colon in response to feeding. In humans it appears to reduce appetite.It is also known as PYY, Peptide Tyrosine Tyrosine, or Pancreatic Peptide YY3-36....
and activated by ghrelinGhrelinGhrelin is a 28 amino acid peptide and hormone that is produced mainly by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates hunger. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone...
.
- Centrally-projecting neurons that contain peptidePeptidePeptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
products of pro-opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcriptCocaine- and amphetamine-regulated transcriptCocaine and amphetamine regulated transcript also known as CART is a protein that in humans is encoded by the CARTPT gene. CART appears to have roles in reward, feeding, and stress, and it has the functional properties of an endogenous psychostimulant.- Function :CART is a neuropeptide that also...
(CART). These neurons have widespread projections to many brain areas, including to all nuclei in the hypothalamus. These cells are important in the regulation of appetiteAppetiteThe appetite is the desire to eat food, felt as hunger. Appetite exists in all higher life-forms, and serves to regulate adequate energy intake to maintain metabolic needs. It is regulated by a close interplay between the digestive tract, adipose tissue and the brain. Decreased desire to eat is...
, and, when activated, they inhibit feeding. These neurons are activated by circulating concentrations of leptinLeptinLeptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones...
and insulinInsulinInsulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
, and they are directly innervated and inhibited by the NPY neurons. POMC neurons that project to the medial preoptic nucleus are also involved in the regulation of sexual behavior in both males and females. The expression of POMC is regulated by gonadal steroids. The release of a POMC product, beta-endorphin is regulated by NPY.
- Centrally-projecting neurons that make somatostatin; the neurosecretory somatostatin neurons that regulate growth hormone secretion are a different population, located in the periventricular nucleus.
Other
- A small population of neurons that synthesise ghrelinGhrelinGhrelin is a 28 amino acid peptide and hormone that is produced mainly by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates hunger. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone...
. The role of this population is not known; many neurons in the arcuate nucleus express receptors for ghrelin, but these are thought to respond mainly to blood-borne ghrelin.
The arcuate nucleus also contains a population of specialized ependymal cells, called tanycytes
Tanycytes
Tanycytes are special ependymal cells located in the floor of the third ventricle having processes extending deep into the hypothalamus. It is possible that their function is to transfer chemical signals from CSF to CNS....
.