Vagus nerve

From Wikipedia, the free encyclopedia

Vagus nerve
Plan of the upper portions of the glossopharyngeal, vagus, and accessory nerves.
Course and distribution of the glossopharyngeal, vagus, and accessory nerves.
Details
Innervates	Levator veli palatini, Salpingopharyngeus, Palatoglossus, Palatopharyngeus, Superior pharyngeal constrictor, Middle pharyngeal constrictor, visceratyppi, Inferior pharyngeal constrictor, viscera rass
Identifiers
Latin	nervus vagus
MeSH	D014630
NeuroNames	702
TA	A14.2.01.153
FMA	5731

The vagus nerve, historically cited as the pneumogastric nerve, is the tenth cranial nerve or CN X, and interfaces with the parasympathetic control of the heart, lungs, and digestive tract. The vagus nerves are paired but are normally referred to in the singular. It is the longest nerve of the autonomic nervous system in the human body.

Structure

Upon leaving the medulla oblongata between the pyramid and the inferior cerebellar peduncle, the vagus nerve extends through the jugular foramen, then passes into the carotid sheath between the internal carotid artery and the internal jugular vein down to the neck, chest, and abdomen, where it contributes to the innervation of the viscera, reaching all the way to the colon. Besides giving some output to various organs, the vagus nerve comprises between 80% and 90% of afferent nerves mostly conveying sensory information about the state of the body's organs to the central nervous system. The right and left vagus nerves descend from the cranial vault through the jugular foramina, penetrating the carotid sheath between the internal and external carotid arteries, then passing posterolateral to the common carotid artery. The cell bodies of visceral afferent fibers of the vagus nerve are located bilaterally in the inferior ganglion of the vagus nerve (nodose ganglia). 

The right vagus nerve gives rise to the right recurrent laryngeal nerve, which hooks around the right subclavian artery and ascends into the neck between the trachea and esophagus. The right vagus then crosses anterior to the right subclavian artery, runs posterior to the superior vena cava, descends posterior to the right main bronchus, and contributes to cardiac, pulmonary, and esophageal plexuses. It forms the posterior vagal trunk at the lower part of the esophagus and enters the diaphragm through the esophageal hiatus. 

The left vagus nerve enters the thorax between left common carotid artery and left subclavian artery and descends on the aortic arch. It gives rise to the left recurrent laryngeal nerve, which hooks around the aortic arch to the left of the ligamentum arteriosum and ascends between the trachea and esophagus. The left vagus further gives off thoracic cardiac branches, breaks up into the pulmonary plexus, continues into the esophageal plexus, and enters the abdomen as the anterior vagal trunk in the esophageal hiatus of the diaphragm.

• Pharyngeal nerve
• Superior laryngeal nerve
• Superior cervical cardiac branches of vagus nerve
• Inferior cervical cardiac branch
• Recurrent laryngeal nerve
• Thoracic cardiac branches
• Branches to the pulmonary plexus
• Branches to the esophageal plexus
• Anterior vagal trunk
• Posterior vagal trunk
• Hering-Breuer reflex in alveoli

The vagus runs parallel to the common carotid artery and internal jugular vein inside the carotid sheath.

Nuclei

The vagus nerve includes axons which emerge from or converge onto four nuclei of the medulla:

1. The dorsal nucleus of vagus nerve – which sends parasympathetic output to the viscera, especially the intestines
2. The nucleus ambiguus – which gives rise to the branchial efferent motor fibers of the vagus nerve and preganglionic parasympathetic neurons that innervate the heart
3. The solitary nucleus – which receives afferent taste information and primary afferents from visceral organs
4. The spinal trigeminal nucleus – which receives information about deep/crude touch, pain, and temperature of the outer ear, the dura of the posterior cranial fossa and the mucosa of the larynx

Development

The motor division of the vagus nerve is derived from the basal plate of the embryonic medulla oblongata, while the sensory division originates from the cranial neural crest.

Function

The vagus nerve supplies motor parasympathetic fibers to all the organs (except the adrenal glands), from the neck down to the second segment of the transverse colon. The vagus also controls a few skeletal muscles, including:

• Cricothyroid muscle
• Levator veli palatini muscle
• Salpingopharyngeus muscle
• Palatoglossus muscle
• Palatopharyngeus muscle
• Superior, middle and inferior pharyngeal constrictors
• Muscles of the larynx (speech).

This means that the vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve). It also has some afferent fibers that innervate the inner (canal) portion of the outer ear (via the auricular branch, also known as Alderman's nerve) and part of the meninges.

Efferent vagus nerve fibers innervating the pharynx and back of the throat are responsible for the gag reflex. In addition, 5-HT₃ receptor-mediated afferent vagus stimulation in the gut due to gastroenteritis is a cause of vomiting. Stimulation of the vagus nerve in the cervix uteri (as in some medical procedures) can lead to a vasovagal response. 

The vagus nerve also plays a role in satiation following food consumption. Knocking out vagal nerve receptors has been shown to cause hyperphagia (greatly increased food intake).

Vagus nerve and the heart

H&E stained fibers of the vagus nerve (bottom right) innervate the sinoatrial node tissue (middle left)

 

Parasympathetic innervation of the heart is partially controlled by the vagus nerve and is shared by the thoracic ganglia. Vagal and spinal ganglionic nerves mediate the lowering of the heart rate. The right vagus branch innervates the sinoatrial node. In healthy people, parasympathetic tone from these sources are well-matched to sympathetic tone. Hyperstimulation of parasympathetic influence promotes bradyarrhythmias. When hyperstimulated, the left vagal branch predisposes the heart to conduction block at the atrioventricular node. 

At this location, neuroscientist Otto Loewi first demonstrated that nerves secrete substances called neurotransmitters, which have effects on receptors in target tissues. In his experiment, Loewi electrically stimulated the vagus nerve of a frog heart, which slowed the heart. Then he took the fluid from the heart and transferred it to a second frog heart without a vagus nerve. The second heart slowed down without an electrical stimulation. Loewi described the substance released by the vagus nerve as vagusstoff, which was later found to be acetylcholine. Drugs that inhibit the muscarinic receptors (anticholinergics) such as atropine and scopolamine, are called vagolytic because they inhibit the action of the vagus nerve on the heart, gastrointestinal tract, and other organs. Anticholinergic drugs increase heart rate and are used to treat bradycardia.

Physical and emotional effects

Excessive activation of the vagal nerve during emotional stress, which is a parasympathetic overcompensation for a strong sympathetic nervous system response associated with stress, can also cause vasovagal syncope due to a sudden drop in cardiac output, causing cerebral hypoperfusion. Vasovagal syncope affects young children and women more than other groups. It can also lead to temporary loss of bladder control under moments of extreme fear.

Research has shown that women having had complete spinal cord injury can experience orgasms through the vagus nerve, which can go from the uterus, cervix, and, it is presumed, the vagina to the brain.

Insulin signaling activates the adenosine triphosphate (ATP)-sensitive potassium (KATP) channels in the arcuate nucleus, decreases AgRP release, and through the vagus nerve, leads to decreased glucose production by the liver by decreasing gluconeogenic enzymes: Phosphoenolpyruvate carboxykinase, Glucose 6-phosphatase.

Clinical significance

Vagus nerve stimulation

Vagus nerve stimulation (VNS) therapy using a neurostimulator implanted in the chest is a treatment used since 1997 to control seizures in epilepsy patients and has been approved for treating drug-resistant cases of clinical depression. A non-invasive VNS device that stimulates an afferent branch of the vagus nerve is also being developed and will soon undergo trials.

Clinical trials have started in Antwerp, Belgium, using VNS for the treatment of tonal tinnitus after a breakthrough study published in early 2011 by researchers at the University of Texas - Dallas showed successful tinnitus-suppression in rats when tones were paired with brief pulses of stimulation of the vagus nerve.

VNS may also be achieved by one of the vagal maneuvers: holding the breath for a few seconds, dipping the face in cold water, coughing, or tensing the stomach muscles as if to bear down to have a bowel movement. Patients with supraventricular tachycardia, atrial fibrillation, and other illnesses may be trained to perform vagal maneuvers (or find one or more on their own).

Vagus nerve blocking (VBLOC) therapy is similar to VNS but used only during the day. In a six-month open-label trial involving three medical centers in Australia, Mexico, and Norway, vagus nerve blocking helped 31 obese participants lose an average of nearly 15 percent of their excess weight. As of 2008 a year-long 300-participant double-blind, phase II trial had begun.

Vagotomy

Vagotomy (cutting of the vagus nerve) is a now-obsolete therapy that was performed for peptic ulcer disease. Vagotomy is currently being researched as a less invasive alternative weight-loss procedure to gastric bypass surgery. The procedure curbs the feeling of hunger and is sometimes performed in conjunction with putting bands on patients' stomachs, resulting in an average of 43% of excess weight lost at six months with diet and exercise.

One serious side effect of a vagotomy is a vitamin B₁₂ deficiency later in life – perhaps after about 10 years – that is similar to pernicious anemia. The vagus normally stimulates the stomach's parietal cells to secrete acid and intrinsic factor. Intrinsic factor is needed to absorb vitamin B₁₂ from food. The vagotomy reduces this secretion and ultimately leads to the deficiency, which, if left untreated, causes nerve damage, tiredness, dementia, paranoia, and ultimately death.

Researchers from Aarhus University and Aarhus University Hospital have demonstrated that vagotomy prevents (halves the risk of) the development of Parkinson's disease, suggesting that Parkinson's disease begins in the gastrointestinal tract and spreads via the vagus nerve to the brain. Or giving further evidence to the theory that dysregulated environmental stimuli, such as that received by the vagus nerve from the gut, may have a negative effect on the dopamine reward system of the substantia nigra, thereby causing Parkinsons disease.

Chagas disease

The neuropathy in Chagas disease spreads in part via the major parasympathetic branches of the vagus nerve.

History

Etymology

The Latin word vagus means literally "wandering" (the words vagrant, vagabond, and vague come from the same root). Sometimes the right and left branches together are spoken of in the plural and are thus called vagi (/ˈveɪdʒaɪ/VAY-jy). The vagus was also historically called the pneumogastric nerve since it innervates both the lungs and the stomach.

National Academies of Sciences, Engineering, and Medicine

From Wikipedia, the free encyclopedia

The National Academies of Sciences, Engineering, and Medicine

The Keck Center of the National Academies in Washington, D.C.
Formation	1863 (as National Academy of Sciences), 1916 (as National Research Council), 2015 (as National Academies of Sciences, Engineering, and Medicine)
Headquarters	Washington, D.C., USA
Membership	Scientists, engineers, and health professionals
Official language	English

The National Academies of Sciences, Engineering, and Medicine (also known as NASEM or the National Academies) is the collective scientific national academy of the United States. The name is used interchangeably in two senses: (1) as an umbrella term for its three quasi-independent honorific member organizations (the National Academy of Sciences (NAS), the National Academy of Engineering (NAE), and the National Academy of Medicine (NAM)). And (2) as the brand for studies and reports issued by the operating arm of the three academies, the National Research Council (NRC). The NRC was first formed in 1916 as an activity of the NAS. Now jointly governed by all three academies, it produces some 200 publications annually which are published by the National Academies Press.

History

The US National Academy of Sciences was created by an Act of Incorporation dated March 3, 1863, which was signed by then President of the United States Abraham Lincoln The Act stated that "... the Academy shall, whenever called upon by any department of the Government, investigate, examine, experiment, and report upon any subject of science or art . ... " With the American civil war raging, the new Academy was presented with few problems to solve, but it did address matters of "... coinage, weights and measures, iron ship hulls, and the purity of whiskey ..." All subsequently affiliated organizations have been created under this same overall congressional charter, including the two younger academies, National Academy of Engineering (NAE) (created in 1964) and NAM (created as the Institute of Medicine in 1970 and rechartered as NAM in 2015).

Under this same charter, the National Research Council was created in 1916. On June 19 of that year, then US President Woodrow Wilson requested that the National Academy of Sciences organize a "National Research Council". The purpose of the Council (at first called the National Research Foundation) was in part to foster and encourage "the increased use of scientific research in the development of American industries ... the employment of scientific methods in strengthening the national defense ... and such other applications of science as will promote the national security and welfare."

At the time, the Academy's effort to support national defense readiness, the Committee on Nitric Acid Supply, was approved by Secretary of War Newton D. Baker. Nitric acid was the substance basic in the making of propellants such as cordite, high explosives, dyes, fertilizers, and other products but availability was limited due to World War I. The NRC, through its committee, recommended importing Chilean saltpeter and the construction of four new ordinance plants. These recommendations were accepted by the War Department in June 1917, although the plants were not completed prior to the end of the war.

In 1918, Wilson formalized the NRC's existence under Executive Order 2859. Wilson's order declared the function of the NRC to be in general:

"(T)o stimulate research in the mathematical. physical, and biological sciences. and in the application of these sciences to engineering, agriculture. medicine. and other useful arts. with the object of increasing knowledge, of strengthening the national defense, and of contributing in other ways to the public welfare."

During World War I, the United States was at war, the NRC operated as the Department of Science and Research of the Council of National Defense as well as the Science and Research Division of the United States Army Signal Corps. When war was first declared, the Council had organized committees on antisubmarine and gas warfare.

On June 1, 1917, the council convened a meeting of scientific representatives of the United Kingdom and France with interested parties from the U.S. on the subject of submarine detection. Another meeting with the British and French was held in Paris in October 1918, at which more details of their work was disclosed. As a result of these meetings, the NRC recommended that scientists be brought together to work on the problems associated with submarine detection. Due to the success of council-directed research in producing a sound-based method of detecting submarines, as well as other military innovations, the NRC was retained at the end of the war, though it was gradually decoupled from the military. 

NRC's Articles of Organization have been changed only three times: in 1956, January 1993, and July 2015.

Honorific societies

The National Academy of Sciences, National Academy of Engineering and National Academy of Medicine are honorary membership organizations, each of which has its own governing Council, and each of which elects its own new members. The membership of the three academies totals more than 6,300 scientists, engineers, and health professionals. New members for each organization are elected annually by current members, based on their distinguished and continuing achievements in original research. By the terms of the original 1863 Congressional charter, the three academies serve pro bono as "advisers to the nation on science, engineering, and medicine."

Program units

The program units, formerly known as the National Research Council, are collectively the operating arm of the three academies for the purpose of providing objective policy advice. Although separately chartered (see above), it falls legally under the overall charter of the National Academy of Sciences, whose ultimate fiduciary body is the NAS Council. In actual practice, the NAS Council delegates governing authority to a Governing Board of the National Research Council that is chaired jointly by the presidents of the three academies, with additional members chosen by them or specified in the charters of the academies. 

Under this three-academy umbrella, the program units produce reports that shape policies, inform public opinion, and advance the pursuit of science, engineering, and medicine.

There are seven major divisions: Division of Behavioral and Social Sciences and Education, Division of Earth and Life Studies, Division of Engineering and Physical Sciences, Health and Medicine Division, Policy and Global Affairs Division, Transportation Research Board, and the Gulf Research Program.

Division on Engineering and Physical Sciences

The Division on Engineering and Physical Sciences has activities organized around:

• Government missions in defense, space, and aerospace
• National infrastructure challenges such as energy and environmental systems, information and telecommunications, manufacturing and engineering design, civil engineering, and the built or constructed environment inclusive of its Board on Infrastructure and Constructed Environment or BICE
• Science and engineering disciplines such as physics, astronomy, computer science and engineering, materials science and engineering, and the mathematical sciences and their applications.
• Continuing assessments of federal government laboratories and research programs.

The study process

The National Academies attempt to obtain authoritative, objective, and scientifically balanced answers to difficult questions of national importance. Top scientists, engineers, health professionals, and other experts (not limited to those in academies membership) are enlisted to address the scientific and technical aspects of some of society's problems. These experts are volunteers who serve on study committees that are convened to answer specific sets of questions. All committee members serve without pay. NASEM itself does not perform original research; rather it provides independent advice. Federal agencies are the primary financial sponsors of the Academies' work; additional studies are funded by state agencies, foundations, other private sponsors, and the National Academies endowment. The external sponsors have no control over the conduct or results of a study, once the statement of task and budget are finalized. Study committees gather information from many sources in public meetings but deliberate in private in order to avoid political, special interest, and sponsor influence. 

All reports go through an extensive external review facilitated by the internal Report Review Committee (also consisting of members from the NAS, NAE, and NAM).

Through this study process, the National Academies produce around 200 reports each year. Recent reports cover such topics as addressing the obesity epidemic, the use of forensics in the courtroom, invasive plants, pollinator collapse, underage drinking, the Hubble Telescope, vaccine safety, the hydrogen economy, transportation safety, climate change, and homeland security. Many reports influence policy decisions; some are instrumental in enabling new research programs; others provide independent program reviews. The National Academies Press is the publisher for the National Academies of Sciences, Engineering, and Medicine, and makes its publications available for free online reading, and the full book PDFs have been available for free download since 2011.

Notable reports

Reports on climate change

In 2001, the NRC published the report Climate Change Science: An Analysis of Some Key Questions, which emphasized the fact that national policy decisions made now and in the long-term future will influence the extent of any damage suffered by vulnerable human populations and ecosystems later in this century. The report endorsed findings of the Intergovernmental Panel on Climate Change (IPCC) as representing the views of the scientific community:

The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability. Human-induced warming and associated sea level rise are expected to continue through the 21st century ... The IPCC's conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community on this issue.

In 2013, the NRC published the report Abrupt Impacts of Climate Change: Anticipating Surprises, which provided an updated look at the issue of abrupt climate change and its potential impacts. This study differed from previous treatments of abrupt changes by focusing on abrupt climate changes and also abrupt climate impacts that have the potential to severely affect the physical climate system, natural systems, or human systems, often affecting multiple interconnected areas of concern.

Report on sexual assault

In 2013, the NRC published the report Estimating the Incidence of Rape and Sexual Assault, which pointed out that approximately 80 percent of sexual assaults go unreported to law enforcement. The report recommends that the National Crime Victimization Survey adopt new approaches to interviews of rape victims, including changing the wording of questions. 

In an article about the report, Amber Stevenson, clinical supervisor and therapist at the Nashville Sexual Assault Center, said that victim-blaming was the main issue preventing victims from coming forward:

As long as we as a community continue to make victim-blaming statements, such as, "She put herself in this situation,"..."She didn't fight back, she must have wanted it," we will continue to see rapes go unreported ... We have to stop blaming the victim. The conversation needs to shift to the person who chose to rape.

Report on integrity in research

The 1992 report, Responsible Science: Ensuring the Integrity of the Research Process was updated in 2017 by the report, Fostering Integrity in Research:

... as experience has accumulated with various forms of research misconduct, detrimental research practices, and other forms of misconduct, as subsequent empirical research has revealed more about the nature of scientific misconduct, and because technological and social changes have altered the environment in which science is conducted, it is clear that the framework established more than two decades ago needs to be updated.

One of the report's main concerns is that a growing percentage of recently published research turns out to be not reproducible due in part to inadequate support of standards of transparency in many fields as well as to various other detrimental research practices.

Other programs

The Christine Mirzayan Science and Technology Policy Fellowship is an annual program for current or recent graduate students to spend three months working in the National Academies. The Academies also administered the Marian Koshland Science Museum in downtown Washington until its closing in 2017; the Museum has since been replaced by LabX, a program of online resources and nationwide public events that aims to increase awareness of scientific and evidence-based solutions to community problems.

Anterior pituitary

From Wikipedia, the free encyclopedia

Anterior pituitary gland
Median sagittal through the hypophysis of an adult monkey. Semidiagrammatic.
Details
Precursor	oral mucosa (Rathke's pouch)
Artery	superior hypophyseal
Vein	hypophyseal
Identifiers
Latin	lobus anterior hypophysis
MeSH	D010903
NeuroNames	407
NeuroLex ID	birnlex_1581
TA	A11.1.00.002
FMA	74627

A major organ of the endocrine system, the anterior pituitary (also called the adenohypophysis or pars anterior), is the glandular, anterior lobe that together with the posterior lobe (posterior pituitary, or the neurohypophysis) makes up the pituitary gland (hypophysis). The anterior pituitary regulates several physiological processes including stress, growth, reproduction and lactation. Proper functioning of the anterior pituitary and of the organs it regulates can often be ascertained via blood tests that measure hormone levels.

Structure

The anterior pituitary complex

 

The pituitary gland is a pea-sized gland that sits in a protective bony enclosure called the sella turcica (Turkish chair/saddle). It is composed of three lobes: the anterior, intermediate, and posterior lobes. In many animals, these lobes are distinct. However, in humans, the intermediate lobe is but a few cell layers thick and indistinct; as a result, it is often considered as part of the anterior pituitary. In all animals, the fleshy, glandular anterior pituitary is distinct from the neural composition of the posterior pituitary. 

The anterior pituitary is composed of three regions:

Pars distalis

Microanatomy of the pars distalis showing chromophobes, basophils and acidophils

The pars distalis, (distal part), comprises the majority of the anterior pituitary and is where the bulk of pituitary hormone production occurs. The pars distalis contains two types of cells including chromophobe cells and chromophil cells. The chromophils can be further divided into acidophils (alpha cells) and basophils (beta cells). These cells all together produce hormones of the anterior pituitary, and release them into the blood stream.

Nota bene: The term "Basophil" and "Acidophil" is used by some books, whereas others prefer to not use these terms. This is due to the possible confusion with white blood cells, where one may also find Basophils and Acidophils.

Pars tuberalis

The pars tuberalis, (tubular part), forms a part of the sheath extending up from the pars distalis which joins with the pituitary stalk (also known as the infundibular stalk or infundibulum), arising from the posterior lobe. (The pituitary stalk does not connect the hypothalamus to the posterior pituitary). The function of the pars tuberalis is poorly understood. However it has been seen to be important in receiving the endocrine signal in the form of TSHB (a β subunit of TSH) informing the pars tuberalis of the photoperiod (length of day). The expression of this subunit is regulated by the secretion of melatonin in response to light information transmitted to the pineal gland. Earlier studies have shown a localisation of melatonin receptors in this region.

Pars intermedia

The pars intermedia, (intermediate part), sits between the pars distalis and the posterior pituitary, forming the boundary between the anterior and posterior pituitaries. It is very small and indistinct in humans.

Development

The anterior pituitary is derived from the ectoderm, more specifically from that of Rathke’s pouch, part of the developing hard palate in the embryo. 

The pouch eventually loses its connection with the pharynx, giving rise to the anterior pituitary. The anterior wall of Rathke's pouch proliferates, filling most of the pouch to form the pars distalis and the pars tuberalis. The posterior wall of the anterior pituitary forms the pars intermedia. Its formation from the soft tissues of the upper palate contrasts with the posterior pituitary, which originates from neuroectoderm.

Function

The anterior pituitary contains five types of endocrine cell, and they are defined by the hormones they secrete: somatotropes (GH); Lactotropes (PRL); gonadotropes (LH and FSH); corticotropes (ACTH) and thyrotropes (TSH). It also contains non-endocrine folliculostellate cells which are thought to stimulate and support the endocrine cell populations.

Hormones secreted by the anterior pituitary are trophic hormones (Greek: trophe, “nourishment”) and tropic hormones. Trophic hormones directly affect growth either as hyperplasia or hypertrophy on the tissue it is stimulating. Tropic hormones are named for their ability to act directly on target tissues or other endocrine glands to release hormones, causing numerous cascading physiological responses.

Role in the endocrine system

Hypothalamic control

Hormone secretion from the anterior pituitary gland is regulated by hormones secreted by the hypothalamus. Neuroendocrine cells in the hypothalamus project axons to the median eminence, at the base of the brain. At this site, these cells can release substances into small blood vessels that travel directly to the anterior pituitary gland (the hypothalamo-hypophyseal portal vessels).

Other Control Mechanisms

Aside from hypothalamic control of the anterior pituitary, other systems in the body have been shown to regulate the anterior pituitary’s function. GABA can either stimulate or inhibit the secretion of luteinizing hormone (LH) and growth hormone (GH) and can stimulate the secretion of thyroid-stimulating hormone (TSH). Prostaglandins are now known to inhibit adrenocorticotropic hormone (ACTH) and also to stimulate TSH, GH and LH release. GABA, through action with the hypothalamus, has been shown experimentally to influence the level of GH secretion. Clinical evidence supports the experimental findings of the excitatory and inhibitory effects GABA has on GH secretion, dependent on GABA’s site of action within the hypothalamic-pituitary unit.

Effects of the anterior pituitary

Thermal homeostasis

The homeostatic maintenance of the anterior pituitary is crucial to our physiological well being. Increased plasma levels of TSH induce hyperthermia through a mechanism involving increased metabolism and cutaneous vasodilation. Increased levels of LH also result in hypothermia but through a decreased metabolism action. ACTH increase metabolism and induce cutaneous vasoconstriction, increased plasma levels also result in hyperthermia and prolactin decreases with decreasing temperature values. follicle-stimulating hormone (FSH) also may cause hypothermia if increased beyond homeostatic levels through an increased metabolic mechanism only.

Gonadal function

Gonadotropes, primarily luteinising hormone (LH) secreted from the anterior pituitary stimulates the ovulation cycle in female mammals, whilst in the males, LH stimulates the synthesis of androgen which drives the ongoing will to mate together with a constant production of sperm.

HPA axis

The anterior pituitary plays a role in stress response. Corticotropin releasing hormone (CRH) from the hypothalamus stimulates ACTH release in a cascading effect that ends with the production of glucocorticoids from the adrenal cortex.

Behavioral effects

Development

The release of GH, LH, and FSH are required for correct human development, including gonadal development.

Breast-feeding

Release of the hormone prolactin is essential for lactation.

Stress

Operating through the hypothalamic-pituitary-adrenal axis (HPA), the anterior pituitary gland has a large role in the neuroendocrine system’s stress response. Stress induces a release of corticotropin-releasing hormone (CRH) and vasopressin from the hypothalamus, which activates the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary gland. Then, this acts on the adrenal cortex to produce glucocorticoids such as cortisol. These glucocorticoids act back on the anterior pituitary gland and the hypothalamus with negative feedback to slow the production of CRH and ACTH. Increased cortisol under stress conditions can cause the following: metabolic effects (mobilization of glucose, fatty acids, and amino acids), bone re-absorption (calcium mobilization), activation of the sympathetic nervous system response (fight or flight), anti-inflammatory effects, and inhibition of reproduction/growth. When the anterior pituitary gland is removed (hypophysectomy) in rats, their avoidance learning mechanisms were slowed down, but injections of ACTH restored their performance. In addition, stress may delay the release of reproductive hormones such as luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This shows that the anterior pituitary gland is involved in behavioral functions as well as being part of a larger pathway for stress responses. It is also known that (HPA) hormones are related to certain skin diseases and skin homeostasis. There is evidence linking hyperactivity of HPA hormones to stress-related skin diseases and skin tumors.

Aging

Operating through the hypothalamic-pituitary-gonadal axis, the anterior pituitary gland also affects the reproductive system. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone. Then the gonads produce estrogen and testosterone. The decrease in release of gonadotropins (LH and FSH) caused by normal aging may be responsible for impotence and frailty in elderly men because of the eventual decrease in production of testosterone. This lower level of testosterone can have other effects, such as reduced libido, well-being and mood, muscle and bone strength, and metabolism.

Tactile responding

It has been shown that infant mice who were stroked with a paintbrush (simulating motherly care) had more release and binding of growth hormone (GH) from the anterior pituitary gland.

Circadian rhythms

Light information received by the eyes is transmitted to the pineal gland via the circadian pacemaker (the suprachiasmatic nucleus). Diminishing light stimulates the release of melatonin from the pineal gland which can also affect the secretion levels in the hypothalamic-pituitary-gonadal axis. Melatonin can lower levels of LH and FSH, which will decrease levels of estrogen and testosterone. In addition, melatonin may affect production of prolactin.

Clinical significance

Increased activity

Hyperpituitarism is the condition where the pituitary secretes excessive amounts of hormones. This hypersecretion often results in the formation of a pituitary adenoma (tumour), which are benign apart from a tiny fraction. There are mainly three types of anterior pituitary tumors and their associated disorders. For example, acromegaly results from excessive secretion of growth hormone (GH) often being released by a pituitary adenoma. This disorder can cause disfigurement and possibly death and can lead to gigantism, a hormone disorder shown in “giants” such as André the Giant, where it occurs before the epiphyseal plates in bones close in puberty. The most common type of pituitary tumour is a prolactinoma which hypersecretes prolactin. A third type of pituitary adenoma secretes excess ACTH, which in turn, causes an excess of cortisol to be secreted and is the cause of Cushing's disease.

Decreased activity

Hypopituitarism is characterized by a decreased secretion of hormones released by the anterior pituitary. For example, hypo-secretion of GH prior to puberty can be a cause of dwarfism. In addition, secondary adrenal insufficiency can be caused by hypo-secretion of ACTH which, in turn, does not signal the adrenal cortex to produce a sufficient amount of cortisol. This is a life-threatening condition. Hypopituitarism could be caused by the destruction or removal of the anterior pituitary tissue through traumatic brain injury, tumor, tuberculosis, or syphilis, among other causes. This disorder used to be referred to as Simmonds' disease but now according to the Diseases Database it is called Sheehan syndrome. If the hypopituitarism is caused by the blood loss associated with childbirth, the disorder is referred to as Sheehan syndrome.

History

Etymology

The anterior pituitary is also known as the adenohypophysis, meaning "glandular undergrowth", from the Greek adeno- ("gland"), hypo ("under"), and physis ("growth").

Additional images

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  The anterior pituitary is the anterior, glandular lobe of the pituitary gland.
  
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