Volcanic Caldera

A volcanic caldera is a large bowl-shaped volcanic depression, which is formed as a result of subsidence. As the magma founders, so the center of the volcano collapses. The Plateau of Giant Craters in Tanzania contains many impressive calderas, including Ngorongoro, which attains a diameter of 22 km. A volcanic caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption such as the ones at Yellowstone National Park in the US and Glen Coe in Scotland. They are sometimes confused with volcanic craters. The word comes from Spanish caldera, and this from Latin CALDARIA, meaning "cooking pot". In some texts the English term cauldron is also used.

Calderas occur primarily in three different volcanic settings, each of which affects their shape and evolution: basaltic shield cones, stratovolcanoes, and volcanic centers consisting of preexisting clusters of volcanoes. These last calderas, associated with broad, large-volume andesitic to rhyolitic ignimbrite sheets, are generally the largest and most impressive, and are those generally denoted by the term.

A collapse is triggered by withdrawal of magma from an underlying chamber some 2.4 – 3.6 miles beneath the surface, resulting in foundering of the roof into the chamber. If enough magma is ejected, the emptied chamber is unable to support the weight of the volcanic edifice above it. A roughly circular fracture - the "Ring Fault" develops around the edge of the chamber. These ring fractures serve as feeders for fault intrusions which are also known as ring dykes. Secondary volcanic vents may form above the ring fracture. As the magma chamber empties, the center of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a single cataclysmic eruption, or it may occur in stages as the result of a series of eruptions. The total area that collapses may be hundreds or thousands of square kilometers.

In 1815, the German geologist Leopold von Buch visited the Las Cañadas Caldera Teide, Tenerife, and the Caldera de Taburiente, La Palma, both in the Canary Islands. When he published his memoirs he introduced the term "caldera" into the geological vocabulary.

Formation of a volcanic caldera

Volcanic Caldera on Fernandina Island in the Galapagos

Lava Dome Volcanoes

A lava dome volcano is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. The geochemistry of lava domes can vary from basalt to rhyolite although most preserved domes tend to have high silica content. The characteristic dome shape is attributed to high viscosity that prevents the lava from flowing very far. This high viscosity can be obtained in two ways: by high levels of silica in the magma, or by degassing of fluid magma. Since viscous basaltic and andesitic domes weather fast and easily break apart by further input of fluid lava, most of the preserved domes have high silica content and consists of rhyolite or dacite.

Lava domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance; consequently, on extrusion, the lava piles over and around its vent. A dome grows largely by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Some domes form craggy knobs or spines over the volcanic vent, whereas others form short, steep-sided lava flows known as "coulees." Volcanic domes commonly occur within the craters or on the flanks of large composite volcanoes. The nearly circular Novarupta Dome that formed during the 1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and 200 feet high. The internal structure of this dome--defined by layering of lava fanning upward and outward from the center--indicates that it grew largely by expansion from within. Viscous magma that is made from dacite or rhyolite forms into lava domes. Viscous lava does not flow easily and cools and crystalizes before it goes very far from a vent. They can be made up of more than one lava flow.

Lava Domes

Cinder Cone Volcano

A cinder cone volcano is a steep conical hill of volcanic fragments which accumulate around and downwind from a volcanic vent. The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles "frozen" into place as magma exploded into the air and then cooled quickly. Cinder cones range in size from tens to hundreds of meters tall. Cinder cones are made of pyroclastic material.

Many cinder cone volcanoes have a bowl-shaped crater at the summit. Lava flows are usually erupted by cinder cones, either through a breach on one side of the crater or from a vent located on a flank. If the crater is fully breached, the remaining walls form an amphitheatre or horseshoe shape around the vent. Lava rarely issues from the top (except as a fountain) because the loose, uncemented cinders are too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent.

Cinder cones are commonly found on the flanks of shield volcanoes, stratovolcanoes, and calderas. For example, geologists have identified nearly 100 cinder cones on the flanks of Mauna Kea, a shield volcano located on the island of Hawaii. These cones are also referred to as scoria cones, cinder, and spatter cones. Perhaps the most famous cinder cone is Parícutin, in Mexico in 1943 from a new vent. Eruptions continued for 9 years, built the cone to a height of 424 meters, and produced lava flows that covered 25 km².

The Earth's most historically active cinder cone is Cerro Negro in Nicaragua. It is part of a group of four young cinder cones NW of Las Pilas volcano. Since it was born in 1850, it has erupted more than 20 times, most recently in 1992 and 1995.

Cerro Negro (Volcano)

Cerro Negro is a cinder cone volcano which is situated in the Maribios mountain range in Nicaragua, about 10 km from the village of Malpaisillo. It is 675-meter high and is the youngest volcano in Central America, having first appeared in April 1850. Cerro Negro is composed of a gravelly basaltic cinder cone, which contrasts greatly with the surrounding verdant hillsides, and gives rise to its name, which means Black Hill. Cerro Negro has erupted frequently since its first eruption. One unusual aspect of several eruptions has been the emission of ash from the top of the cone, while lava erupts from fractures at the base.

Cerro Negro is a polygenetic cinder cone that is part of the Central American Volcanic Arc, which formed as a result of the Cocos plate subducting under the Caribbean plate, at a rate of 9 cm/year. It has erupted more than 20 times since its birth in 1850. Explosive eruptions from the central crater are often accompanied by lava flows from the base of the cone. It is the youngest of four cinder cones scattered along a 20 km line east-southeast of Telica. It is the largest and southernmost of four cinder cones that have formed along a NW-SE trend line in the Cordillera de los Maribios mountain range. Despite its youth, Cerro Negro has been one of the most active volcanoes in Nicaragua, with its latest eruption occurring in 1999. Since its birth in 1850, it has erupted approximately 23 times, making Cerro Negro the most active cinder cone volcano in the history of our planet.

Paricutin Volcano

Paricutin is a cinder cone volcano in the Mexican state of Michoacan. It lies close to a lava-covered village of the same name. Paricutin is part of the Michoacan-Guanajuato Volcanic Field, which covers much of west central Mexico. The volcano began as a fissure in a cornfield owned by a P'urhépecha farmer, Dionisio Pulido on February 20, 1943. Pulido, his wife, and their son all witnessed the initial eruption of ash and stones first-hand as they plowed the field.

The Paricutin cinder cone volcano grew quickly, reaching five stories tall in just a week, and it could be seen from afar in a month. Much of the volcano's growth occurred during its first year, while it was still in the explosive pyroclastic phase. Nearby villages Paricutín (after which the volcano was named) and San Juan Parangaricutiro were both buried in lava and ash; the residents relocated to vacant land nearby. At the end of this phase, after roughly one year, the volcano had grown 336 meters (1,102.36 ft) tall. For the next eight years the volcano would continue to erupt, although this was dominated by relatively quiet eruptions of lava that would scorch the surrounding 25 km² (9.65 mi²) of land.

Paricutin's activity would slowly decline during this period until the last six months of the eruption, during which violent and explosive activity was frequent. In 1952 the eruption ended and Parícutin went quiet, attaining a final height of 424 meters (1,391.08 ft) above the cornfield from which it was born. The volcano has been quiet since. Like most cinder cones, Parícutin is a monogenetic volcano, which means that it will never erupt again. Any new eruptions in a monogenetic volcanic field erupt in a new random location.

Mount Pinatubo

Mount Pinatubo is an active stratovolcano which is situated on the island of Luzon, in the Philippines. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is 42 km (26 mi) west of the dormant and more prominent Mount Arayat, occasionally mistaken for Pinatubo. Before 1991, the Mount Pinatubo was inconspicuous and heavily eroded and was covered in dense forest which supported a population of several thousand indigenous people, the Aeta.

The Pinatubo eruption in June 1991 produced the second largest terrestrial eruption of the 20th century (after the 1912 eruption of Novarupta) and the largest eruption in living memory. The colossal 1991 eruption had a Volcanic Explosivity Index (VEI) of 6, and came some 450–500 years after the volcano's last known eruptive activity (estimated as VEI 5, the level of the 1980 eruption of Mount St. Helens), and some 1000 years after previous VEI 6 eruptive activity. Successful predictions of the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives, but surrounding areas were severely damaged by pyroclastic flows, ash deposits, and later by lahars caused by rainwater remobilizing earlier volcanic deposits: thousands of houses and other buildings were destroyed.

The effects of Mt. Pinatubo eruption were felt worldwide. It ejected roughly 10 billion metric tonnes of magma, and 20 million tons of SO2, bringing vast quantities of minerals and metals to the surface environment. It injected large amounts of aerosols into the stratosphere—more than any eruption since that of Krakatoa in 1883. Over the following months, the aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C (0.9 °F), and ozone depletion temporarily increased substantially.

Eruption of Mount Pinatubo June 1991 (video)

Stratovolcano

Also known as composite volcano, a stratovolcano is a tall, conical volcano with many layers of hardened lava, tephra, volcanic ash, and a crater, or craters, at the summit. They are characterized by a steep profile and periodic, explosive eruptions. A stratovolcano is composed of intermixed layers of lava flows and poorly consolidated volcanic material such as ash and cinders. Stratovolcanos are typically located on continental crust above subducting plates and are easily recognized by their often beautiful symmetrically-shaped cones.

Stratovolcanoes erupt explosively as they are usually found at a destructive plate margin. They have also been responsible for the largest, most destructive historic eruptions due to retention and build-up of volatile gases within the volcanos and their magmas during their pre-eruptive phases. During any given year, a few dozen stratovolcanos are active, and around 500 have erupted during historic times.

Stratovolcanoes are also called "composite volcanoes" because of their composite layered structure built up from sequential outpourings of eruptive materials. They are among the most common types of volcanoes, in contrast to the less common shield volcanoes. A famous stratovolcano is Krakatoa, best known for the eruption in 1883.

Cross Section of Stratovolcano

Composite Volcano on an Aleutian Island

Stratovolcano video

Shield Volcano

A shield volcano is a tall, broad volcano with gently sloping sides. Shield volcanoes are usually built up over time by flow after flow of fluid basaltic lava that comes out from vents or fissures on the surface of the volcano, resulting in their relatively flat, broad profile. In contrast, steeply sloped stratovolcanoes better match the popular stereotype of a volcano. Some of the largest volcanoes on Earth are shield volcanoes. The name derives from a translation of "Skjaldbreiður", an Icelandic shield volcano whose name means "broad shield", from its resemblance to a warrior's shield. Shield volcanoes are characteristic of the Hawaiian chain; Manua Kea is an example, with a basal diameter of around 200?km, 4000?m beneath the sea.

Shield volcanoes can be so large that they are sometimes considered to be a mountain range, such as the Ilgachuz Range and the Rainbow Range, both of which are located in Canada. These shield volcanoes formed when the North American Plate moved over a hotspot similar to the one feeding the Hawaiian Islands, called the Anahim hotspot. Shield volcanoes can be found in many places around the world, including Australia, Ethiopia, and the Galapagos Islands. The Piton de la Fournaise, on Reunion Island, is one of the most active shield volcanoes on earth, with one eruption per year on average.

Mount Okmok, Aleutian Islands (A Shield Volcano)

A Shield Volcano Kilauea, Hawaii (Video)

Nociception

Nociception is the neural processes of encoding and processing noxious stimuli. In other words, nociception is the detection of tissue damage by specialized transducers (nociceptors) attached to “A delta” and “C” peripheralnerve fibers. The term "Nociception" is often used interchangably with the term "Pain", but technically refers to the transmission of nociceptive information to the brain without reference to the production of emotional or other types of response to the noxious stimulus. A nociceptor is a primary afferent neuron that is preferentiallysensitive to a noxious stimulus.

Nociception is an afferent activity which is produced in the peripheral and central nervous system by stimuli that have the potential to damage tissue. This activity is initiated by nociceptors, which are pain receptors, that can detect mechanical, thermal or chemical changes above a set threshold. Once stimulated, a nociceptor transmits a signal along the spinal cord, to the brain. Nociception triggers a variety of autonomic responses and may also result in the experience of pain in sentient beings.

Splanchnic Nerves

The splanchnic nerves are paired nerves which contribute to the innervation of the viscera, carrying efferent fibers of the autonomic nervous system as well as sensory afferent fibers from the visceral organs. All carry sympathetic fibers except for the pelvic splanchnic nerves, which carry parasympathetic fibers.

The term splanchnic nerves include the thoracic splanchnic nerves, the lumbar splanchnic nerves, sacral splanchnic nerves, and cardiopulmonary nerves.

Thoracic splanchnic nerves are splanchnic nerves that arise from the sympathetic trunk in the thorax and travel inferiorly to provide sympathetic innervation to the abdomen. The nerves contain preganglionic sympathetic and visceral afferent fibers.

Graves' Disease

Graves' disease is an autoimmune disorder where the thyroid is diffusely enlarged and overactive, producing an excessive amount of thyroid hormones. This metabolic imbalance is known as hyperthyroidism, and the enlargement of the thyroid gland caused by the Graves' disease is called goiter. The changes are caused by autoantibodies to the TSH-receptor (TSHR-Ab) that activate that TSH-receptor, thereby stimulating thyroid hormone synthesis and secretion and thyroid growth (causing a diffuse goiter). The resulting state of hyperthyroidism causes a dramatic constellation of neuropsychological and physical signs and symptoms, and has profound effects on the heart and cadiovascular system. Graves’ disease is the most common cause of severe hyperthyroidism and is accompanied by more clinical signs and symptoms and laboratory abnormalities as compared with milder forms of hyperthyroidism.

Graves' disease affects up to 2% of the female population, sometimes appears after childbirth, and has a female:male incidence of 5:1 to 10:1. It has a powerful hereditary component and frequently presents itself during early adolescence. About 20-25% of people with Graves' disease will also suffer from Graves' ophthalmopathy (a protrusion of one or both eyes), caused by inflammation of the eye muscles by attacking autoantibodies.

Diagnosis is usually made on the basis of symptoms, although thyroid hormone tests may be useful, particularly to monitor treatment. However, Graves’ thyrotoxicosis is a disease which has gradually affected the life of the patients, usually for many months, but sometimes years, prior to the diagnosis. Patients suffer from a wide range of symptoms and have major impairment in most areas of HRQL (health-related quality of life) in the untreated phase. For many patients, quality of life also stays substantially impaired for a long time during and after treatment.

Toxic Multinodular Goiter

Toxic multinodular goiter is an enlarged thyroid gland which contains a small rounded growth or growths called nodules. These nodules produce too much thyroid hormone. Toxic multinodular goiter arises from an existing simple goiter. It occurs most often in the elderly. Risk factors include being female and over 60 years old. It is the second most common cause of hyperthyroidism after Graves disease. Because toxic multinodular goiter is mainly a disease of the elderly, other chronic health problems may influence the outcome of this condition. The elderly person may be less able to tolerate the effect of hyperthyroidism on the heart.

Hyperthyroidism

Hyperthyroidism is a medical condition in which the thyroid gland becomes overactive, synthesizing too much hormones. It occurs when the thyroid releases too much of its hormones in the blood stream over a short (acute) or long (chronic) period of time, causing thyrotoxicosis. Thyroid hormone is important at a cellular level, affecting nearly every type of tissue in the body.

Thyroid hormone is critical to normal function of the cell as it functions as a stimulus to metabolism. In excess, it both overstimulates metabolism and exacerbates the effect of the sympathetic nervous system, causing "speeding up" of various body systems and symptoms resembling an overdose of epinephrine (adrenaline). These include fast heart beat and symptoms of palpitations, nervous system tremor and anxiety symptoms, digestive system hypermotility (diarrhea), and weight loss. On the other hand, a lack of functioning thyroid tissue results in a symptomatic lack of thyroid hormone, termed hypothyroidism.

Symptoms of hyperthyroidism include weight loss, increased heart rate, anxiety, hyperactivity, irritability, tremors, sweating, etc. The treatment depends on the cause and the severity of symptoms. It usually involves initial temporary use of suppressive thyrostatics medication, and possibly later use of permanent surgical or radioisotope therapy. All approaches may cause under active thyroid function (hypothyroidism) which is easily managed with levothyroxine supplementation.

Professor explains hyperthyroidism (video)

Hypothyroidism

Hypothyroidism is a medical condition which is caused by insufficient production of thyroid hormones by the thyroid gland. Cretinism is a form of hypothyroidism found in infants.

The most common cause of hypothyroidism worldwide is iodine deficiency. In iodine-replete individuals hypothyroidism is generally caused by Hashimoto's thyroiditis, or otherwise as a result of either an absent thyroid gland or a deficiency in stimulating hormones from the hypothalamus or pituitary.

Other common cause of hypothyroidism is inflammation of the thyroid gland, which damages the gland's cells. As a result the pituitary gland does not secrete enough thyroid-stimulating hormone (TSH) to induce the thyroid gland to produce enough thyroxine and triiodothyronine.

The characteristic symptoms of hypothyroidism are: poor muscle tone (muscle hypotonia); fatigue; cold intolerance; increased sensitivity to cold; thin, brittle fingernails; thin, brittle hair; depression; decreased sweating; muscle cramps and joint pain.

Treatment: hypothyroidism is treated with the levorotatory forms of thyroxine (L-T4) and triiodothyronine (L-T3). Both synthetic and animal-derived thyroid tablets are available and can be prescribed for patients in need of additional thyroid hormone. Thyroid hormone is taken daily, and doctors can monitor blood levels to help assure proper dosing.

Endometrial Hyperplasia

Endometrial hyperplasia is a medical condition which is characterized by abnormal thickening of the inner lining of the uterus. This is due to an excessive proliferation of the cells of the endometrium which predisposes the woman to tissue changes in the uterus. Endometrial hyperplasia initially represents a physiological response of endometrial tissue to the growth-promoting actions of estrogen. Nevertheless, the gland-forming cells of a hyperplastic endometrium may also undergo changes over time which predispose them to cancerous transformation.

Usually, endometrial hyperplasia result from high levels of estrogens, combined with insufficient levels of the progesterone-like hormones which ordinarily counteract estrogen's proliferative effects on this tissue. This may occur in a number of settings, including polycystic ovary syndrome, estrogen producing tumours and certain formulations of estrogen replacement therapy. Endometrial hyperplasia is a significant risk factor for the development of endometrial cancer so careful monitoring and treatment of women with this disorder is essential.

Neuroblast

A neuroblast is an undifferentiated embryonic cell which develops into a functional neuron. In humans, neuroblasts produced by stem cells in the adult subventricular zone migrate into damaged areas after brain injuries. However, they are restricted to the subtype of small interneuron-like cells, and it is unlikely that they contribute to functional recovery of stratial circuits.

Endocrine System

The endocrine system is a system of glands, each of which secretes a type of hormone to regulate the body. The endocrine system is an information signal system much like the nervous system. Hormones regulate many functions of an organism, including mood, growth and development, tissue function, and metabolism. The field of study that deals with disorders of endocrine glands is endocrinology, a branch of the wider field of internal medicine.

The endocrine system is made up of a series of ductless glands that produce chemicals called hormones. A number of glands that signal each other in sequence is usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis. Typical endocrine glands are the pituitary, thyroid, and adrenal glands. Features of endocrine glands are, in general, their ductless nature, their vascularity, and usually the presence of intracellular vacuoles or granules storing their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen. Also controls metabolism in our body system.

The nervous system sends electrical messages to control and coordinate the body. The endocrine system has a similar job, but uses chemicals to “communicate”. These chemicals are known as hormones. A hormone is a specific messenger molecule synthesized and secreted by a group of specialized cells called an endocrine gland. These glands are ductless, which means that their secretions (hormones) are released directly into the bloodstream and travel to elsewhere in the body to target organs, upon which they act. Note that this is in contrast to our digestive glands, which have ducts for releasing the digestive enzymes.

Corticosteroids

Corticosteroids are a class of steroid hormones which are secreted by the adrenal cortex. Corticosteroids participate in a wide range of physiologic systems such as stress response, immune response and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte levels, and behavior.

The corticosteroids are synthesized from cholesterol within the adrenal cortex. Most steroidogenic reactions are catalysed by enzymes of the cytochrome P450 family. They are located within the mitochondria and require adrenodoxin as a cofactor (except 21-hydroxylase and 17a-hydroxylase). Aldosterone and corticosterone share the first part of their biosynthetic pathway. The last part is either mediated by the aldosterone synthase (for aldosterone) or by the 11ß-hydroxylase (for corticosterone). These enzymes are nearly identical (they share 11ß-hydroxylation and 18-hydroxylation functions) but aldosterone synthase is also able to perform an 18-oxidation. Moreover, aldosterone synthase is found within the zona glomerulosa at the outer edge of the adrenal cortex; 11ß-hydroxylase is found in the zona fasciculata and reticularis.

Corticosteroids are also known as glucocorticoids and mineralocorticoids. They control carbohydrate, fat and protein metabolism. They are also anti-inflammatory by preventing phospholipid release, decreasing eosinophil action and a number of other mechanisms. Mineralocorticoids such as aldosterone control electrolyte and water levels, mainly by promoting sodium retention in the kidney. Some common natural hormones are corticosterone (C21H30O4), cortisone (C21H28O5, 17-hydroxy-11-dehydrocorticosterone) and aldosterone.

Corticosteroids were once thought to be almost miraculous. In 1948, at the Mayo Clinic in Rochester, Minnesota, a group of arthritis patients were given daily injections of a corticosteroid. The results were so striking and the improvement so dramatic that it was thought that the "cure" for arthritis had been discovered. Nevertheless, as the use of corticosteroids expanded over the years, side effects emerged and it was realized that high doses given over prolonged periods of time turned steroids into "scare-oids". Patients were warned of the potential problems, the use of corticosteroids became more conservative, and some patients were so frightened of them they even declined treatment. Corticosteroids are powerful drugs that can have valuable effect if administered within proper guidelines. Understanding how they work and how they can be safely taken is very important. Two of the side effects of using too much corticosteroids are hypertension (high blood pressure) and tumors.

Cortisol

Cortisol is a corticosteroid hormone synthesized by the zona fasciculata of the adrenal cortex, which is a part of the adrenal gland. Cortisol is called the "stress hormone" since it is involved in response to stress and anxiety, controlled by corticotropin-releasing hormone (CRH). Its primary function is to increase blood sugar and stores of sugar in the liver as glycogen, and also suppresses the immune system. Various synthetic forms of cortisol are used to treat a variety of different illnesses. The most well-known of these is a natural metabolic intermediary of cortisol called hydrocortisone. When first introduced as a treatment for rheumatoid arthritis, hydrocortisone was referred to as Compound E.

Cortisol takes part in the following functions: proper glucose metabolism; regulation of blood pressure; insulin release for blood sugar maintanence; immune function; inflammatory response.

The cortisol awakening response (CAR) is an increase of about 50% in cortisol levels occurring 20 to 30 minutes after awakening in the morning in some people. This rise is superimposed upon the late-night rise in cortisol which occurs before awakening. It is thought to be linked to the hippocampus' preparation of the hypothalamic-pituitary-adrenal axis (HPA) to face anticipated stress.

Corticosterone

Corticosterone is a 21-carbon steroid hormone of the corticosteroid type produced in the cortex of the adrenal glands. Classified as a glucocorticoid, corticosterone is a corticosteroid which functions in the metabolism of carbohydrates and proteins. In humans, corticosterone is produced primarily in the zona fasciculata of the adrenal cortex. It has only weak glucocorticoid and mineralocorticoid potencies in humans and is important mainly as an intermediate in the steroidogenic pathway from pregnenolone to aldosterone.

Corticosterone is converted to aldosterone by aldosterone synthase, found only in the mitochondria of glomerulosa cells. Glomerulosa cells are found in the Zona glomerulosa, which is the most superficial region of endocrine cells in the adrenal cortex.

Hypophyseal Portal System

The hypophyseal portal system is the system of blood vessels which links the hypothalamus and the anterior pituitary in the brain. Also known as hypothalamo-hypophyseal portal system, it can be said that it is the vascular anatomy of the pituitary gland which provides the route for hypothalamic regulating hormones to enter and control the secretion of the adenohypophyseal hormones.

The hypophyseal portal system makes it possible for an endocrine communication between the two structures. It is part of the hypothalamic-pituitary-adrenal axis. The anterior pituitary receives releasing and inhibitory hormones in the blood. Using these, the anterior pituitary is able to fulfill its function of regulating the other endocrine glands. It is one of only a few portal systems of circulation in the body; that is, it involves two capillary beds connected in series by venules.

A key to understanding the endocrine relationship between hypothalamus and anterior pituitary is to appreciate the vascular connections between these organs. As will be emphasized in later sections, secretion of hormones from the anterior pituitary is under strict control by hypothalamic hormones. These hypothalamic hormones reach the anterior pituitary through the following route: 1) a branch of the hypophyseal artery ramifies into a capillary bed in the lower hypothalamus, and hypothalmic hormones destined for the anterior pituitary are secreted into that capillary blood; 2) blood from those capillaries drains into hypothalamic-hypophyseal portal veins, with the hypothalamic-hypophyseal portal veins branching again into another series of capillaries within the anterior pituitary; 3) capillaries within the anterior pituitary, which carry hormones secreted by that gland, coalesce into veins that drain into the systemic venous blood.

Hypophyseal Portal System

Hypothalamic-pituitary-adrenal Axis

The hypothalamic-pituitary-adrenal axis (HPA axis) is a complex set of direct influences and feedback interactions among the hypothalamus, the pituitary gland, and the adrenal glands (small, conical organs on top of the kidneys). The interactions among these organs constitute the hypothalamic-pituitary-adrenal axis, a major part of the neuroendocrine system that controls reactions to stress and regulates many body processes, including digestion, the immune system, mood and emotions, sexuality, and energy storage and expenditure. A wide variety of species, from the most ancient organisms to humans, share components of the HPA axis. It is the common mechanism for interactions among glands, hormones, and parts of the midbrain that mediate the general adaptation syndrome.

The hypothalamic-pituitary-adrenal axis is a feedback. The main hormones that activate the HPA axis are corticotropin-releasing hormone (CRH), arginine vasopressin (AVP) and adrenocorticotropin hormone (ACTH). The loop is completed by the negative feedback of cortisol on the hypothalamus and pituitary. The simultaneous release of cortisol into the circulation has a number of effects, including elevation of blood glucose for increased metabolic demand. Cortisol also negatively affects the immune system and prevents the release of immunotransmitters. Interference from other brain regions (eg hippocampus and amygdala) can also modify the HPA axis, as can neuropeptides and neurotransmitters.

The key elements of the HPA axis are the paraventricular nucleus of the hypothalamus, which contains neuroendocrine neurons that synthesize and secrete vasopressin and corticotropin-releasing hormone (CRH). These two peptides regulate the anterior lobe of the pituitary gland. corticotropin-releasing hormone and vasopressin are released from neurosecretory nerve terminals at the median eminence. They are transported to the anterior pituitary through the portal blood vessel system of the hypophyseal stalk. There, CRH and vasopressin act synergistically to stimulate the secretion of stored ACTH from corticotrope cells. ACTH is transported by the blood to the adrenal cortex of the adrenal gland, where it rapidly stimulates biosynthesis of corticosteroids such as cortisol from cholesterol. Cortisol is a major stress hormone and has effects on many tissues in the body, including on the brain.

Release of CRH from the hypothalamus is influenced by stress, by blood levels of cortisol and by the sleep/wake cycle. In healthy individuals, cortisol rises rapidly after wakening, reaching a peak within 30–45 minutes. It then gradually falls over the day, rising again in late afternoon. Cortisol levels then fall in late evening, reaching a trough during the middle of the night. An abnormally flattened circadian cortisol cycle has been linked with chronic fatigue syndrome (MacHale, 1998), insomnia (Backhaus, 2004) and burnout (Pruessner, 1999).

Adrenal Glands

The adrenal glands are orange-colored endocrine glands which are situated on top of the kidneys. They are triangle-shaped, measuring about one-half inch in height and 3 inches in length.

The adrenal gland is composed of two parts: 1) the cortex, which secretes steroid hormones such as cortisol, aldosterone, and testosterone; 2) the medulla, which is the inner part that produces epinephrine and norepinephrine, which are commonly called adrenaline and noradrenaline respectively. When the glands produce more or less hormones than your body needs, you can become sick.

The cortex of the adrenal gland is devoted to the synthesis of corticosteroid hormones from cholesterol. Some cells belong to the hypothalamic-pituitary-adrenal axis and are the source of cortisol and corticosterone synthesis. Under normal unstressed conditions, the human adrenal glands produce the equivalent of 35 to 40 mg of cortisone acetate per day. Other cortical cells produce androgens such as testosterone, while some regulate water and electrolyte concentrations by secreting aldosterone. In contrast to the direct innervation of the medulla, the cortex is regulated by neuroendocrine hormones secreted by the pituitary gland and hypothalamus, as well as by the renin-angiotensin system.

Adrenal Glands Animation

Estrogen

Estrogen is any of several steroid hormones produced mainly by the ovaries and responsible for promoting estrus and the development and maintenance of female secondary sex characteristics. Estrogen is an essential part of a woman’s reproductive process. It regulates the menstrual cycle and prepares the uterus for pregnancy by enriching and thickening the endometrium. Estrogens are steroid hormones which readily diffuse across the cell membrane. Once inside the cell, they bind to and activate estrogen receptors which in turn up-regulate the expression of many genes. Additionally, estrogens have been shown to activate a G protein-coupled receptor, GPR30.

Estrogens are used in estrogen replacement therapy for postmenopausal women, and as part of some oral contraceptives. The three major naturally occurring estrogens in women are estrone (E1), estradiol (E2), and estriol (E3). Estradiol (E2) is the predominant form in nonpregnant females, estrone is produced during menopause, and estriol is the primary estrogen of pregnancy. In the body these are all produced from androgens through actions of enzymes.

Testosterone

Testosterone is a steroid hormone which is secreted by the testes of male. It is responsible for the proper development of male sexual characteristics. It also maintains muscle bulk, adequate levels of red blood cells, bone density, sense of well-being, and sexual and reproductive function. Small amounts are also produced by the adrenal glands.

In men, testosterone plays a key role in the development of male reproductive tissues such as the testis and prostate as well as promoting secondary sexual characteristics such as increased muscle and bone mass and hair growth. In addition, testosterone is essential for health and well-being as well as preventing osteoporosis.

On average, an adult human male body produces about ten times more testosterone than an adult human female body, but females are, from a behavioral perspective rather than from an anatomical or biological perspective, more sensitive to the hormone. However, the overall ranges for male and female are very wide, such that the ranges actually overlap at the low end and high end respectively.

Progestin

Progestin is a synthetic form of the hormone progesterone. The two most frequent uses of progestins are for hormonal contraception (either alone or with an estrogen), and to prevent endometrial hyperplasia from unopposed estrogen in hormone replacement therapy. Progestins are also used to treat secondary amenorrhea, dysfunctional uterine bleeding and endometriosis, and as palliative treatment of endometrial cancer, renal cell carcinoma, breast cancer, and prostate cancer. High dose megestrol acetate is used to treat anorexia, cachexia and AIDS-related wasting. Progesterone, or sometimes the progestin dydrogesterone or 17a-hydroxyprogesterone caproate, is used for luteal support in IVF protocols, questionably for treatment of recurrent pregnancy loss, and for prevention of preterm birth in pregnant women with a history of at least one spontaneous preterm birth.

Scientists have discovered that the most effective method of contraception was with a combination of estrogen and progestin. This can be done in a monophasic, biphasic, or in a triphasic manner. In the monophasic method, both an estrogen and a progestin are administered for 20 or 21 days and stopped for a 7 or 8 day period that includes the 5 day menstrual period. Sometimes, a 28 day regimen is used that includes 6 or 7 inert tablets. Newer biphasic and triphasic methods are now used to more closely simulate the normal menstrual cycle. Yet another method is to administer a small dose of progestin only (no estrogen) in order to decrease certain risks associated with administering estrogen, but a major side effect is irregular bleeding that is usually observed during the first 18 months of such therapy.