Friday, August 19, 2011

In a classic series of papers from the early 1950's, A.L. Hodgkin and A.F. Huxley performed a painstaking series of experiments on the giant axon of the squid. Based on their observations, Hodgkin and Huxley constructed a mathematical model to explain the electrical excitability of neurons in terms of discrete Na+ and K+ currents. A Java version of their Nobel prize winning model (as described in J. Physiol., 1952, 117: 500-544) is presented below:

Source code

What does the model show?
What equations does the model use?
Where can I learn more?
Known bugs
E-mail the author

What does the model show?

The model simulates an electrical signal called an action potential that passes through the axon of a neuron. Action potentials allows neurons to communicate with one another and with muscle cells. This electrical communication makes possible all of our brain's activity and all muscle movement. After you start the model and hit the stimulate button, enough current to raise the voltage +15 mV is injected into the axon. The first time you do this, you'll observe an action potential. If you hit the stimulate button again immediately after the action potential has fired, you'll notice that another action potential does not occur. If you wait a bit longer, however, and again hit the stimulate button, an action potential will again fire. This demonstrates the "refractory period". After a neuron fires, it needs to "rest" before it can fire again. If you experiment a bit with the stimulate button, you'll notice that if you hit it quickly many times in a row you can overcome the refractory period and cause the neuron to fire.

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What equations does the model use?

The first 4 equations above are the differential equations which the program repeatedly steps through. You can go look up what all the symbols mean in Hodgkin and Huxley's 1952 paper (J. Physiol., 117:500-544, p. 518)!

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Where can I learn more?

The classic book on this stuff is "Ionic Channels of Excitable Membranes" by Bertil Hille. The first few chapters cover the Hodgkin-Huxley model in detail. If you are looking for something a little more introductory, I am partial to "Molecular Biology of the Cell" by Alberts, Bray, Lewis, Raff, Roberts and Watson. The book covers all of cell biology (not just neurons and the like) so it is big and expensive, but it very well written and has pretty pictures. Alternatively, you could just go to graduate school!

A much more comprehensive suite of models in Java is online at http://pb010.anes.ucla.edu/

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Known bugs

Like all Java applets, the model may not run from behind a firewall. And obviously, the applet will not run if the browser is not Java-enabled.

On some browsers scrolling up and down can cause the model to become scrambled. I've notice this happening on applets at http://java.sun.com as well, so it seems to be a Java problem. Hitting the refresh button on the browser will reload and unscramble the applet.

If you have any comments, criticisms or bugs to report, I would be very grateful for feedback. You can e-mail me at anthony.fodor@gmail.com.

Tuesday, February 22, 2011

at 1:10 AM Labels: Posted by ELECTRIC MACHINES 0 comments

Endocrine Vs Nervous System

The endocrine system acts with nervous system to coordinate the body's activities.

Both systems enable cells to communicate with others by using chemical messengers.

The endocrine system uses chemical messengers called hormones that are transported by the circulatory system (blood). They act on target cells that may be anywhere in the body.

The endocrine system is slower than the nervous system because hormones must travel through the circulatory system to reach their target.

Target cells have receptors that are specific to the signaling molecules. The binding of hormones to the receptors on or within the target cell produces a response by the target cell.

receptor.gif (2684 bytes)

The chemical messengers used by the nervous system are neurotransmitters. Neurotransmitters travel across a narrow space (the synaptic cleft) and bind to receptors on the target cell.

The nervous system conducts signals much quicker than the endocrine system.

Endocrine Vs Exocrine glands

Endocrine glands do not have ducts. Exocrine glands have ducts that carry their secretions to specific locations.

Two Kinds of Hormones

Peptide Hormones

Peptide hormones are composed of amino acids.

A peptide hormone binds to a cell-surface receptor, it does not enter the cell.

The resulting complex activates an enzyme that catalyzes the synthesis of cyclic AMP from ATP. Cyclic AMP activates other enzymes that are inactive.

Cyclic AMP is a second messenger; the hormone is the first messenger. Other second messengers have been discovered.

Steroid Hormones

Steroid hormones enter the cell and bind to receptors in the cytoplasm.

The hormone-receptor complex enters the nucleus where it binds with chromatin and activates specific genes. Genes (DNA) contain information to produce protein as diagrammed below. When genes are active, protein is produced.

cent_dog.gif (1999 bytes)

Steroid hormones act more slowly than peptide hormones because of the time required to produce new proteins as opposed to activating proteins that are already present.

Hypothalamus

The hypothalamus is part of the brain. It maintains homeostasis (constant internal conditions) by regulating the internal environment (examples: heart rate, body temperature, water balance, and the secretions of the pituitary gland).

Pituitary Gland

The pituitary contains two lobes. Hormones released by the posterior lobe are synthesized by neurons in the hypothalamus. Unlike the posterior lobe, the anterior lobe produces the hormones that it releases.

Refer to the diagram below as you read about the hypothalamus, pituitary, and each of the glands they control.

Posterior pituitary

The posterior pituitary contains axons of neurons that extend from the hypothalamus. Hormones are stored in and released from axon endings in the posterior lobe of the pituitary.

Oxytocin

Oxytocin stimulates the uterine contractions of labor that are needed to move the child out through the birth canal.

The hormone stimulates the release of milk from the mammary glands by causing surrounding cells to contract. After birth, stimulation of the breast by the infant feeding stimulates the posterior pituitary to produce oxyticin.

Antidiuretic Hormone (ADH)

Antidiuretic hormone increases the permeability of the distal convoluted tubule and collecting duct of the kidney nephron resulting in less water in the urine. The urine becomes more concentrated as water is conserved.

The secretion of ADH is controlled by a negative feedback mechanism as follows:

concentrated blood (too little water) ® hypothalamus ® ADH ® kidney ® reabsorbs water, makes blood more dilute

Below: Within the kidney, fluid and dissolved substances are filtered from the blood and pass through tubules where some of the water and dissolved substances are reabsorbed. The remaining liquid and wastes form urine. Details of this process are discussed in the chapter on the excretory system.

The presence of too much blood in the circulatory system stimulates the heart to produce a hormone called atrial natriuretic factor (ANF). This hormone inhibits the release of ADH by the posterior pituitary causing the kidneys to excrete excess water.

Alcohol inhibits the release of ADH, causing the kidneys to produce dilute urine.

Control of the Anterior Pituitary

The hypothalamus produces hormones that travel in blood vessels to the anterior pituitary, stimulating it to produce other hormones.

The hormones produced by the hypothalamus are called hypothalamic-releasing hormones.

The anterior pituitary produces at least six different hormones. Each one is produced in response to a specific hypothalamic-releasing hormone.

The blood vessel that carries hypothalamic-releasing hormones from the hypothalamus to the pituitary is called a portal vein because it connects two capillary beds. One capillary bed is in the hypothalamus and the other is in the anterior pituitary.

Release-inhibiting hormones produced by the hypothalamus inhibit the pituitary from secreting its hormones.

Example

The pituitary is stimulated to release growth hormone (GH) by growth hromone releasing hormone (GHRH) produced in the hypothalamus. It is inhibited from releasing growth hormone by growth hormone release-inhibiting hormone(GHRIH), also produced by the hypothalamus.

Six different hormones produced by the anterior lobe will be studied here. Three of these have direct effects on the body, the other three control other glands.

Anterior Pituitary Hormones that Directly Affect the Body

Growth Hormone (GH or Somatotropic Hormone)

Growth hormone stimulates body cells to grow. If too little hormone is produced, pituitary dwarfism results. The secretion of too much hormone results in a pituitary giant.

Acromegaly is a genetic disease in which growth hormone is produced throughout a persons lifetime.

Prolactin

Prolactin is produced in quantity after childbirth.

It stimulates the development of the mammary glands and the production of milk.

It is also involved in the metabolism of fats and carbohydrates.

Melanocyte-Stimulating Hormone (MSH)

This hormone causes skin color changes in some fishes, amphibians, and reptiles.

In humans, it stimulates the melanocytes to synthesize melanin.

Anterior pituitary hormones that regulate other glands

The pituitary also controls other glands and is often referred to as the "master gland".

Three kinds of pituitary hormones that regulate other glands are discussed below. The glands that they regulate will be discussed in the following section.

Thyroid Stimulating Hormone (TSH) ® thyroid ® thyroxin

Adrenocorticotropic Hormone (ACTH) ® adrenal cortex ® cortisol

Gonadotropic Hormones (FSH and LH) ® ovaries and testes ® sex hormones; controls gamete production

Negative Feedback Inhibition

Hormone secretions by glands that are under the control of the hypothalamus are controlled by negative feedback. When the hormone levels are high, they inhibit the hypothalamus and anterior pituitary, resulting in a decline in their levels.

Thyroid gland

The thyroid produces thyroxin (also called T4 because it contains 4 iodine atoms) and triiodothyronine (also called T3 because it contains 3 iodine atoms).

Both T4 and T3 have similar effects on target cells. In most target tissues, T4 is converted to T3. They influence metabolic rate, growth, and development.

Thyroxin production is regulated by a negative feedback mechanism in which it inhibits the hypothalamus from stimulating the thyroid.

Hypothyroidism occurs when the thyroids produce too little hormone. In adults, it results in lethargy and weight gain. In infants, it causes cretinism, which is characterized by dwarfism, mental retardation, and lack of sexual maturity. Administering thyroid hormones treats these affects.

Too much T3 and T4 (hyperthyroidism) increases heart rate and blood pressure, and causes weight loss.

Iodine is needed to manufacture thyroid hormones. A deficiency in iodine prevents the synthesis of thyroid hormones which, in turn, results in an excess of thyroid stimulating hormone being produced by the anterior pituitary. A goiter results when constant stimulation of the thyroid causes it to enlarge.

Calcitonin

The thyroid gland also secretes calcitonin, which stimulates calcium deposition in the bones. This is the opposite of the action of parathyroid hormone (see below).

Calcitonin production is not regulated by the anterior pituitary. It's secretion is stimulated by high calcium levels in the blood.

Parathyroid glands

The parathyroid glands are 4 small glands embedded in posterior surface of the thyroid gland.

They secrete parathyroid hormone (PTH), which increases blood levels of Ca++.

Bone tissue acts as a storage reservoir for calcium; PTH stimulates the removal of calcium from the bone to increase levels in the blood.

PTH also increases the kidney’s reabsorption of Ca++ so that less is lost in urine and it activates vitamin D which enhances Ca++ absorption from food in the gut.

Secretion is regulated by the Ca++ level in the blood, (not hypothalamic or pituitary hormones).

Adrenal Cortex

The outer layer of an adrenal gland is the adrenal cortex.

It produces three kinds of steroid hormones. These are glucocorticoids, mineralocorticoids, and small amounts of sex hormones. The major glucocorticoid is cortisol and the major mineralocorticoid is aldosterone.

Cortisol (A Glucocorticoid)

Glucocorticoids are produced in response to stress.

Cortisol raises the level of glucose in the blood by stimulating the liver to produce glucose from stored non-carbohydrate sources such as proteins and lipids and to release it into the blood.

Cortisol reduces swelling by inhibiting the immune system. Swelling of tissues due to injury or infection is discussed in the chapter on the immune system. The drug prednisone, derived from cortisol, is used to treat inflammation.

Negative feedback control of cortisol level is diagrammed below.

cortisol.gif (4342 bytes)

Aldosterone (A Mineralocorticoid)

Aldosterone secretion is not under the control of the anterior pituitary.

It acts primarily on the kidney to promote absorption of sodium and excretion of potassium.

Increased sodium levels contributes to the retention of water and thus increased blood volume. In the absence of aldosterone, sodium is excreted and the lower sodium levels result in decreased blood volume and lower blood pressure.

The presence of too much blood in the circulatory system stimulates the heart to produce atrial natriuretic factor. This hormone inhibits the release of aldosterone by the adrenal cortex and ADH by the posterior pituitary causing the kidneys to excrete excess water. The loss of water and sodium contribute to lowering the blood volume.

Adrenal Medulla

The adrenal medulla is composed of modified neurons that secrete epinephrine and norepinephrine (adrenaline and noradrenaline) under conditions of stress.

These hormones are released in response to a variety of stresses and stimulate the fight- or- flight response of the sympathetic nervous system. It results in a faster heart rate, faster blood flow, and dilated airways to facilitate oxygen flow to the lungs. In addition, the level of glucose in the blood is increased to make energy more available.

Their secretion is controlled by brain centers (including hypothalamus) via sympathetic nerves, not by pituitary hormones.

Gonads

LH and FSH from the anterior pituitary stimulate the gonads (ovaries and testes).

LH stimulates the testes to produce several kinds of steroid hormones called androgens. One of these androgens is testosterone, the main sex hormone in males.

LH stimulates the ovaries produce estrogen and progesterone, the female sex hormones.

Sex hormones are responsible for the development of secondary sex characteristics, which develop at puberty. Some examples of secondary sex characteristics in males are deepening of the voice (due to a large larynx), growth of facial hair, and muscle development. Some secondary sex characteristics in females are development of the breasts and broadening of the pelvis. Both sexes show increased activity of sweat glands and sebaceous glands (oil glands in the skin), and growth of pubic and axillary (armpit) hair.

FSH controls gamete (egg or sperm) production.

img015.gif (2623 bytes)

Pancreas

The pancreas is a digestive gland that secretes digestive enzymes into the duodenum through the pancreatic duct.

The islets of Langerhans are groups of cells within the pancreas that secrete insulin and glucagon. The islets are endocrine glands because they are ductless; the circulatory system carries their hormones to target cells.

Insulin

Insulin promotes the removal of glucose from the blood for storage as glycogen (muscle, liver), fats (fat cells), and protein.

It promotes the buildup of fats and proteins and inhibits their use as an energy source.

Glucagon

Glucagon is produced in the islets of Langerhans but by different cells than those that produce insulin.

The effects of glucagon are opposite those of insulin. It raises the level of glucose in the blood.

It is normally secreted between meals to maintain the concentration of glucose in the blood.

Diabetes Mellitus

Diabetes mellitus is a disease in which glucose is not sufficiently metabolized. This results in high glucose levels in blood and glucose in the urine.

Cells can starve because glucose is not being metabolized.

Type I

Type I diabetes is also called "juvenile-onset diabetes" or "insulin-dependent diabetes" because the symptoms usually appear during childhood and insulin injections are necessary to treat it.

It usually occurs after a viral infection triggers an immune response that results in the body destroying its own insulin-producing cells.

Because the disease is caused by a lack of insulin, it can be treated with insulin injections.

Type II

Type II diabetes is more common than type I.

Type II diabetes is caused by a deficiency in insulin production or by changes in insulin receptors on the target cells. In either case, blood glucose level may be high because cells do not receive the message to metabolize glucose.

This form of diabetes usually becomes noticeable in middle age.

It is treated with a low fat, low sugar diet, regular exercise, weight control. Another treatment is oral medications that make the cells more sensitive to the effects of insulin or that stimulate more insulin production.

Thymus Gland

The thymus grows during childhood but gradually decreases in size after puberty.

Lymphocytes that have passed through the thymus are transformed into T cells.

Lymphocytes are white blood cells that function to fight infection. There are two kinds of lymphocytes: B cells and T cells. T cells participate in identifying and destroying body cells that are infected.

Thymus hormones called thymosins stimulate the development and differentiation of T lymphocytes. They play a role in regulating the immune system by stimulating other kinds of immune cells as well.

thymus.gif (2615 bytes)

Pineal Gland

Fish and Amphibians

The pineal gland of fish and amphibians is located near the skin and functions to detect light.

Birds and Mammals

In birds, it is located on the brain but still receives direct light stimulus through the skull.

In mammals, it is located within the brain and therefore cannot receive light stimulation directly. Light from the eyes stimulates the gland via the optic nerve.

Melatonin is produced when the pineal gland is in the dark. During the winter, nights are longer and as a result the level of melatonin in the blood is higher. The level of melatonin in the blood therefore varies with season and can be used to help animals time events such as when to breed, nest, migrate, etc.

These annual cycles are called circannual rhythms. Melatonin may also participate in producing 24-hour cycles called circadian rhythms.

In humans, the gland may be involved in sexual development.

Flashcards

The next five pages contain flashcards that can be used to learn the glands and their secretions. Use scissors to cut out the flashcards.

1) Eleven of the cards contain gland names written on one side. Write the name of the secretion on the other side. Go through these cards by viewing the gland name and trying to identify the secretion. Then, go through the cards by viewing the secretion name and trying to identify the gland name. Continue going through all of the cards until you have learned all of the glands and their secretions.

2) Twenty-three cards have secretions written on one side. Write the following information on the other side:

-The name of the gland that produces the hormone

-How the hormone affects the body

-How production of the hormone is controlled

Go through these cards by viewing the secretion name and trying to state the effect of the hormone and then telling how the hormone is controlled. Continue going through the cards until you have learned how the hormones affect the body and how the hormones are controlled.

Glands

adrenal cortex

adrenal medulla

anterior pituitary

ovaries

testes

pancreas

parathyroid

pineal

posterior pituitary

thymus

thyroid

Secretions

adrenocorticotropic hormone

aldosterone

antidiuretic hormone

calcitonin

cortisol

epinephrine, norepinephrine

estrogen

FSH (follicle stimulating hormone)

glucagon

gonadotropic hormones (FSH, LH)

growth hormone

insulin

LH (leutinizing hormone)

melatonin

oxytocin

parathyroid hormone

prolactin

progesterone

testosterone

thymosins

thyroid stimulating hormone

thyroxin

triiodothyronine

The Biology Web Home page

source:http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/endocrine%20system/endocrin.htm

Glands are small but powerful organs that are located throughout the body. They control very important body functions by releasing hormones.

The following list of glands make up the endocrine system.

Pituitary Gland
The pituitary gland is sometimes called the "master gland" because of its great influence on the other body organs. Its function is complex and important for overall well-being.pituitary

The pituitary gland is divided into two parts, front (anterior) and back (posterior).

The anterior pituitary produces several types of hormones:

  • Prolactin or PRL - PRL stimulates milk production from a woman's breasts after childbirth and can affect sex hormone levels from the ovaries in women and the testes in men.
  • Growth hormone or GH - GH stimulates growth in childhood and is important for maintaining a healthy body composition. In adults it is also important for maintaining muscle mass and bone mass. It can affect fat distribution in the body. (For more information go to the Growth section on this site)
  • Adrenocorticotropin or ACTH - ACTH stimulates production of cortisol by the adrenal glands. Cortisol, a so-called "stress hormone," is vital to survival. It helps maintain blood pressure and blood glucose levels.
  • Thyroid-stimulating hormone or TSH - TSH stimulates the thyroid gland to make thyroid hormones, which, in turn, control (regulate) the body's metabolism, energy, growth and development, and nervous system activity.
  • Luteinizing hormone or LH - LH regulates testosterone in men and estrogen in women.
  • Follicle-stimulating hormone or FSH - FSH promotes sperm production in men and stimulates the ovaries to release eggs (ovulate) in women. LH and FSH work together to allow normal function of the ovaries or testes.

The posterior pituitary produces two hormones:

  • Oxytocin - Oxytocin causes milk letdown in nursing mothers and contractions during childbirth.
  • Antidiuretic hormone or ADH - ADH, also called vasopressin, is stored in the back part of the pituitary gland and regulates water balance. If this hormone is not secreted properly, this can lead to problems of sodium (salt) and water balance, and could also affect the kidneys so that they do not work as well.

In response to over- or underproduction of pituitary hormones, the target glands affected by these hormones can produce too many or too few hormones of their own, leading to hormone imbalance. For example, too much growth hormone can cause gigantism, or excessive growth (referred to as acromegaly in adults), while too little GH may cause dwarfism, or very short stature.

Additional Resources


Hypothalamus
The hypothalamus is part of the brain that lies just above the pituitary gland. It releases hormones that start and stop the release of pituitary hormones. The hypothalamus controls hormone production in the pituitary gland through several "releasing" hormones. Some of these are growth hormone-releasing hormone, or (controls GH release); thyrotropin-releasing hormone, or TRH (controls TSH release); and corticoptropin-releasing hormone, or CRH (controls ACTH release). Gonadotropin-releasing hormone (GnRH) tells the pituitary gland to make luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are important for normal puberty.
Thymus
ThymusThe thymus is a gland needed early in life for normal immune function. It is very large just after a child is born and weighs its greatest when a child reaches puberty. Then its tissue is replaced by fat. The thymus gland secretes hormones called humoral factors. These hormones help to develop the lymphoid system, which is a system throughout the body that help it to reach a mature immune response in cells to protect them from invading bodies, like bacteria.
Pineal Gland
Pineal glandScientists are still learning how the pineal gland works. They have found one hormone so far that is produced by this gland: melatonin. Melatonin may stop the action of (inhibit) the hormones that produce gonadotropin, which causes the ovaries and testes to develop and function. It may also help to control sleep patterns.
Testes
Testes Males have twin reproductive glands, called testes, that produce the hormone testosterone. Testosterone helps a boy develop and then maintain his sexual traits. During puberty, testosterone helps to bring about the physical changes that turn a boy into an adult male, such as growth of the penis and testes, growth of facial and pubic hair, deepening of the voice, increase in muscle mass and strength, and increase in height. Throughout adult life, testosterone helps maintain sex drive, sperm production, male hair patterns, muscle mass, and bone mass.

Testicular cancer, which is the most common form of cancer for males between ages 15 and 35, may need to be treated by surgical removal of one or both testicles. The resulting decrease or absence of testosterone may cause decreased sexual drive, impotence, altered body image, and other symptoms.
Ovaries
OvariesThe two most important hormones of a woman's twin reproductive glands, the ovaries, are estrogen and progesterone. These hormones are responsible for developing and maintaining female sexual traits, as well as maintaining a pregnancy. Along with the pituitary gonadotropins (luteinizing hormone or LH and follicle-stimulating hormone or FSH), they also control the menstrual cycle. The ovaries also produce inhibin, a protein that curbs (inhibits) the release of follicle-stimulating hormone from the anterior pituitary and helps control egg development.

The most common change in the ovarian hormones is caused by the start of menopause, part of the normal aging process. It also can occur when ovaries are removed surgically. Loss of ovarian function means loss of estrogen, which can lead to symptoms of menopause including hot flashes, thinning vaginal tissue, lack of menstrual periods, mood changes and bone loss, or osteoporosis.

A condition called polycystic ovary syndrome (PCOS) is caused by overproduction of male hormones in females. PCOS can affect menstrual cycles, fertility, and hormone levels, as well as cause acne, facial hair growth, and male pattern balding.

Additional Resources

  • Menopause information
  • Osteoporosis information
  • Polycystic Ovary Syndrome information
  • Educational Resources
Thyroid

thyroidThe thyroid is a small gland inside the neck, located in front of your breathing airway (trachea) and below your Adam's apple. The thyroid hormones control your metabolism, which is the body's ability to break down food and store it as energy and the ability to break down food into waste products with a release of energy in the process. The thyroid produces two hormones, T3 (called tri-iodothyronine) and T4 (called thyroxine).

Thyroid disorders result from an underactive or overactive thyroid producing, respectively, too little or too much thyroid hormone. Symptoms of hypothyroidism (too little hormone) include decreased energy, slow heart rate, dry skin, constipation, and feeling cold all the time. In children, hypothyroidism most commonly leads to slowed growth. Infants born with hypothyroidism can have delayed development and mental retardation if not treated. In adults, this disorder often causes weight gain. An enlarged thyroid, or goiter, may develop.

Hyperthyroidism (too much hormone) may impact normal thyroid size and result in exophthalmic goiter, or Grave's disease. Symptoms of this thyroid disease include anxiety, fast heart rate, diarrhea, and weight loss. An enlarged thyroid gland (goiter) and swelling behind the eyes that causes the eyes to push forward, or bulge out, are common.

Additional Resources

Adrenal Glands
undefinedEach adrenal gland is actually two endocrine organs. The outer portion is called the adrenal cortex. The inner portion is called the adrenal medulla. The hormones of the adrenal cortex are essential for life. The types of hormones secreted by the adrenal medulla are not.

The adrenal cortex produces glucocorticoids (such as cortisol) that help the body control blood sugar, increase the burning of protein and fat, and respond to stressors like fever, major illness, and injury. The mineralcorticoids (such as aldosterone) control blood volume and help to regulate blood pressure by acting on the kidneys to help them hold onto enough sodium and water. The adrenal cortex also produces some sex hormones, which are important for some secondary sex characteristics in both men and women.

Two important disorders caused by problems with the adrenal cortex are Cushing's syndrome and Addison's disease. Cushing's syndrome is the result of too much cortisol, and Addison's disease occurs when there is too little cortisol.

The adrenal medulla produces epinephrine (adrenaline), which is secreted by nerve endings and increases the heart rate, opens airways to improve oxygen intake, and increases blood flow to muscles, usually when a person is scared, excited, or under stress.

Norepinephrine also is made by the adrenal medulla, but this hormone is more related to maintaining normal activities as opposed to emergency reactions. Too much norepinephrine can cause high blood pressure.

Additional Resources

Parathyroid

Parathyroid glandLocated behind the thyroid gland are four tiny parathyroid glands. These make hormones that help control calcium and phosphorous levels in the body. The parathyroid glands are necessary for proper bone development. In response to too little calcium in the diet, the parathyroid glands make parathyroid hormone, or PTH, that takes calcium from bones so that it will be available in the blood for nerve conduction and muscle contraction.

If the parathyroids are removed during a thyroid operation, low blood calcium will result in symptoms such as irregular heartbeat, muscle spasms, tingling in the hands and feet, and possibly difficulty breathing. A tumor or chronic illness can cause too much secretion of PTH and lead to bone pain, kidney stones, increased urination, muscle weakness, and fatigue.


Pancreas
PancreasThe pancreas is a large gland behind your stomach that helps the body to maintain healthy blood sugar (glucose) levels. The pancreas secretes insulin, a hormone that helps glucose move from the blood into the cells where it is used for energy. The pancreas also secretes glucagon when the blood sugar is low. Glucagon tells the liver to release glucose, stored in the liver as glycogen, into the bloodstream.

Diabetes, an imbalance of blood sugar levels, is the major disorder of the pancreas. There are two types of diabetes. Type I, and Type II diabetes. Type I diabetes occurs when the pancreas does not produce enough insulin. Type II diabetes occurs when the body is resistant to the insulin in the blood). Without enough insulin to keep glucose moving through the metabolic process, the blood glucose level rises too high.

In Type I diabetes, a patient must take insulin shots. In Type II diabetes, a patient may may not necessarily need insulin and can sometimes control blood sugar levels with exercise, diet and other medications.

A condition called hyperinsulinism (HI) is caused by too much insulin and leads to hypoglycemia (low blood sugar). The inherited form, called congenital HI, causes severe hypoglycemia in infancy. Sometimes it can be treated with medication but often requires surgical removal of part or all of the pancreas. An insulin-secreting tumor of the pancreas, or insulinoma, is a less common cause of hypoglycemia. Symptoms of low blood sugar include anxiety, sweating, increased heart rate, weakness, hunger, and light-headedness. Low blood sugar stimulates release of epinephrine, glucagon and growth hormone, which help to return the blood sugar to normal.

Additional Resources

source: www.hormone.org/Endo101/page2.cfm#CP_JUMP_825