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.

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. 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. 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. 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. 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. 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
• Hypothalmus
• Thymus
• Pineal Gland
• Testes
• Ovaries
• Thyroid
• Adrenal Glands
• Parathyroid
• Pancreas

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.

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

• Growth Information
• Pituitary Gland Information
• Educational 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

The 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

Scientists 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

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

The 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

The 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

• Thyroid information
  
• Educational Resources

Adrenal Glands

Each 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

• Pituitary Gland information
• Educational Resources

Parathyroid

Located 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

The 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

• Diabetes information
• Educational Resources

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

What's in this article? (click to view)

• What Is the Endocrine System?
• Hypothalamus
• Pituitary
• Thyroid
• Parathyroids
• Adrenal Glands
• Pineal
• Reproductive Glands
• What Does the Endocrine System Do?
• Things That Can Go Wrong With the Endocrine System

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Ever dozed through chemistry class and wondered what chemistry had to do with you? A lot! Your body produces its own chemicals and uses them to control certain functions, and the main system that coordinates these chemicals is called the endocrine system.

What Is the Endocrine System?

Although we rarely think about the endocrine system, it influences almost every cell, organ, and function of our bodies. The endocrine system is instrumental in regulating mood, growth and development, tissue function, metabolism, and sexual function and reproductive processes.

In general, the endocrine system is in charge of body processes that happen slowly, such as cell growth. Faster processes like breathing and body movement are controlled by the nervous system. But even though the nervous system and endocrine system are separate systems, they often work together to help the body function properly.

The foundations of the endocrine system are the hormones and glands. As the body's chemical messengers, hormones (pronounced: hor-moanz) transfer information and instructions from one set of cells to another. Many different hormones move through the bloodstream, but each type of hormone is designed to affect only certain cells.

A gland is a group of cells that produces and secretes, or gives off, chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body.

Some types of glands release their secretions in specific areas. For instance, exocrine (pronounced: ek-suh-krin) glands, such as the sweat and salivary glands, release secretions in the skin or inside the mouth. Endocrine glands, on the other hand, release more than 20 major hormones directly into the bloodstream where they can be transported to cells in other parts of the body.

The major glands that make up the human endocrine system include the:

• hypothalamus
• pituitary gland
• thyroid
• parathyroids
• adrenal glands
• pineal body
• reproductive glands (which include the ovaries and testes)

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Hypothalamus

The hypothalamus (pronounced: hi-po-tha-luh-mus), a collection of specialized cells that is located in the lower central part of the brain, is the main link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.

Pituitary

Although it is no bigger than a pea, the pituitary (pronounced: puh-too-uh-ter-ee) gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands.

The production and secretion of pituitary hormones can be influenced by factors such as emotions and changes in the seasons. To accomplish this, the hypothalamus provides information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.

The tiny pituitary is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. The anterior lobe produces hormones such as:

• growth hormone, which stimulates the growth of bone and other body tissues and plays a role in the body's handling of nutrients and minerals
• prolactin (pronounced: pro-lak-tin), which activates milk production in women who are breastfeeding
• thyrotropin (pronounced: thy-ruh-tro-pin), which stimulates the thyroid gland to produce thyroid hormones
• corticotropin (pronounced: kor-tih-ko-tro-pin), which stimulates the adrenal gland to produce certain hormones

The pituitary also secretes endorphins (pronounced: en-dor-fins), chemicals that act on the nervous system and reduce feelings of pain. In addition, the pituitary secretes hormones that signal the reproductive organs to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.

The posterior lobe of the pituitary releases antidiuretic (pronounced: an-ty-dy-uh-reh-tik) hormone, which helps control the balance of water in the body. The posterior lobe also produces oxytocin (pronounced: ahk-see-toe-sin), which triggers the contractions of the uterus in a woman having a baby.

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Thyroid

The thyroid (pronounced: thy-royd), located in the front part of the lower neck, is shaped like a bow tie or butterfly and produces the thyroid hormones thyroxine (pronounced: thy-rahk-sin) and triiodothyronine (pronounced: try-eye-oh-doe-thy-ruh-neen). These hormones control the rate at which cells burn fuels from food to produce energy.

The production and release of thyroid hormones is controlled by thyrotropin (pronounced: thy-ruh-tro-pin), which is secreted by the pituitary gland. The more thyroid hormone there is in a person's bloodstream, the faster chemical reactions occur in the body.

Why are thyroid hormones so important? There are several reasons — for example, they help kids' and teens' bones grow and develop, and they also play a role in the development of the brain and nervous system in kids.

Parathyroids

Attached to the thyroid are four tiny glands that function together called the parathyroids (pronounced: par-uh-thy-roydz). They release parathyroid hormone, which regulates the level of calcium in the blood with the help of calcitonin (pronounced: kal-suh-toe-nin), which is produced in the thyroid.

Adrenal Glands

The body also has two triangular adrenal (pronounced: uh-dree-nul) glands, one on top of each kidney.

The adrenal glands have two parts, each of which produces a set of hormones and has a different function:

1. The outer part, the adrenal cortex, produces hormones called corticosteroids (pronounced: kor-tih-ko-ster-oydz) that influence or regulate salt and water balance in the body, the body's response to stress, metabolism, the immune system, and sexual development and function.
2. The inner part, the adrenal medulla (pronounced: muh-duh-luh), produces catecholamines (pronounced: kah-tuh-ko-luh-meenz), such as epinephrine (pronounced: eh-puh-neh-frun). Also called adrenaline, epinephrine increases blood pressure and heart rate when the body experiences stress.

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Pineal

The pineal (pronounced: pih-nee-ul) body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin (pronounced: meh-luh-toe-nin), a hormone that may help regulate when you sleep at night and when you wake in the morning.

Reproductive Glands

The gonads are the main source of sex hormones. Most people don't realize it, but both guys and girls have gonads.

In guys the male gonads, or testes (pronounced: tes-teez), are located in the scrotum. They secrete hormones called androgens (pronounced: an-druh-junz), the most important of which is testosterone (pronounced: teh-stass-tuh-rone). These hormones tell a guy's body when it's time to make the changes associated with puberty, like penis and height growth, deepening voice, and growth in facial and pubic hair. Working with hormones from the pituitary gland, testosterone also tells a guy's body when it's time to produce sperm in the testes.

A girl's gonads, the ovaries (pronounced: oh-vuh-reez), are located in her pelvis. They produce eggs and secrete the female hormones estrogen (pronounced: es-truh-jen) and progesterone (pronounced: pro-jes-tuh-rone). Estrogen is involved when a girl begins to go through puberty. During puberty, a girl will experience breast growth, will begin to accumulate body fat around the hips and thighs, and will have a growth spurt. Estrogen and progesterone are also involved in the regulation of a girl's menstrual cycle. These hormones also play a role in pregnancy.

Although the endocrine glands are the body's main hormone producers, some other organs not in the endocrine system — such as the brain, heart, lungs, kidneys, liver, and skin — also produce and release hormones.

The pancreas (pronounced: pan-kree-us) is also part of the body's hormone-secreting system, even though it is also associated with the digestive system because it produces and secretes digestive enzymes. The pancreas produces (in addition to others) two important hormones, insulin (pronounced: in-suh-lin) and glucagon (pronounced: gloo-kuh-gawn). They work together to maintain a steady level of glucose, or sugar, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy.

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What Does the Endocrine System Do?

Once a hormone is secreted, it travels from the endocrine gland that produced it through the bloodstream to the cells designed to receive its message. These cells are called target cells. Along the way to the target cells, special proteins bind to some of the hormones. These proteins act as carriers that control the amount of hormone that is available for the cells to use.

The target cells have receptors that latch onto only specific hormones, and each hormone has its own receptor, so that each hormone will communicate only with specific target cells that have receptors for that hormone. When the hormone reaches its target cell, it locks onto the cell's specific receptors and these hormone-receptor combinations transmit chemical instructions to the inner workings of the cell.

When hormone levels reach a certain normal amount, the endocrine system helps the body to keep that level of hormone in the blood. For example, if the thyroid gland has secreted the right amount of thyroid hormones into the blood, the pituitary gland senses the normal levels of thyroid hormone in the bloodstream. Then the pituitary gland adjusts its release of thyrotropin, the hormone that stimulates the thyroid gland to produce thyroid hormones.

Another example of this process is parathyroid hormone. Parathyroid hormone increases the level of calcium in the blood. When the blood calcium level rises, the parathyroid glands sense the change and reduce their secretion of parathyroid hormone. This turnoff process is called a negative feedback system.

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Things That Can Go Wrong With the Endocrine System

Too much or too little of any hormone can be harmful to your body. For example, if the pituitary gland produces too much growth hormone, a teen may grow excessively tall. If it produces too little, a teen may be unusually short. Doctors can often treat problems with the endocrine system by controlling the production of hormones or replacing certain hormones with medication.

Some endocrine problems that affect teens include:

Adrenal insufficiency. This condition occurs when the adrenal glands don't produce enough corticosteroids. The symptoms of adrenal insufficiency may include weakness, fatigue, abdominal pain, nausea, dehydration, and skin changes. Doctors treat adrenal insufficiency with medications to replace corticosteroid hormones.

Type 1 diabetes. When the pancreas fails to produce enough insulin, type 1 diabetes (pronounced: dy-uh-be-teez and previously known as juvenile diabetes) occurs. In kids and teens, type 1 diabetes is usually an autoimmune disorder, which means that some parts of the body's immune system attack and destroy the cells of the pancreas that produce insulin. To control their blood sugar levels and reduce the risk of developing diabetes problems, kids and teens with this condition need regular injections of insulin.

Type 2 diabetes. Unlike type 1 diabetes, in which the body can't produce normal amounts of insulin, in type 2 diabetes the body can't respond to insulin normally. Kids and teens with the condition tend to be overweight. Some kids and teens can control their blood sugar level with dietary changes, exercise, and oral medications, but many will need to take insulin injections like people with type 1 diabetes.

Growth hormone problems. Too much growth hormone in kids and teens who are still growing will make their bones and other body parts grow excessively. This rare condition (sometimes called gigantism) is usually caused by a pituitary tumor and can be treated by removing the tumor. The opposite can happen when a kid or teen has a pituitary glad that doesn't produce enough growth hormone. Doctors may treat these growth problems with medication.

Hyperthyroidism. Hyperthyroidism (pronounced: hy-per-thy-roy-dih-zum) is a condition in which the levels of thyroid hormones in the blood are very high. In kids and teens, the condition is usually caused by Graves' disease, an immune system problem that causes the thyroid gland to become very active. Doctors may treat hyperthyroidism with medications, surgery, or radiation treatments.

Hypothyroidism. Hypothyroidism (pronounced: hy-po-thy-roy-dih-zum) is a condition in which the levels of thyroid hormones in the blood are very low. Thyroid hormone deficiency slows body processes and may lead to fatigue, a slow heart rate, dry skin, weight gain, and constipation. Kids and teens with this condition may also grow more slowly and reach puberty at a later age. Hashimoto's thyroiditis is an immune system problem that often causes problems with the thyroid and blocks the production of thyroid hormone. Doctors often treat this problem with medication.

Precocious puberty. If the pituitary glands release hormones that stimulate the gonads to produce sex hormones too early, some kids may begin to go through puberty at a very young age. This condition is called precocious puberty. Kids and teens who are affected by precocious puberty can be treated with medication that will help them develop at a normal rate.

Reviewed by: Steven Dowshen, MD
Date reviewed: August 2009

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The nervous system is the major controlling, regulatory, and communicating system in the body. Its activities can be grouped together as three general, overlapping functions:

• Sensory
• Integrative
• Motor

Millions of sensory receptors detect changes, called stimuli, which occur inside and outside the body. They monitor such things as temperature, light, and sound from the external environment. Inside the body, the internal environment, receptors detect variations in pressure, pH, carbon dioxide concentration, and the levels of various electrolytes. All of this gathered information is called sensory input.

Sensory input is converted into electrical signals called nerve impulses that are transmitted to the brain. There the signals are brought together to create sensations, to produce thoughts, or to add to memory. Decisions are made each moment based on the sensory input. This is integration.

Based on the sensory input and integration, the nervous system responds by sending signals to muscles, causing them to contract, or to glands, causing them to produce secretions. Muscles and glands are called effectors because they cause an effect in response to directions from the nervous system. This is the motor output or motor function.

Nerve Tissues

Although the nervous system is very complex, there are only two main types of cells in nerve tissue. The actual nerve cell is the neuron. It is the "conducting" cell that transmits impulses and the structural unit of the nervous system. The other type of cell is neuroglia, or glial, cell. The word "neuroglia" means "nerve glue." These cells are nonconductive and provide a support system for the neurons. They are a special type of "connective tissue" for the nervous system.

Neurons

Neurons, or nerve cells, carry out the functions of the nervous system by conducting nerve impulses. They are highly specialized and amitotic. This means that if a neuron is destroyed, it cannot be replaced because neurons do not go through mitosis. The image below illustrates the structure of a typical neuron.

Each neuron has three basic parts: cell body (soma), one or more dendrites, and a single axon.

Cell Body

In many ways, the cell body is similar to other types of cells. It has a nucleus with at least one nucleolus and contains many of the typical cytoplasmic organelles. It lacks centrioles, however. Because centrioles function in cell division, the fact that neurons lack these organelles is consistent with the amitotic nature of the cell.

Dendrites

Dendrites and axons are cytoplasmic extensions, or processes, that project from the cell body. They are sometimes referred to as fibers. Dendrites are usually, but not always, short and branching, which increases their surface area to receive signals from other neurons. The number of dendrites on a neuron varies. They are called afferent processes because they transmit impulses to the neuron cell body. There is only one axon that projects from each cell body. It is usually elongated and because it carries impulses away from the cell body, it is called an efferent process.

Axon

An axon may have infrequent branches called axon collaterals. Axons and axon collaterals terminate in many short branches or telodendria. The distal ends of the telodendria are slightly enlarged to form synaptic bulbs. Many axons are surrounded by a segmented, white, fatty substance called myelin or the myelin sheath. Myelinated fibers make up the white matter in the CNS, while cell bodies and unmyelinated fibers make the gray matter. The unmyelinated regions between the myelin segments are called the nodes of Ranvier.

In the peripheral nervous system, the myelin is produced by Schwann cells. The cytoplasm, nucleus, and outer cell membrane of the Schwann cell form a tight covering around the myelin and around the axon itself at the nodes of Ranvier. This covering is the neurilemma, which plays an important role in the regeneration of nerve fibers. In the CNS, oligodendrocytes produce myelin, but there is no neurilemma, which is why fibers within the CNS do not regenerate.

Functionally, neurons are classified as afferent, efferent, or interneurons (association neurons) according to the direction in which they transmit impulses relative to the central nervous system. Afferent, or sensory, neurons carry impulses from peripheral sense receptors to the CNS. They usually have long dendrites and relatively short axons. Efferent, or motor, neurons transmit impulses from the CNS to effector organs such as muscles and glands. Efferent neurons usually have short dendrites and long axons. Interneurons, or association neurons, are located entirely within the CNS in which they form the connecting link between the afferent and efferent neurons. They have short dendrites and may have either a short or long axon.

Neuroglia

Neuroglia cells do not conduct nerve impulses, but instead, they support, nourish, and protect the neurons. They are far more numerous than neurons and, unlike neurons, are capable of mitosis.

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The nervous system as a whole is divided into two subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

The Central Nervous System

The brain and spinal cord are the organs of the central nervous system. Because they are so vitally important, the brain and spinal cord, located in the dorsal body cavity, are encased in bone for protection. The brain is in the cranial vault, and the spinal cord is in the vertebral canal of the vertebral column. Although considered to be two separate organs, the brain and spinal cord are continuous at the foramen magnum. In addition to bone, the CNS is surrounded by connective tissue membranes, called meninges, and by cerebrospinal fluid.

Meninges

There are three layers of meninges around the brain and spinal cord. The outer layer, the dura mater, is tough white fibrous connective tissue. The middle layer of meninges is arachnoid, which resembles a cobweb in appearance, is a thin layer with numerous threadlike strands that attach it to the innermost layer. The space under the arachnoid, the subarachnoid space, is filled with cerebrospinal fluid and contains blood vessels. The pia mater is the innermost layer of meninges. This thin, delicate membrane is tightly bound to the surface of the brain and spinal cord and cannot be dissected away without damaging the surface.

Meningiomas are tumors of the nerve tissue covering the brain and spinal cord. Although meningiomas are usually not likely to spread, physicians often treat them as though they were malignant to treat symptoms that may develop when a tumor applies pressure to the brain.

Brain

The brain is divided into the cerebrum, diencephalons, brain stem, and cerebellum.

Cerebrum

The largest and most obvious portion of the brain is the cerebrum, which is divided by a deep longitudinal fissure into two cerebral hemispheres. The two hemispheres are two separate entities but are connected by an arching band of white fibers, called the corpus callosum that provides a communication pathway between the two halves.

Each cerebral hemisphere is divided into five lobes, four of which have the same name as the bone over them: the frontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. A fifth lobe, the insula or Island of Reil, lies deep within the lateral sulcus.

Diencephalon

The diencephalon is centrally located and is nearly surrounded by the cerebral hemispheres. It includes the thalamus, hypothalamus, and epithalamus. The thalamus, about 80 percent of the diencephalon, consists of two oval masses of gray matter that serve as relay stations for sensory impulses, except for the sense of smell, going to the cerebral cortex. The hypothalamus is a small region below the thalamus, which plays a key role in maintaining homeostasis because it regulates many visceral activities. The epithalamus is the most dorsal portion of the diencephalon. This small gland is involved with the onset of puberty and rhythmic cycles in the body. It is like a biological clock.

Brain Stem

The brain stem is the region between the diencephalon and the spinal cord. It consists of three parts: midbrain, pons, and medulla oblongata. The midbrain is the most superior portion of the brain stem. The pons is the bulging middle portion of the brain stem. This region primarily consists of nerve fibers that form conduction tracts between the higher brain centers and spinal cord. The medulla oblongata, or simply medulla, extends inferiorly from the pons. It is continuous with the spinal cord at the foramen magnum. All the ascending (sensory) and descending (motor) nerve fibers connecting the brain and spinal cord pass through the medulla.

Cerebellum

The cerebellum, the second largest portion of the brain, is located below the occipital lobes of the cerebrum. Three paired bundles of myelinated nerve fibers, called cerebellar peduncles, form communication pathways between the cerebellum and other parts of the central nervous system.

Spinal Cord

The spinal cord extends from the foramen magnum at the base of the skull to the level of the first lumbar vertebra. The cord is continuous with the medulla oblongata at the foramen magnum. Like the brain, the spinal cord is surrounded by bone, meninges, and cerebrospinal fluid.

The spinal cord is divided into 31 segments with each segment giving rise to a pair of spinal nerves. At the distal end of the cord, many spinal nerves extend beyond the conus medullaris to form a collection that resembles a horse's tail. This is the cauda equina. In cross section, the spinal cord appears oval in shape.

The spinal cord has two main functions:

• Serving as a conduction pathway for impulses going to and from the brain. Sensory impulses travel to the brain on ascending tracts in the cord. Motor impulses travel on descending tracts.
• Serving as a reflex center. The reflex arc is the functional unit of the nervous system. Reflexes are responses to stimuli that do not require conscious thought and consequently, they occur more quickly than reactions that require thought processes. For example, with the withdrawal reflex, the reflex action withdraws the affected part before you are aware of the pain. Many reflexes are mediated in the spinal cord without going to the higher brain centers.

The Peripheral Nervous System

The peripheral nervous system consists of the nerves that branch out from the brain and spinal cord. These nerves form the communication network between the CNS and the body parts. The peripheral nervous system is further subdivided into the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of nerves that go to the skin and muscles and is involved in conscious activities. The autonomic nervous system consists of nerves that connect the CNS to the visceral organs such as the heart, stomach, and intestines. It mediates unconscious activities.

Structure of a Nerve

A nerve contains bundles of nerve fibers, either axons or dendrites, surrounded by connective tissue. Sensory nerves contain only afferent fibers, long dendrites of sensory neurons. Motor nerves have only efferent fibers, long axons of motor neurons. Mixed nerves contain both types of fibers.

A connective tissue sheath called the epineurium surrounds each nerve. Each bundle of nerve fibers is called a fasciculus and is surrounded by a layer of connective tissue called the perineurium. Within the fasciculus, each individual nerve fiber, with its myelin and neurilemma, is surrounded by connective tissue called the endoneurium. A nerve may also have blood vessels enclosed in its connective tissue wrappings.

Cranial Nerves

Twelve pairs of cranial nerves emerge from the inferior surface of the brain. All of these nerves, except the vagus nerve, pass through foramina of the skull to innervate structures in the head, neck, and facial region.

The cranial nerves are designated both by name and by Roman numerals, according to the order in which they appear on the inferior surface of the brain. Most of the nerves have both sensory and motor components. Three of the nerves are associated with the special senses of smell, vision, hearing, and equilibrium and have only sensory fibers. Five other nerves are primarily motor in function but do have some sensory fibers for proprioception. The remaining four nerves consist of significant amounts of both sensory and motor fibers.

Spinal Nerves

Thirty-one pairs of spinal nerves emerge laterally from the spinal cord. Each pair of nerves corresponds to a segment of the cord and they are named accordingly. This means there are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve.

Each spinal nerve is connected to the spinal cord by a dorsal root and a ventral root. The cell bodies of the sensory neurons are in the dorsal root ganglion, but the motor neuron cell bodies are in the gray matter. The two roots join to form the spinal nerve just before the nerve leaves the vertebral column. Because all spinal nerves have both sensory and motor components, they are all mixed nerves.

Autonomic Nervous System

The autonomic nervous system is a visceral efferent system, which means it sends motor impulses to the visceral organs. It functions automatically and continuously, without conscious effort, to innervate smooth muscle, cardiac muscle, and glands. It is concerned with heart rate, breathing rate, blood pressure, body temperature, and other visceral activities that work together to maintain homeostasis.

The autonomic nervous system has two parts, the sympathetic division and the parasympathetic division. Many visceral organs are supplied with fibers from both divisions. In this case, one stimulates and the other inhibits. This antagonistic functional relationship serves as a balance to help maintain homeostasis.

source: SEER's Training Website.

Cranial Nerves

The cranial nerves are 12 pairs of nerves that can be seen on the ventral (bottom) surface of the brain. Some of these nerves bring information from the sense organs to the brain; other cranial nerves control muscles; other cranial nerves are connected to glands or internal organs such as the heart and lungs.

Cranial Nerves
Number	Name	Function	Location
I	Olfactory Nerve	Smell
II	Optic Nerve	Vision
III	Oculomotor Nerve	Eye movement; pupil constriction
IV	Trochlear Nerve	Eye movement
V	Trigeminal Nerve	Somatosensory information (touch, pain) from the face and head; muscles for chewing.
VI	Abducens Nerve	Eye movement
VII	Facial Nerve	Taste (anterior 2/3 of tongue); somatosensory information from ear; controls muscles used in facial expression.
VIII	Vestibulocochlear Nerve	Hearing; balance
IX	Glossopharyngeal Nerve	Taste (posterior 1/3 of tongue); Somatosensory information from tongue, tonsil, pharynx; controls some muscles used in swallowing.
X	Vagus Nerve	Sensory, motor and autonomic functions of viscera (glands, digestion, heart rate)
XI	Spinal Accessory Nerve	Controls muscles used in head movement.
XII	Hypoglossal Nerve	Controls muscles of tongue
Note: the olfactory "nerve" is composed of the rootlets of olfactory hair cells in the nasal mucosa and is not visible on the ventral surface of the brain. The rootlets end in the olfactory bulb. The olfactory tract contains nerve fibers projecting out of the olfactory bulb to the brain. The images in this table have been adapted from those in the Slice of Life project.

Hear IT!	Olfactory	Optic	Oculomotor	Trochlear
	Trigeminal	Abducens	Facial	Vestibulocochlear
	Glossopharyngeal	Vagus	Spinal Accessory	Hypoglossal

Can't remember the names of the cranial nerves? Here is a handy-dandy mnemonic for you: On Old Olympus Towering Top A Famous Vocal German Viewed Some Hops. The bold letters stand for: olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, spinal accessory, hypoglossal. Still can't remember the cranial nerves? Perhaps you need some Cranial Nerve Bookmarks to help you study! After you print the bookmarks, cut them into three individual bookmarks and use them to mark your place when you study.

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Test Your Cranial Nerves Now that you know the names and functions of the cranial nerves, let's test them. These tests will help you understand how the cranial nerves work. These tests are not meant to be a "clinical examination" of the cranial nerves. You will need to get a partner to help...both of you can serve as the experimenter (tester) and the subject. Record your observations of what your partner does and says. Olfactory Nerve (I) Gather some items with distinctive smells (for example, cloves, lemon, chocolate or coffee). Have your partner smell the items one at a time with each nostril. Have your partner record what the item is and the strength of the odor. Now you be the one who smells the items...have your partner use different smells for you. Optic Nerve (II) Make an eye chart (a "Snellen Chart") like the one on the right. It doesn't have to be perfect. Have your partner try to read the lines at various distances away from the chart. Oculomotor Nerve (III), Trochlear Nerve (IV) and Abducens Nerve (VI) These three nerves control eye movement and pupil diameter. Hold up a finger in front of your partner. Tell your partner to hold his or her head still and to follow your finger, then move your finger up and down, right and left. Do your partner's eyes follow your fingers? Check the pupillary response (oculomotor nerve): look at the diameter of your partner's eyes in dim light and also in bright light. Check for differences in the sizes of the right and left pupils. Trigeminal Nerve (V) The trigeminal nerve has both sensory and motor functions. To test the motor part of the nerve, tell your partner to close his or her jaws as if he or she was biting down on a piece of gum. To test the sensory part of the trigeminal nerve, lightly touch various parts of your partner's face with piece of cotton or a blunt object. Be careful not to touch your partner's eyes. Although much of the mouth and teeth are innervated by the trigeminal nerve, don't put anything into your subject's mouth. Facial Nerve (VII) The motor part of the facial nerve can be tested by asking your partner to smile or frown or make funny faces. The sensory part of the facial nerve is responsible for taste on the front part of the tongue. You could try a few drops of sweet or salty water on this part of the tongue and see if your partner can taste it. Vestibulocochlear Nerve (VIII) Although the vestibulocochlear nerve is responsible for hearing and balance, we will only test the hearing portion of the nerve here. Have your partner close his or her eyes and determine the distance at which he or she can hear the ticking of a clock or stopwatch. Glossopharyngeal Nerve (IX) and Vagus Nerve (X) Have your partner drink some water and observe the swallowing reflex. Also the glossopharyngeal nerve is responsible for taste on the back part of the tongue. You could try a few drops of salty (or sugar) water on this part of the tongue and see if your partner can taste it. Spinal Accessory Nerve (XI) To test the strength of the muscles used in head movement, put you hands on the sides of your partner's head. Tell your partner to move his or her head from side to side. Apply only light pressure when the head is moved. Hypoglossal Nerve (XII) Have your partner stick out his or her tongue and move it side to side.

Try it!

Do you like interactive word search puzzles? Make sure your browser is "java-enabled" and try this one: Cranial Nerve Puzzle

Cranial Nerve Information from Yale University. Examination of the Cranial Nerves More details about the cranial nerves. A great cranial nerve review with quizes.

source:http://faculty.washington.edu/chudler/cranial.html