Endocrine System: - Weebly



Endocrine System:

Animals coordinate activities of specialized parts via the nervous and endocrine systems. The endocrine system uses chemical messengers that travel to specific organs. These chemicals are called hormones.

Note: Exocrine Glands produce a variety of substances, such as, sweat, mucus, and enzymes. These substances then through ducts. Endocrine Glands produce a variety of substances, but move these substances without ducts and so are often referred to as ductless glands.

Many endocrine glands have specialized neurosecretory cells. These cells produce hormones and act as nerve cells. These cells can receive signals from the nervous system, produce hormones and put the hormones in the blood. You’ll also see that hormones can affect the nervous system, e.g. epinephrine.

Feedback is a big part of the endocrine system. Negative feedback is the main way the endocrine uses to maintain homeostasis. However, some positive feedback is used, milk production and child birth to maintain homeostasis.

Hormones:

Hormones are chemical messengers that are formed by endocrine glands in one part of the body and travel to other parts of the body and cause chemical changes.

There are more than 50 known hormones. They are regulatory devices that make the body behave in a coordinated fashion and can affect behavior. Only certain types of cells, TARGET CELLS, are affected by a particular hormone.

Each hormone has a specific shape that is recognized by the hormone's target cells. The first step in a hormone's action is the binding of the hormone to a HORMONE RECEPTOR. A hormone receptor is a protein on or in the cell membrane. Once bound, the target cell responds to the hormone signal. Opposing, or antagonistic, hormones often counter the effects of hormones. For example, insulin and glucagon. Insulin removes glucose from the blood and glucagon adds glucose to the blood. Hormones may stimulate the synthesis of an enzyme or protein by activating the appropriate gene. They may increase or decrease the synthesis of enzyme proteins by changing the rate of transcription or translation, or they may turn an enzyme or protein off by changing its shape.

Variations on Hormones: other chemical messengers:

A) Phermones are chemical signals that work like hormones except they are communication signals between animals of the same species. Phermones attract mates, mark territories, and alarm organisms. Phermones are small, volatile molecules that disperse easily into the environment and are active in small amounts.

B) Local Regulators are chemical messengers that affect target cells near their point of secretion. Below are some examples of local regulators.

1) Growth Factors are proteins that must be present in order for cells to grow and develop normally. Target cells of the growth factors also have receptors on the cell membrane. Some oncogenes have membrane proteins that mimic growth factor receptors. There are many different types of growth factors. These are produced by cells and affect local cells only.

2) Prostaglandins are modified fatty acids that act as local regulators. There are 16 different discovered prostaglandins. They are active in the female reproductive system (found in semen, it stimulates uterine contractions). They are also local regulators in the defense system, for example, fever, inflammation and pain intensity are caused by prostaglandins. Aspirin inhibits the formation of prostaglandins. Again, these only affect local cells.

3) Many cells produce the gas nitric oxide (NO). NO affects targets in a few seconds and breaks down quickly. Secreted by neurons, NO functions as a neurotransmitter. Secreted by the WBC, it can kill certain cancer and bacterial cells. NO released by the endothelial cells of the blood vessels cause the smooth muscle to relax, allowing them to dilate.

There are three groups of hormones:

1) Lipid derived hormones: There are two classes of these hormones: Eicosanoids, which are derived from arachidonic acid (20 carbon fatty acid). These small molecules usually act as local regulators, but can serve a secondary function as a hormone. Steroids are the second class of lipid derived hormones. They have the usual four-ring structure. Each ring may have different side groups. Steroids are derived from cholesterol. In the blood, steroid hormones are bound to specific transport proteins. Common steroids are the sex hormones progesterone, testosterone, and estrogen. Other steroids are aldosterone, corticosterone, and cortisol.

2) Peptides are composed of amino acid chains. Two of the simplest are the antidiuretic hormone (ADH) and oxytocin, which are made up of 9 amino acids each. Insulin has 51 amino acids.

3) Modified Amino Acids: There are only a few of these, epinephrine and norepinephrine. The two modified amino acids are tyrosine and tryptohan.

Hormone interaction: This is how the hormones work together.

1) Two hormones may have antagonistic (opposing) effects. For example: calcitonin/parathyroid hormone, or glucagon/insulin. These hormones maintain a balance.

2) Two hormones nay have an additive effect; the net results are greater than the individual results. This is called a synergistic effect. For example, glucose sparing action of GH and Glucocorticoids.

3) One hormone is needed in order for the second to work—permissive effect. For example, epinephrine doesn’t change energy consumption unless the thyroid hormones are present.

4) Hormones can produce different, but complimentary results—integrative effect. For example, calcitonin and parathyroid hormones have different effects, but complement each other in calcium metabolism.

How Hormones work:

Hormones are usually released into capillaries. Within the blood, hormones may circulate freely or bound to specific carrier proteins. A freely circulating hormone remains active for less than an hour. It is activated when 1) it diffuses out of the blood and binds to specific cell receptors, 2) the liver or kidney absorbs it and breaks it down, or 3) it is broken down by an enzyme of the plasma or interstitial tissue/fluid.

Hormones act in low concentrations and can affect different target cells within an organism. However, how a hormone triggers specific changes in target cells can be broken down into one of two methods:

1) Lipid derived hormones enter the nucleus and influence the expression of the cells genes.

2) Most non-steroid hormones attach to the cell surface and through a second messenger affect the cell. Both of these methods are called SIGNAL TRANSDUCTION PATHWAYS.

Lipid derived hormones:

We’ll focus on steroid hormones. Steroids pass through the target cell membrane and enter the nucleus. A steroid hormone will bind to a receptor protein in the nucleus. This receptor protein recognizes specific DNA regions. These receptor proteins are probably transcription factors that stimulate transcription of specific genes by attaching to an enhancer sequence of DNA. Steroids can initiate transcription, produce mRNA; eventually a protein is produced.

Thyroid hormones (lipid derived) can bind to receptors on the mitochondria and DNA (in the nucleus).

Peptide and Modified AA Hormones:

Peptide hormones and modified AA are unable to pass through the cell membrane. These bind to hormone receptors of the target cell. This binding will cause a variety of biochemical reactions through a second messenger. There are two types of second messengers: cAMP (cyclic AMP) and Inositol Triphosphate. NOTE: secondary messengers are small non-protein compounds of the signal transduction pathway.

The second messenger:

Cyclic AMP is so important it is called the second messenger. The hormones are the first messengers. They bring the message to the target cells. Within the cell, the actual response of the cell is triggered by cAMP.

Most signal molecules are large, water-soluble molecules that can’t pass through the cell membrane. Once the hormones bind to the receptor, it can activate a secondary messenger. One type of cell receptor is called the G protein-linked receptor. This receptor is helped by the G protein (which is made up of seven alpha helices spanning the membrane). The G protein (on the cytoplasmic side) functions as a switch depending on whether GDP or GTP is attached to the protein. When GDP is bound, G protein is inactive. If the GTP is bound, the G protein is active. When the hormone binds to the receptor, GTP displaces GDP on the G protein. This activates the G protein and it binds to another enzyme, in some cases, adenyl cyclase, which changes ATP to cAMP. The cAMP molecule stimulates protein kinase A, which begins the enzyme cascade that leads to a cellular response.

For example: Epinephrine is released from the adrenal medulla and carried by the blood to a target cell, such as the liver. The liver has a receptor site on the cell membrane. Epinephrine fixes onto the membrane and activates the enzyme adenyl cyclase through the G protein, which is provided by GTP. When the enzyme is activated it converts ATP to cAMP. cAMP stimulates the activity of glycogen phosphorylase which hydrolyzes glycogen. cAMP activates a cAMP-dependent protein kinase which transfers a phosphate group from ATP to a protein. This stimulates another kinase, which adds a phosphate group to glycogen phosphorylase, which hydrolyzes glycogen. cAMP starts this ENZYME CASCADE. One benefit from an enzyme cascade is that it amplifies the response of the hormone. The number of activated products increases at each step.

Inositol Triphosphate and Calcium Ions:

Many chemical messengers induce responses in target cells by increasing the cytoplasmic concentration of Ca++. In fact, calcium is used even more than cAMP as a secondary messenger. Calcium can function as a secondary messenger because the concentration outside of the cell is higher than the concentration inside of the cell (usually by 10,000 times). The calcium ions are actively transported out of the cell and into the endoplasmic reticulum. The release of calcium from the ER is usually involves the secondary messengers diacylglycerol (DAG) and inositol triphosphate (IP3). These secondary messengers are formed when a specific phospholipid (Phospholipid PIP2) is cleaved.

Phospholipase C + Phospholipid PIP2 ( DAG and IP3

Hormone binds to a G protein-linked receptor, which activates the G protein (when GTP displaces GDP). The G protein activates Phospholipase C that cleaves Phospholipid PIP2 to form DAG and IP3. IP3 binds to calcium gates on the ER and opens the gates. Calcium flows into the cytoplasm and can act on a variety of proteins that will cause a cell response.

Invertebrate Hormones:

All invertebrates have hormones that function in maintaining homeostasis. Invertebrate hormones regulate water balance, help with reproduction, regulate growth and signal when to molt.

Endocrine system in vertebrates:

The evolution of the hormones and nervous system is important in adjusting the animals to the environment. The endocrine system is essentially the same in all mammals.

In the endocrine system more than a dozen tissues secrete hormones. The changes occur when the hormone encounters the target area. Hormones are degradable; they do their jobs and disappear.

There are two main types of hormones:

1) Some hormones directly affect tissues.

2) TROPHIC HORMONES affect other endocrine glands.

The Endocrine Glands:

Hypothalamus: I call this the ‘Big Boss.’ This gland controls the ‘master gland.’

1) It secretes trophic hormones, which controls the anterior pituitary. The anterior pituitary controls other glands.

2) The hypothalamus releases two hormones, which are stored in the posterior pituitary.

3) It exerts a direct neural control over the adrenal medulla. The message is carried from the hypothalamus to the adrenal gland directly by nerves.

The hypothalamus manufactures hormones and releasing factors, which control the release of hormones from the anterior pituitary. The hypothalamus plays a role in integrating the endocrine and nervous systems. The hormone releasing cells are specialized neurons that are different from both the secretory cells of the endocrine system and other neurons. These cells are neurosecretory cells. They receive impulses from other neurons and secrete hormones as a response. The hypothalamus acts as the bodies monitoring station. It monitors the blood pH, blood temperature, blood hormone concentrations, blood solute concentration… As it is monitoring, it controls how you respond to those conditions. One way to control how you respond is through the production of hormones and trophic hormones (that control the production of other hormones). There are two sets of secretory cells that:

1) Produce releasing hormones for the anterior pituitary. These hormones cause the anterior pituitary to produce and release hormones, or stop producing and releasing the hormones. For example, Gonadotropin-releasing hormone (GnRH) tells the anterior pituitary to release FSH and LH.

2) Produces inhibiting hormones to stop the anterior pituitary.

3) Produce hormones for the posterior pituitary: oxytocin and antidiuretic hormone.

The Pituitary:

This is called the 'Master Gland' because it influences other glands. The hormones from the pituitary activate other glands. It is a tiny structure (12-13 mm in diameter), about the size of a kidney bean. If you want to find out where it is point your finger into your ear, between your eyes, where the two lines intersect, will be your pituitary.

The gland lies in a pocket of bone and attaches to a stalk, which emerges from the base of the brain just below the hypothalamus.

The pituitary is divided into the anterior (front) and the posterior lobes. The anterior lobe is primarily glandular and secretes seven important hormones: Adrenocorticotrophic hormone (ACTH), Growth hormone (GH), Prolactin (PRL), Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH), Thyroid Stimulating Hormone (TSH), Melanocyte Stimulating Hormone (MSH). The posterior lobe is really part of the brain (it is directly connected to the brain); its blood circulation is separated from the anterior lobe. The posterior does not manufacture hormones, but it stores two peptide hormones, oxytocin and ADH, that are made in the hypothalamus. All these hormones use cAMP, as it’s secondary messenger. The anterior pituitary releases the hormones into fenestrated capillaries. These hormones are also trophic hormones.

Anterior Pituitary (Adenohypophysis) Hormones:

ACTH (adrenocorticotrophic hormone), a peptide hormone, which targets the outer layer of cells in the adrenal gland (adrenal cortex). When these cells are stimulated, the adrenal cortex secretes a battery of hormones, most of them steroids. As the levels rise, the increased levels of the released hormones inhibit the ACTH production. ACTH regulates the metabolism of fats.

GH (growth hormone) is a peptide hormone of almost 200 amino acids. GH stimulates growth, the production of other growth factors (acts like a trophic hormone), stimulates the liver to produce a hormone insulin-like growth factors which stimulates bones and cartilage to grow, and assists amino acids across the cell membrane (allows the liver to be classified as an endocrine gland). This increases the acceleration of protein synthesis, decreases carbohydrate utilization, increases blood glucose and stimulates the release of insulin. Proper levels are needed for proper growth. Increased levels cause giantism. Decreased levels cause midgets to form. We can clone GH in E. coli. A sudden increase in GH causes acromegaly or growth surges in restricted areas.

Thyroid Stimulating Hormone (TSH), a glycoprotein, stimulates the thyroid to release thyroxin, triiodothyronine, and calcitonin. These regulate the metabolic rate of the body and calcium ion levels of the blood.

Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH), both are glycoproteins, are gonadotropins, which stimulate the activity of male and female gonads. Both are glycoproteins. In females: FSH will stimulate the production of the egg. LH will cause ovulation and stimulate the production of estrogen. In males: FSH will stimulate the production of sperm. LH will stimulate the production of testosterone. These two hormones are controlled by GnRH of the hypothalamus.

Melanocyte Stimulating Hormone (MSH) regulates the activity of pigment containing cells present in some vertebrates.

Similar MSH are ENDORPHINS. Endorphins are the body's opiate, which inhibits the perception of pain. Heroin and other opiates mimic endorphins. The 'runner's high' is caused by the release of endorphins to counteract the pain.

Prolactin promotes milk production in mammals. Similar to the growth factor, it regulates fat metabolism and reproduction in birds, and delays metamorphosis in amphibians.

Posterior Pituitary (Neurohypophysis) Hormones:

Oxytocin and ADH are stored here. ADH is released when the osmotic pressure of blood is down. This causes retention of fluids in the blood. The increased fluid levels increase the osmotic pressure inside the blood vessels. ADH also causes you to be really thirsty. Oxytocin is stimulated by the stimulation of the nipples and sexual intercourse. Oxytocin stimulates prolactin, which stimulates milk secretion; it also stimulates the uterus to contract during childbirth and orgasm. In males, it may stimulate the smooth muscle contraction of the ductus deferens for ejaculation.

FYI: Both these hormones are found on chromosome 20. Both these hormones are responsible for maintaining water balance in the body. ADH: increases water reabsorption. Oxytocin: Increases NaCl excretion by the body (as well as, stimulating the uterus contractions and milk production).

Oxytocin and ADH can affect the brain. There are oxytocin and ADH receptors on brain cells. These receptors and the response to oxytocin and ADH contribute to monogamy and love.

If there are no oxytocin receptors in the brain, mice have no social memory—they cannot recognize another mouse it had just met. If oxytocin is injected into the amygdala of the brain, the mouse remembers. The hypothesis states that since mating releases oxytocin and ADH, they allow you to remember your mate.

The more receptors, the more oxytocin response you get. The receptors are proteins controlled by genes. These genes are controlled by T.F. and promoters. In organisms with longer promoter sequences (about 350 to 550 DNA bases in length), the more oxytocin receptors you have.

It may be that once you mate, you release oxytocin and ADH, you remember you mate. In addition, the stimulated portion of the brain, overlaps the area stimulated by cocaine. Maybe love is addicting!

Thyroid Gland:

This is shaped like a bow tie with two lobes, located on the ventral surface of the trachea. The thyroid is located in front and slightly below the larynx. It weighs about 30 grams. The thyroid produces thyroxine (T4—4 iodines) and triiodothyronine (T3—3 iodines). T3 is usually more active, although both have the same effects on the target cells. Both hormones control the cell metabolism. They will increase cellular respiration, increase oxygen consumption, and increase the amount of ATP/Energy produced. The thyroid plays a huge role in the development and maturation of vertebrates. The thyroid maintains normal blood pressure, heart rate, muscle tone, digestion, and reproduction.

About 75% of T4 and 70% of T3 molecules become attached to thyroid binding globulins, which are the transport proteins.

An overactive thyroid causes hyperthyroidism with the following symptoms: nervousness, hyperactivity, insomnia, and weight loss. An underactive thyroid causes hypothyroidism, which has the opposite symptoms. A goiter is an enlargement of the thyroid gland; the thyroid cells are deficient in iodine. Cretins are people that have lacked thyroxine since birth. Mental and physical retardation are a result of the low level of thyroxine.

Thyroxine is controlled by the thyroid-stimulating hormone of the anterior pituitary gland. TSH is influenced by a releasing factor from the hypothalamus.

Calcitonin is a thyroid protein, which regulates blood calcium concentration by decreasing the blood calcium levels. Calcitonin targets the intestine (decreases calcium absorption), the kidney (increases calcium excretion), and the bones (increases the amount of calcium added to the bone).

Parathyroid Gland and Parathormone:

Parathyroids are four pea-sized bodies embedded in the thyroid.

The parathyroid produces Parathyroid Hormone (PTH). Made up of 81 amino acids, PTH increases calcium concentration. Calcitonin decreases the calcium concentration. PTH targets three known tissues: intestine, kidneys and bone cells.

Intestines: PTH increases absorption of calcium into the blood.

Kidney: PTH inhibits the secretion of calcium from the body.

Bone: PTH increases the activity of osteoblasts, which dissolves calcium phosphate; the calcium ions are then released into the blood.

Low amounts of PTH give muscle convulsions, which can lead to death. Vitamin D is essential for PTH to function. Vitamin D is in the skin in the inactive form. When struck by U.V. light, it becomes active and acts like a hormone.

Pancreas and the Islets of Langerhans:

The pancreas is an exocrine gland and manufactures digestive enzymes for the intestine. The pancreas is about 20-25 cm (8-10 inches) long. It is about 80 grams (2.8 ounces). 99% of the cells in the pancreas are exocrine cells and produce digestive enzymes or alkaline solutions that will travel to the duodenum.

That leaves 1% of the cells in the pancreas to be endocrine cells. Scattered through out the pancreas are groups of true endocrine cells that secrete products directly into the blood stream. These cells are called the Islets of Langerhans. The Islets are made up of four types of cells:

1) Alpha cells produce glucagon. This increases blood glucose levels by increasing glycogen break down.

2) Beta cells produce insulin. This decreases blood glucose levels by increasing the rate of glucose uptake and utilization by body cells. There is also an increase in glycogen synthesis.

3) Delta cells produce a hormone identical to growth hormone-inhibiting hormone; this suppresses the release of glucagon and insulin by the other cells in the islets. There is a decrease in the rate of food absorption and enzyme secretion into the digestive tract.

4) F cells produce pancreatic polypeptide. PP inhibits gall bladder contractions and regulates the production of pancreatic enzymes. This may control the rate of nutrient absorption by the digestive tract.

Glucagon stimulates liver to break down glycogen into glucose and which is released when blood sugar is low. Glucagon increases the conversion of amino acids and fatty acids to sugar.

Insulin (normal levels: 70-100 mg/dl) also stimulates many tissues to take up glucose from the blood. When there is high blood sugar, insulin stimulates the liver to remove glucose from the blood and convert it to glycogen. Insulin also stimulates many tissues to take up glucose from the blood. It slows liver breakdown of glycogen and inhibits the conversion of amino acids and fatty acids to sugar.

If you have low blood sugar, then you are hypoglycemic.

If you have high blood sugar, then you are a diabetic. There are two types of diabetes. Type I diabetes mellitus (insulin dependent/juvenile onset) is caused by an autoimmune disorder where the immune system attacks pancreatic cells. Such diabetics require a daily injection of insulin. Type II diabetes mellitus (adult onset/insulin independent) is caused either by target cells responding less to insulin or the production of less insulin. This form of diabetes is treated through a restrictive diet.

Adrenal Glands:

The adrenal glands are closely associated with the kidney. There are two glands in one. There is an outer layer called the cortex and an inner layer called the medulla.

Adrenal Cortex:

The adrenal cortex can be stimulated by stress. ACTH stimulates the adrenal cortex to synthesize and secrete a family of steroids called CORTICOSTEROIDS. Increased levels of these steroids suppress the release of ACTH. There are two types of corticosteroids.

1) Mineralocorticoids, such as aldosterone, target cells on the renal tubule of the kidney. This increases the recovery of sodium and increases the excretion of potassium and hydrogen ions into the urine. Aldosterone works with ADH to keep fluids of the body relatively constant. These hormones basically maintain the electrolyte level of the blood.

2) Glucocorticoids (stimulated by ACTH), cortisol (AKA: hydrocortisone) and corticosterone, are partly responsible for elevating glucose metabolism. There is an increase of glucose synthesis and glycogen formation. Adipose cells will release fat into the blood. Tissues will break down fatty acids and proteins, not glucose. They also increase protein and fat metabolism. Glucocorticoids have an important role in the sexual development of an individual. The adrenal cortex makes the same hormones (estrogen and testosterone) as the testes and ovaries, and these produce the secondary sex characteristics. Also show anti-inflammatory effects. They inhibit the activities of WBC. They slow the migration of phagocytic cells into injured sites. Cells are less likely to release histamine; this also weakens the immune response.

Adrenal Medulla: Produces epinephrine (affects heart and metabolic rates) and norepinephrine (affects blood pressure). These are CALECHOLAMINES, which are synthesized from the amino acid tyrosine. These hormones increase the level of energy sources, increases the rate and stroke of the heartbeat, causes smooth muscles of some blood vessels to constrict and others to relax which prevents blood from going to the skin, gut and kidney and increases blood flow the heart, brain and skeletal muscles. Both of these give you the ‘fight or flight’ response.

Ovaries and Testes:

The ovary produces the two hormones estrogen and progesterone.

The testes produce androgens; the main one is testosterone.

They are important in reproductive functions and also stimulates the lengthening of long bones.

Pineal body:

The pineal body is located above the brain stem and near the center of the brain; it secretes MELATONIN (a modified amino acid), which, along with MSH, controls the skin pigmentation in vertebrates. Melatonin is secreted at night; the amount secreted depends on the length of night. This allows people to think that Melatonin is associated with biological rhythms or the biological clock. The main target of Melatonin is the suprachiasmatic nucleus (SCN), which functions as our biological clock.

In the winter, you will usually eat more carbohydrates. Why? You need to produce Melatonin from serotonin. You need to increase the tryptophane intake to produce serotonin (a modified AA). You need to increase your insulin secretions in order to absorb the tryptophane absorption into the brain. You’ll need to eat more carbohydrates to counteract this increase in insulin.

Thymus: At puberty the thymus begins to diminish and almost disappears by adulthood. The Thymus secretes THYMOSIN, which stimulates the development and differentiation of T lymphocytes.

Endocrine Tissues of Other systems:

Kidney:

1) Calcitriol is a steroid hormone that is secreted in the presence of Parathyroid hormone. Cholecalciterol (vitamin D3) is converted to calcitriol (not directly), which stimulates the absorption of calcium and phosphate.

2) Erythropoietin: EPO—peptide hormone released in response to low oxygen levels in the kidney. EPO will stimulate the bone marrow to produce RBC.

3) Renin: Stimulates the renin-angiotensin system. Renin changes antiotensinogen to angiotensin 1…

Heart: Produces natriuretic peptides, which is part of the ANF system. This hormone will decrease blood volume by opposing the renin-angiotensin system.

Adipose Tissues:

1) Produce leptin: adipose cells secrete Leptin and helps control appetite. The leptin tells the hypothalamus that you are satiated. With a decrease in adipose cells, there is a decrease in leptin, the hypothalamus picks this up and tells you that you want to eat. Leptin must be present for normal GnRH levels. This is why females with low body fat enter puberty later and stop menstruating.

2) Resistin: This decreases insulin sensitivity throughout the body. This may be a link to type 2 diabetes.

FYI: How does stress affect the hormones and immune system?

When you’re stressed, the hypothalamus will stimulate the anterior pituitary to produce ACTH. The ACTH will stimulate the Adrenal Cortex to produce Cortisol. Cortisol turns on TCF. TCF will suppress interleukin 2 (immune system). The suppression of interleukin 2 will decrease your T cells, which decreases your ability to fight diseases.

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