Download Notes on the Endocrine System

Notes on the Endocrine System
Chemical signals coordinate body function.
A hormone is a regulatory chemical that travels through the blood from its production
site and affects other sites in the body. They are made and secreted by organs called
endocrine glands. Secretory vesicles in the cells of the endocrine gland release hormone
molecules directly into the blood system where they travel to target cells that respond to
the hormone. Neurosecretory cells are a second type of hormone-secreting cells that not
only conduct nerve signals but also make and secrete hormones. The endocrine systems
often works closely with the nervous system. The nerve system sends signals to effectors
which can be either muscle cells or endocrine cells. The nerve system uses
neurotransmitters to bridge the synapse between neurons which are referred to as a
local regulator. Other local regulators include prostaglandins, which in the placenta,
signal the uterus to contract during labour. Hormones, on the other hand, often have their
effect over a great distance in the body. Whereas the nerve system offers a quick response
to the environment, the endocrine system is slower and controls such things as metabolic
rate, growth, maturation and reproduction.
Hormones affect target cells by two main signaling mechanisms.
Although vertebrate make over 50 different kinds of hormones, there are two
mechanisms by which they trigger responses in target cells. For example, the hormone
epinephrine (adrenaline) causes liver cells to breakdown
stored glycogen into glucose by first binding to a receptor
protein on the membrane of a liver cell. The binding
activates the receptor which initiates a multistep signaltransduction pathway in the cell. A series of relay
molecules are activated in sequence. The final relay
molecule activates an enzyme that converts glycogen
into glucose. Hormones that bind to plasma membrane
receptors are all made from amino acids. Amine
hormones are modified version of a single amino acid.
Peptide hormones are short chains (maybe only 3) of
amino acids and protein hormones are long polypeptide
chains of amino acids.
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Steroid hormones are lipids made from cholesterol such as the sex hormones
testosterone and estrogen. Being non-polar,
they are able to pass through the phospho-lipid
membrane of a cell and enter the cytoplasm.
In a target cell they bind to a receptor protein
in the cytoplasm which carries out the
transduction by itself. It becomes a gene
activator which attaches to a specific site on the
cell’s DNA in the nucleus. This stimulates
the transcription of DNA into messenger RNA
which is than translated into a new protein or
enzyme in the endoplasmic reticulum of the
cell.
Overview: The vertebrate endocrine system.
Hormones have a wide range of
targets. The sex hormones have
wide-ranging effects on most
tissues of the body. Other
hormones, such as glucagon
from the pancreas, have only a
few kinds of target cells in the
liver. Some hormones have other
endocrine glands as their targets.
For example, the pituitary gland
produces thyroid-stimulating
hormone (TSH) which stimulates
further activity in the thyroid
gland. The hypothalamus of the
brain demonstrates the close
association between the nervous
regulatory system and the
hormonal regulatory system.
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The hypothalamus, closely tied to the pituitary, connects the nervous and
endocrine systems.
As part of the brain, the hypothalamus receives information from nerves about the
external and internal environment. It can respond to those conditions by sending out
either nervous or hormonal signals. It is directly connected to the pituitary gland which
also secretes hormones that influence numerous bodily functions. The posterior lobe of
the pituitary is actually an extension of the nervous tissue of the hypothalamus. It stores
and secretes hormones made in the hypothalamus. The anterior lobe is made of nonnervous glandular tissue. It can synthesize and secrete its own hormones, several of
which control other endocrine glands. The hypothalamus exerts control over the anterior
pituitary by secreting two kinds of hormones into the blood. Releasing hormones make
the anterior pituitary secrete hormones, and inhibiting hormones make the anterior
pituitary stop secreting hormones. For example, the hypothalamus secretes a releasing
hormone known as TRH (TSH Releasing Hormone). In turn, TRH makes the anterior
pituitary secrete TSH (Thyroid Stimulating Hormone). Under the influence of TSH the
thyroid gland secrete thyroxine into the blood which increases the metabolic rate of cells
all over the body. Negative feedback controls TRH secretion and consequently the
secretion of TSH and thyroxine. When TSH and thyroxine increase in the blood, they
inhibit TRH secretion. Negative feedback is important throughout the endocrine system.
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The hypothalamus and pituitary have multiple endocrine functions.
The TRH-TSH-thyroxine is one example of a regulatory hormone system in mammals.
Neurosecretory cells extend from the hypothalamus into the posterior pituitary. These
cells synthesize the hormones oxytocin and antidiuretic hormone (ADH). The
hormones travel along the secretory cells and are released into the blood supply of the
posterior pituitary. Oxytocin causes uterine contractions during childbirth and release of
milk during nursing. ADH helps cells of the kidney tubules to reabsorb water thus
producing less urine when the body needs to conserve water.
A second set of neurosecretory cells secrete releasing and inhibiting hormones through
the anterior pituitary. In response to hypothalamic-releasing hormones, the anterior
pituitary synthesizes and releases many peptide and protein hormones. Thyroidstimulating hormone (TSH), adrenocorticotropic hormone (ACTH), folliclestimulating hormone (FSH), and luteinizing hormone (LH) all activate other
endocrine glands. Feedback mechanisms control these hormones too.
The broadest effect is achieved by the protein called growth hormone (GH). GH
promotes protein synthesis and fat metabolism in a wide variety of target cells. Excess
GH in a young person can result in giantism whereas a lack of GH can cause dwarfism.
Genetic engineering has resulted in the production of bacteria with the gene for GH
synthesis inserted into their genome.
Prolactin (PRL), has very different effect in different species. In mammals it stimulates
mammary glands to produce milk. In birds it controls fat metabolism and regulates
reproduction. In amphibians it regulates larval development and in freshwater fish it
regulates salt and water balance. These diverse effects suggest that prolactin is an ancient
hormone whose functions diversified during vertebrate evolution.
Also from the anterior pituitary are the endorphins. These hormones are sometimes
called the body’s natural painkillers. They have pain inhibiting effect upon the nervous
system similar to morphine. They are produced by the brain and the anterior pituitary.
They are responsible for the “runners high” and have also been produced during
meditation and acupuncture treatments.
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The thyroid regulates development and metabolism
The thyroid produces two amine hormones. Thyroxin is often called T4 because it
contains four iodine atoms; the other, triiodothyronine, is called T3 because it contains
three iodine atoms. They have crucial roles in development a maturation. For example, in
bullfrogs, they trigger the profound reorganization that takes place a tadpoles develop
into frogs. In mammals they are involved in bone and nerve development. A congenital
thyroid deficiency known as cretinism results in retarded skeletal growth and poor
mental development. In adults, T3 and T4 help maintain normal blood pressure, heart rate,
muscle tone, digestion, and reproductive functions. They tend to increase the rate of
oxygen consumption and cellular metabolism. An excess of T3 and T4 in the blood
(hyperthyroidism) can make a person overheat, sweat profusely, become irritable,
develop high blood pressure, and lose weight. Hypothyroidism can cause weight gain,
lethargy, and intolerance to cold. Hypothyroidism can result from insufficient iodine in
the diet and can cause a goiter – an enlargement of the thyroid gland. Without iodine the
thyroid cannot synthesize adequate amounts of T3 and T4 hormones. This disrupts a
feedback loop that controls thyroid activity. The blood never carries enough hormone to
shut off the secretion of TRH (TSH-releasing hormone) by the hypothalamus. The
thyroid enlarges because TSH from the anterior pituitary continues to stimulate it.
Hormones from the thyroid and parathyroid maintain calcium homeostasis.
Without calcium ions (Ca2+) nerve signals cannot be transmitted across the synapse,
muscles can’t contract, blood cannot clot, and cells cannot transport molecules across
their membranes. There are four parathyroid glands embedded in the surface of the
thyroid. Two peptide hormones, calcitonium from the thyroid gland and parathyroid
hormone (PTH), secreted by the parathyroids, regulate the blood calcium level. These
two hormones are said to be antagonistic because they have opposite effects.
Calcitonium lowers the calcium level in the blood, whereas PTH raises it.
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Pancreatic hormones manage cellular fuel.
The pancreas produces two hormones. Insulin is a protein hormone produced by beta islet
cells in the pancreas. Alpha islet cells secrete an antagonistic peptide hormone called
glucagon.
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Diabetes is a common endocrine disorder.
Diabetes mellitus is a serious hormonal disease in which the body cells are unable to
absorb glucose from the blood. It affects as many as 5 out of 100 people in the U.S. and
Canada. The disease occurs when there is not enough insulin in the blood or when body
cells do not respond normally to blood insulin. In either case, the cells cannot obtain
enough glucose from the blood, and thus, starved for fuel, they are forced to burn the
body’s supply of fats and proteins. Meanwhile, since the digestive system can continue to
absorb glucose from the diet, the glucose concentration in the blood can become
extremely high – so high that glucose is excreted in the urine. (Normally, the kidney
leaves no glucose in the urine.) There are treatments for diabetes mellitus – insulin
supplements and/or special diets, but no cure. Every year 350,000 Americans die from
the disease or its complications which include severe dehydration, cardiovascular and
kidney disease, nerve damage and gangrene.
There are two types of diabetes mellitus. Type I (insulin dependent) is an autoimmune
disease in which T cells of the immune system destroy the pancreatic beta cells. It usually
develops before age 15. Patients require regular supplements of insulin, often by direct
injection. The insulin is made commercially by genetically engineered bacteria.
Type II diabetes (non-insulin dependent) occurs because the body cells fail to respond
to insulin in the blood. It appears to be genetic and may be due to a faulty insulin receptor
on the cells’ membrane. Type II diabetics account for 90% of the cases in the U.S. It is
almost always associated with obesity and often doesn’t show up until a person is in their
40’s. It can often be managed by controlling sugar intake, exercise, and diets high in
soluble fiber and low in fat and sodium.
Warning signs of diabetes are lack of energy, a craving for sweets, frequent urination,
and persistent thirst. The diagnostic test is a glucose-tolerance test: the person swallows a
sugar solution and then has blood drawn at prescribed time intervals and tested for the
presence of glucose. In the graph below, the normal person is able to maintain a more
even concentration of glucose in the blood.
Another problem with insulin is hypoglycemia due to overactive beta cells. Blood
glucose levels drop to well below normal. 2 to 4 hours after a meal the person may
experience hunger, weakness, sweating and nervousness. In severe cases, convulsions
may occur as the brain runs out of glucose fuel. Hypoglycemia is not very common and
can usually be managed by eating smaller, but more frequent meals.
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The adrenal glands mobilize responses to stress.
The adrenal glands sit atop the kidneys and consist of an outer portion called the adrenal
cortex, and an inner portion called the adrenal medulla. The adrenal medulla produces
the “fight or flight” amine hormones called epinephrine (adrenaline) and
norepinephrine (noradrenaline). Stressful stimuli activate nerve centers in the
hypothalamus which send signals to nerve cells in the spinal cord and then to the adrenal
medulla. Both hormones contribute to the short-term stress response; stimulating liver
cells to release glucose, raising blood pressure, breathing rate, and metabolic rate. Blood
vessels in the brain and muscle dilate to heighten alertness and prepare for action, while
at the same time constricting blood vessels elsewhere.
Hormones secreted by the adrenal cortex provide a slower, longer-lasting response to
stress. The hypothalamus secretes a releasing hormone that stimulates target cells in the
anterior pituitary to secrete the hormone ACTH (adrenocorticotropic hormone). ACTH
stimulates the adrenal cortex to secrete a family of steroid hormones called
corticosteroids. The two main types are mineralocorticoids such as aldosterone which
makes the kidney reabsorb sodium ions and water with the overall effect of increasing the
volume of the blood and raising blood pressure. Long-term continuation can lead to
hypertension. Glucocorticoids stimulate the breakdown of muscle proteins, making
amino acids available for conversion to glucose by the liver. Very high levels of
glucocorticoids can suppress the immune system and the inflammatory response.
Cortisone treatments have been used to treat arthritis and sports injuries, at the cost of
suppressing the immune system.
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The gonads secrete sex hormones.
The glucocorticoids and mineralocorticoids secreted by the adrenal glands are steroid
hormones. Some other steroids, the sex hormones, affect growth and development and
also regulate reproductive cycles and sexual behavior. The gonads (ovaries in the female,
testes in the male) secrete sex hormones, in addition to producing gametes (eggs and
sperm)
Mammals produce three major categories of sex hormones: androgens, estrogens, and
progestins. Both males and females have all three types, but in different proportions.
Females have a high ratio of estrogens to androgens. Estrogen maintains the female
reproductive system and promote the development of such female features as the
generally smaller body size, higher-pitched voice, breasts, and wider hips. Progestins,
such as progesterone, are mostly involved in preparing the uterus to support the embryo.
Androgens stimulate the development and maintenance of the male reproductive system.
Males have a high ratio of androgens to estrogens, the main one being testosterone.
Androgens produced by male embryos early in development stimulate the embryo to
develop into a male rather than a female. High concentrations of androgens trigger the
development of male characteristics: a lower-pitched voice, facial hair, and larger skeletal
muscles.
Anabolic steroids are synthetic variants of the male hormone testosterone. Anabolism is
the building of substances by the body. Although banned, athletes who admit to using
anabolic steroids cite such benefits as increased strength, stamina, muscle growth and
aggressiveness.
The downside is that overdosing can cause violent mood swings (steroid rage) and deep
depression. The liver may be damaged leading to liver cancer. Using steroids can alter
cholesterol levels and lead to high blood pressure, increasing the risk of cardiovascular
problems. Using anabolic steroids, which mimic the structure of testosterone, can cause
the body to reduce its own output of natural male sex hormones which can cause
shrunken testicles, reduced sex drive, infertility, and breast enlargement in men. In teens,
bones may stop growing, stunting growth.
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