Module 10

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Module 10

• Endocrine System

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• The endocrine system is a control system that
  secretes chemicals within the body so as to
  control tissues that are far (separate) from
  source of the secretion.
• The goal of the endocrine system is to regulate
  the body. That is also the goal of the nervous
  system. Thus, these two systems work in tandem
  with one another.

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Similarities of Nervous System and Endocrine
System

• Similarities between Nervous System and Endocrine
  System
• both control centers for feedback systems. When
  the body senses that a parameter is getting
  either too high or too low, it will initiate a
  response to bring that parameter back in line.
  Sometimes, the nervous system controls the
  response. Sometimes, the endocrine system
  controls the response. More often, both systems
  play a part in the response.

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Differences between Nervous System and Endocrine
System

• Differences between Nervous System and Endocrine
  System
• Speed of response Nervous System is fast,
  Endocrine System is slow
• Duration of influence Nervous System is quick,
  Endocrine System is longer
• Effectors they control Nervous System controls
  2 types of effectors (muscles and glands),
  Endocrine System controls practically every cell
  in the body
• Strength of the signal Nervous System, the
  Action Potential is fixed, Endocrine System the
  signal varies
• Repair Nervous System cannot be repaired,
  Endocrine System can

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The Endocrine System as a Whole

• The endocrine system consists of hormone glands
  or hormonal cells which secrete hormones into the
  blood.
• The blood then serves as a transport medium which
  takes the hormones to cells that can be quite
  distant from the cells which secreted them.
• For example, the parathyroid hormone affects
  osteoclasts, stimulating them to break down bone
  (see Figure 4.5). The osteocytes may be very far
  distant from the parathyroid glands, but the
  hormone is transported to the osteoclasts by the
  blood. As the parathyroid hormone is traveling
  through the blood, however, it is exposed to many
  cells. Why is it the osteoclasts that respond to
  the hormone? Because they have the receptors for
  the hormone. The blood will expose many cells to
  the hormone, but if those cells do not have a
  receptor for the hormone, it will not affect them
  in any way

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• The endocrine system's functions begin with the
  endocrine gland.
• The gland will synthesize the hormone and may
  store it until it is needed. Over time, the gland
  will release the appropriate amount of the
  hormone into the blood.
• Once the hormone is in the blood, it might have
  to be bound to a carrier protein. Why? If the
  hormone is fat soluble, it's going to need a
  protein to bind to because blood is watery, and a
  fat-soluble chemical is either not at all soluble
  in water or just partially soluble in water.
• Thus, to become a part of the blood, it will have
  to bind to a protein that is carried in the
  blood. In essence, then, the hormone will hitch
  a ride on a carrier protein since it cannot mix
  with the blood directly.

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• Eventually, the hormone will be released near the
  target cells. Target cells are those cells which
  have receptors for the hormone. The target cells
  will then produce the desired response. The
  target cell may also provide a feedback signal.
• That is, it may tell the original gland that the
  job is done and that there does not need to be
  any more hormone secreted. It may also tell the
  gland that more hormone needs to be secreted.
• The hormone will then eventually be disposed of
  by the liver or the kidney. The kidney will put
  it into the urine, and the liver will usually put
  it into the bile. Bile is secreted into the small
  intestine, and the hormones which are put in the
  bile end up in the feces

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• Urine tests can tell us a great deal about what's
  going on in the endocrine system.
• After all, if the hormone is disposed of in the
  urine, by testing the urine, you can test what
  hormones the body is producing.
• For example, there are many home pregnancy
  testing kits that involve testing a sample of a
  woman's urine. The tests work because they detect
  a particular hormone that's produced only if a
  woman is pregnant. Since the hormone is disposed
  in the urine, it is easy to test for it there.

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• The fact that hormones are often excreted in the
  urine can be handy for something else as well.
• Women beyond puberty produce estrogen, which is a
  sex hormone.
• For various reasons after menopause, some women
  use estrogen as a medication. This is called
  estrogen replacement therapy.
• Well, one of the popular versions of this drug is
  called Premarin. This name stands for pregnant
  mare urine, because the estrogen is distilled
  from female horse urine during the time that the
  horse is pregnant. Pregnant horses produce a lot
  of estrogen, and once it is excreted in the
  urine, it can be extracted and used in estrogen
  replacement therapy!

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Endocrine Glands and Hormones

• Hypothalamus. Although the hypothalamus is a part
  of the CNS, it is also a endocrine gland.
• In fact, it is one of the major points in the
  body in which the nervous system and the
  endocrine system interact.
• The hypothalamus responds to a variety of
  stimuli, and in response, it can secrete hormones
  which regulate the pituitary gland.
• The hypothalamus secretes two type of hormones
• releasing hormones - cause the pituitary gland to
  secrete more hormones
• inhibiting hormones - cause the pituitary gland
  to secrete fewer hormones

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• Although it is small, the pituitary gland is
  really two different glands
• anterior pituitary gland
• posterior pituitary gland
• These are actually two very different endocrine
  glands, both structurally and functionally.
  However, they're stuck together, so we lump them
  into one gland the pituitary gland.

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Anterior Pituitary Gland

• The anterior pituitary produces several major
  hormones
• growth hormone (GH),
• thyroid stimulating hormone (TSH),
• adrenocorticotropic hormone (ACTH),
• luteinizing hormone (LH),
• follicle stimulating hormone (FSH),
• prolactin (PRL),
• melanocyte stimulating hormone (MSH).

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• Growth hormone, GH, stimulates growth in most
  tissues. One of the most important tissues that
  GH affects is bone. GH stimulates bone growth.
• Because growth is more important during youth, GH
  levels are usually higher in young people than in
  adults.
• Although GH is mostly thought of as a hormone
  that stimulates growth, it does perform other
  functions. It is also one of the major regulators
  of metabolism.

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• Thyroid stimulating hormone, TSH, increases the
  secretion of thyroxine (thigh rok' sin) from the
  thyroid.
• Adrenocorticotropic hormone, ACTH, increases
  during times of body stress. It increases the
  cortisol that is secreted from the adrenal
  glands.
• Luteinizing hormone, LH, along with the follicle
  stimulating hormone (FSH), stimulate the ovaries
  or testes, which are part of the reproductive
  system.
• In a reproduction-related matter, prolactin, PRL,
  stimulates milk production in the breasts. PRL is
  also present in males, but it can rise to much
  higher levels in females.
• Melanocyte stimulating hormone, MSH, increases
  the synthesis of melanin.

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• Notice that some of the hormones secreted by the
  anterior pituitary glands actually stimulate
  other endocrine glands.
• TSH, for example, stimulates the thyroid gland,
  and ACTH stimulates the adrenal glands.
• Thus, the pituitary actually controls other
  endocrine glands and is therefore sometimes
  called the master endocrine gland.

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• This master endocrine gland, however, has its own
  master. As mentioned before, the hypothalamus
  regulates the pituitary gland.
• For example, it releases the growth hormone
  releasing hormone (GH-RH). As its name implies,
  this hormone stimulates the anterior pituitary
  gland to release growth hormone.
• Another hormone secreted by the hypothalamus,
  corticotropin releasing hormone (CRH),
  stimulates the anterior pituitary gland to
  release ACTH.
• The gonadotropin releasing hormone (GnRH)
  increases the anterior pituitary gland's
  secretion of FSH and LH. Remember, FSH and LH
  stimulate the ovaries and testes, which are
  called gonads.
• Another releasing hormone, the thyroid
  stimulating hormone releasing hormone (TSH-RH)
  stimulates the anterior pituitary gland to
  release its TSH. The TSH-RH is also called the
  thyrotropin releasing hormone.
• Finally, the hypothalamus also produces an
  inhibiting hormone called the prolactin
  inhibiting hormone (PIH), which reduces the
  amount of PRL that is secreted by the anterior
  pituitary gland.

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Posterior Pituitary Gland

• The posterior pituitary produces its own
  hormones.
• The antidiuretic hormone (ADH) increases the
  amount of water retained in the blood during
  kidney function. As a result, urine production
  decreases.
• The other important hormone released by the
  posterior pituitary gland is oxytocin (OT).
  Oxytocin increases the contractions of the uterus
  during birth as well as promotes the release of
  breast milk.

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Thyroid Gland

• Moving on to the thyroid gland, there are two
  important hormones to consider
• Thyroxine - Thyroxine increases the metabolism
  rate of most cells in the body. This, then, is
  one of the hormones that has a wide-ranging
  effect throughout the body. Excessive secretion
  of thyroxine generally results in weight loss,
  anxiety, and an elevated metabolic rate. A
  significant lack of thyroxine secretion can cause
  diminished mental activity and accumulation of
  mucus material in the skin, which gives the
  person a puffy appearance
• Calcitonin (kal sih toh' nin) - Calcitonin is a
  more focused hormone. It selectively targets
  osteoclasts, lowering their activity so that
  calcium levels in the blood decrease.

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Parathyroid Glands

• The parathyroid glands are embedded in the
  thyroid gland.
• Most people have four parathyroid glands.
• Their job is to secrete the parathyroid hormone
  (PTH). PTH produces the opposite effect of
  calcitonin. While calcitonin inhibits osteoclast
  activity so as to lower the blood calcium level,
  PTH stimulates osteoclast activity in order to
  raise the blood calcium level.
• PTH also stimulates the intestines to absorb more
  calcium and the kidneys to retain calcium, which
  once again raises the blood calcium level.

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Adrenal Gland

• The adrenal gland is actually two glands in one.
  It sits near the very top of each kidney.
• The inside of the gland is the adrenal medulla,
• The outer layer of the gland is the adrenal
  cortex. The adrenal medulla produces epinephrine
  and norepinephrine.
• As you learned in the previous module, these
  hormones increase the response of the sympathetic
  division of the ANS.

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• The adrenal cortex, on the other hand, produces
• Cortisol. The amount of cortisol secreted from
  the adrenal cortex rises during times of stress.
  It does a number of things, but the main thing we
  want you to remember is that it increases the
  breakdown of protein and fat in most tissue. This
  is important because in times of stress, your
  brain needs a good supply of glucose. By
  stimulating the other tissues to break down fats
  and proteins, cortisol saves the glucose so that
  it can be used by the neurons in the brain.
• aldosterone. Stimulates the kidneys to retain
  sodium in the urine. Thus, as the aldosterone
  levels increase, the amount of sodium in the
  urine decreases.

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Pancreas

• The pancreas is a part of both the digestive
  system and the endocrine system.
• Its job in the endocrine system is to produce
  insulin (in' suh lin) and glucagon (gloo' cuh
  jen).
• These hormones are produced by cells on the
  surface of the pancreas called pancreatic islets
  (also known as islets of Langerhans).
• Certain islets, called beta cells, secrete
  insulin. This hormone stimulates most cells to
  take in glucose. The net effect of insulin, then,
  is to lower blood glucose levels.
• Other islets, called alpha cells, secrete
  glucagon, which has the opposite effect. It
  stimulates the liver to release glucose, which in
  turn increases the glucose level of the blood.

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• An interruption of the endocrine function of the
  pancreas leads to the condition known as diabetes
  mellitus (mel' ih tus).
• There are actually two types of diabetes
  mellitus.
• Type 1 diabetes mellitus, called insulin
  dependent diabetes mellitus, is the result of
  the pancreas being unable to produce insulin.
  Typically, people with type 1 diabetes mellitus
  are weak and lethargic, as their cells are forced
  to break down fats and proteins for energy rather
  than burning glucose.
• Type 2 diabetes mellitus, called non-insulin
  dependent diabetes mellitus, is caused by the
  cells being unable to respond to insulin
  efficiently due to few or no insulin receptors.

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• Many people familiar with diabetes mellitus
  simply call it diabetes.
• There is another type of diabetes called diabetes
  insipidus.
• This diabetes is not related to the pancreas,
  however. It is the result of too little ADH,
  People with diabetes insipidus must drink gallons
  of water daily, because their kidneys allow
  literally gallons of water to end up in the
  urine. Compared to diabetes mellitus, however,
  this disease is quite rare.

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• The reproductive system also has endocrine
  glands.
• In females, they are the ovaries.
• The ovaries produce estrogen and progesterone.
• In males, the reproductive endocrine glands are
  the testes. They produce testosterone.

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• The pineal body is in the brain.
• It doesn't get light directly however, nerve
  signals from the eyes profoundly affect the
  output of hormones from the pineal gland.
• When your eyes receive light, the pineal body
  produces serotonin. When the eyes stop detecting
  light, it switches and starts producing its
  hormone melatonin.
• What does that do? It affects day-night cycles
  and reproductive readiness.

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• The last endocrine gland we want to discuss is
  the thymus gland.
• It secretes the hormone thymosin, which is
  involved with the development of the immune
  system.

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Hormone Chemistry

• Thus, it is important to know a bit about the
  chemistry of hormones.
• We can classify hormones into three different
  groups
• Amine - Amines are hormones that have been
  derived from an amino acid.
• Steroid - A steroid, on the other hand, is made
  from cholesterol.
• Protein/peptide. If a hormone is neither an amine
  nor a steroid, it must be a protein or peptide.
  Now remember, a protein is a long chain of amino
  acids. A peptide is just a short chain of amino
  acids. You can therefore think of a peptide as a
  mini-protein.

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• Epinephrine, norepinephrine and thyroxine are all
  amine hormones. Thus, they are derived from amino
  acids.
• If hormones have similar physical
  characteristics, they will tend to be able to fit
  into each other's receptors. As a result, they
  will overlap in their function.
• These three hormonesepinephrine, norepinephrine,
  and thyroxineare all derivatives of the same
  amino acid. As a result, they have similar
  physical characteristics.
• Thus, they have overlapping activity.
• As you already know, epinephrine and
  norepinephrine are very similar in their
  activity. After all, they elevate heart rate,
  elevate blood pressure, and increase blood
  glucose levels. Well, even thyroxine does this.
  If a person's thyroid secretes too much
  thyroxine, the symptoms are elevated heart rate,
  elevated blood pressure, a high metabolism, etc.
  Thus, all three of these similar hormones produce
  very similar effects in the body.

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• Estrogen and progesterone are steroids, as they
  are derived from cholesterol.
• They are also physically similar, so their
  functions overlap somewhat.
• Not surprisingly, the male sex hormone,
  testosterone, is also a steroid.
• In addition, the hormones of the adrenal cortex,
  cortisol and aldosterone, are steroids. If a
  hormone is from the ovary, from the testes, or
  from the adrenal cortex, then, it is a steroid
  hormone.

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• If a hormone is none of the above, it's a peptide
  or protein.
• Thus, all of the other hormones mentioned in the
  previous section (parathyroid hormone, insulin,
  oxytocin, etc.) are peptides or proteins.
• The difference between peptides and proteins is
  based on size.
• Proteins are huge molecules, while peptides are
  much smaller. Thus, a protein hormone is a very
  large molecule, while a peptide hormone is
  smaller.

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How Hormone Secretion is Controlled

• The control scheme of the endocrine system starts
  with the endocrine glands.
• Hormones are synthesized, stored, and secreted by
  these glands.
• The target cells, of course, cannot respond to
  the hormone until it is secreted into the
  bloodstream, so it is interesting to study how
  the gland knows when to secrete a hormone and
  when to stop secreting it.
• This is an important process to understand, since
  you do not want the endocrine system secreting
  hormones at random times.

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Nonhormonal Regulation

• There are three major ways that hormone release
  is controlled in the body. The first is called
  nonhormonal regulation.
• A chemical other than a hormone is involved in
  regulating the release of a hormone.
• An example of this can be found in the pancreas.
  The release of insulin is controlled by the level
  of glucose in the blood. When blood enters the
  pancreas, the blood glucose level directly
  affects the islets. If blood glucose levels are
  high, the islets secrete more insulin. This
  causes most body cells to take in glucose, thus
  lowering blood glucose levels. When the blood
  glucose levels are low, the pancreas secretes
  less insulin, reducing the amount of glucose
  taken in by the cells of the body. This, in turn,
  raises blood glucose levels. This negative
  feedback system, then, is controlled directly by
  the level of glucose in the blood.

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Direct Neural Control

• The second mode of regulation is the direct
  neural control of the hormone gland by the
  nervous system.
• There is a field of biology called
  neuro-endocrinology, which studies how the
  nervous system exerts control over the endocrine
  glands.
• In direct neural control, this field of study has
  shown that the nervous system can control the
  endocrine system via neurotransmitter or
  neurohormone

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• The pituitary gland is made up of two separate
  glands the posterior pituitary gland and the
  anterior pituitary gland. In this figure, we are
  concentrating on the means by which the
  hypothalamus controls the posterior pituitary
  gland. In the hypothalamus, there are neurons
  whose axons travel down the infundibulum (in fun
  dib' you lum), which is the connection between
  the hypothalamus and the pituitary gland. These
  particular axons travel all the way down to the
  posterior pituitary and then secrete chemicals.
  These chemicals, however, are not
  neurotransmitters. They are neurohormones.

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• What are these neurohormones? They are oxytocin
  and ADH.
• Although the neurons originate in the
  hypothalamus, they store the hormones in the
  posterior pituitary gland.
• As you can see in the figure, the hormone is
  released by the neurons into the veins that run
  through the posterior pituitary gland. Since
  these neurons are specialized in that they
  secrete neurohormone rather than
  neurotransmitter, we call them neurosecretory
  cells.Neurosecretory cells - Neurons of the
  hypothalamus that secrete neurohormone rather
  than neurotransmitter

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• The third way that hormone release is controlled
  is called hormonal control.
• A good way to illustrate this is to show how the
  hypothalamus controls the anterior pituitary
  gland.

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• The figure concentrates on the anterior pituitary
  gland. Notice how this figure differs from the
  previous one. In the posterior pituitary gland,
  the neurosecretory cells extended into the gland
  and released the hormones into the bloodstream.
  In this figure, however, the neurons do not
  extend into the gland. Instead, they release
  their hormones into the blood, and the blood
  carries the hormones directly to the anterior
  pituitary gland via a special vein called the
  portal vein. Those hormones then stimulate the
  anterior pituitary gland to release its own
  hormones.

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• Do you now see why we say that the anterior and
  posterior pituitary glands are completely
  different glands?
• The hormones of the posterior pituitary gland are
  secreted into the bloodstream directly by the
  neurosecretory cells of the hypothalamus.
• Thus, the hormones of the posterior pituitary
  gland are, in reality, produced by cells of the
  hypothalamus.
• That is not the case in the anterior pituitary
  gland. In this gland, the hormones are secreted
  by the cells of the gland itself.
• Those cells are simply stimulated by hormones
  released by the neurosecretory cells of the
  hypothalamus.

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Patterns of Hormone Secretion

• It is important to realize that this control is
  designed to cause a given pattern of hormone
  secretion.
• For example, the release of thyroxine is
  increased or decreased by hypothalamic control.
• However, what is the normal mode of thyroxine
  secretion?
• In other words, is thyroxine only secreted when
  the hypothalamus is stimulated, or is there some
  standard level of thyroxine secretion that is
  simply modified by the hypothalamus?
• Well, in the human body, there are three distinct
  patterns of hormonal secretion which the control
  mechanisms then affect in one way or another.

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• The first pattern of secretion is constant
  secretion.
• Hormones which conform to this pattern are
  constantly produced by the body, and the control
  mechanisms which act on these hormones simply
  raise or lower this constant level of secretion.
• TSH-RH is an excellent example of such a hormone
  secretion pattern. If you live indoors, TSH-RH is
  pretty much constantly being secreted by the
  hypothalamus.
• However, the hypothalamus can either raise or
  lower this constant level so as to adjust to
  stresses such as a change in body temperature.

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• The second pattern is called acute response. In
  this pattern of hormone secretion, the hormone is
  at very low levels in the body until a particular
  stimulus occurs.
• Then, the production of hormone increases
  quickly, in response to the stimulus. Once the
  stimulus goes away, the production drops off
  quickly.
• Epinephrine and norepinephrine are excellent
  examples of acute response hormones. These
  hormones are not constantly secreted.
• Instead, they are produced only in response to
  emotional or physical stress. A stress stimulus
  will cause them to be secreted and, the larger
  the stress, the larger the amount of epinephrine
  and norepinephrine.

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• The third pattern of secretion is cyclic
  secretion.
• Hormones which follow this pattern are secreted
  on a regular, predictable cycle.
• For example, estrogen and progesterone in women
  are secreted on a monthly cycle, which is called
  the menstrual cycle.
• The male hormone, testosterone, is secreted on a
  daily cycle. The level of testosterone in a male
  is higher every morning and drops by evening.

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Hormone Receptors in the Body

• Hormones will do no good without receptors.
• After all, type 2 diabetes mellitus is a disease
  caused by a decrease in sensitivity of the
  receptors for insulin.
• Thus, we must have both hormones and receptors in
  order to achieve endocrine control of the body.

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• There are two types of hormone receptors
• Membrane-bound receptors - As the name implies,
  membrane-bound receptors are located on the
  plasma membrane of the target cell. The hormone
  fits into the receptor like a key fits into a
  lock. Unless a chemical has a very similar shape
  to a given hormone, then, it will not affect that
  hormone's receptor, because it cannot fit into
  the lock.

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• The membrane-bound receptor/hormone reaction
  actually fits the lock and key analogy very well.
  
• Consider what happens when you put a key in the
  ignition of a car and turn it. The car starts,
  right?
• The car had everything it needed to start
  (engine, gasoline, etc.) before you turned the
  key. When you turn the key in the ignition, that
  just gives the car a signal to get all of that
  machinery going and start the engine.
• When a hormone interacts with a membrane-bound
  receptor, the same thing happens. The cell
  already has what it needs to produce the
  response. The hormone/receptor interaction is
  just the signal telling the cell to get it all
  going.
• Thus, this interaction simply activates things
  which are ready to go in the cell. Since
  everything is ready to go in the cell, the
  membrane-bound receptor/hormone interaction
  produces a relatively fast response.

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• Intracellular receptors
• In this process, the hormone actually enters the
  cell, travels through the cytoplasm and into the
  nucleus. There, the hormone then binds to a
  receptor protein in the nucleus, and that
  receptor protein activates a specific gene in the
  DNA. What is the result? The activated gene gives
  the cell instructions to make a protein or
  enzyme, according to the protein synthesis
  mechanisms we reviewed back in Module 1. That
  protein or enzyme is what produces the desired
  effect.This kind of interaction, then, helps
  the cell produce something that was not in the
  cell before. Unlike the membrane-bound
  receptor/hormone interaction, this interaction is
  relatively slow, as the hormone must travel to
  the nucleus of the cell, and the cell must
  produce the protein that is needed. Typically,
  steroid hormones stimulate cells via
  intracellular receptors because they're fat
  soluble. Remember from Module 1 that fat-soluble
  molecules can diffuse right through the plasma
  membrane, so steroids have ready access to the
  inside of the cell. Although thyroxine is an
  amine hormone, it also has intracellular
  receptors.

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Prostaglandins

• Prostaglandins - Biologically active lipids which
  produce many effects in the body, including
  smooth muscle contractions, inflammation, and
  pain.
• Prostaglandins are one of several signal
  chemicals in the body that are something other
  than hormones. Unlike hormones, they are secreted
  by virtually all cells in the body. Also, unlike
  hormones, they don't go into the blood and get
  carried far away. Instead, they are secreted into
  the interstitial fluid (fluid surrounding the
  cells) and thus produce only local effects.
  However, because they do have some similarities
  with hormones (both hormones and prostaglandins
  are potent in tiny amounts), they are sometimes
  called parahormones.

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• What do prostaglandins do?
• They increase certain smooth muscle contractions.
  
• They increase blood clotting.
• They control inflammation of the tissue.
• Some decrease inflammation and some increase it.

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• A knowledge of prostaglandins helps us understand
  what aspirin does.
• For a long time, aspirin was considered a drug
  that was wonderful but nobody really knew what it
  was doing.
• Aspirin could take away your toothache, headache,
  or other problem without seeming to affect
  anything else.
• It seemed like aspirin could just go right to the
  problem and take care of it without affecting
  anything else.

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• Aspirin works because it decreases prostaglandins
  in the body. Thus, it decreases the pain and
  inflammation which they can cause.
• The Nobel prize was awarded for the discovery
  that this is how aspirin works.
• Have you noticed lately that aspirin companies
  are promoting the idea that an aspirin a day can
  prevent heart attacks? Why do they say that?
  Since aspirin decreases the prostaglandins in the
  body, it decreases blood clotting, thinning the
  blood. Thinner blood can reduce the risk of
  having a heart attack.
• Of course, you can't allow your blood to get too
  thin, or it won't clot at all. That can lead to
  much more disastrous things, like bleeding to
  death from a minor cut!