Hormones and the Endocrine System

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Chapter 45

• Hormones and the Endocrine System

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Internal Communication

• Animals have 2 systems of internal communication
  and regulation
• 1. The nervous system.
• 2. The endocrine system.

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1. The Nervous System

• The nervous system is the pathway of
  communication involving high speed electrical
  signals.

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2. The Endocrine System

• The endocrine system is all of the animals
  hormone secreting cells.
• The endocrine system coordinates a slow,
  long-lasting response.

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Endocrine Glands

• Endocrine glands are hormone secreting organs.
• They are ductless glands.
• Their product is secreted into extracellular
  fluid and diffuses into circulation.

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Endocrine and Nervous Systems

• It is convenient to think of the nervous system
  and the endocrine as separate.
• They are actually very closely linked.
• Neurosecretory cells are specialized nerve cells
  that release hormones into the blood.
• They have characterisitics of both nerves and
  endocrine cells.

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Neurosecretory Cells

• The hypothalamus and the posterior pituitary
  gland contains neurosecretory cells.
• These produce neurohormones which are
  distinguishable from endocrine hormones.
• Some hormones serve as both endocrine hormones
  and neurotransmitters.

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Neurosecretory Cells

• They can stimulate a response, or they can induce
  a target cell to elicit a response.
• For example, a suckling infant and oxytocin
  release is an example.

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Biological Control Systems

• Recall,
• These are comprised of a receptor/sensor which
  detects a stimulus and sends information to a
  control center that controls an effector.
• The control center processes the information and
  compares it to a set point.
• The control center sends out processed
  information and directs the response of the
  effector.

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3 General Hormonal Pathways

• 1. A simple endocrine pathway.
• 2. A simple neurohormone pathway.
• 3. A simple neuroendocrine pathway.

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1. A Simple Endocrine Pathway

• A stimulus elicits a response on an endocrine
  cell causing a hormone release.
• The hormone diffuses into the blood where it
  reaches a target effector eliciting a response.

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1. A Simple Endocrine Pathway

• For example
• A low glucose level in the blood stimulates the
  pancreas to release glucagon.
• Glucagon acts on liver cells to release glycogen.
• Glycogen breaks down into glucose and gets into
  the blood.

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2. Simple Neurohormone Pathway

• In the simple neurohormone pathway, a stimulus
  travels via a sensory neuron to the
  hypothalamus/posterior pituitary gland.
• Neurosecretory cells here release hormones into
  the blood.
• These hormones travel to the target cells and
  elicit a response.

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2. Simple Neurohormone Pathway

• For example
• A suckling infants stimulation is sent via a
  sensory neuron to the hypothalamus/posterior
  pituitary where oxytocin is made and released
  into the blood.
• The hormones travel to the smooth muscle in the
  breast which responds by contracting and
  releasing milk.

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3. A Simple Neuroendocrine Pathway

• A stimulus sends the signal to the hypothalamus
  via a sensory neuron.
• The neurosecretory cells of the hypothalamus
  release hormones into the blood.
• These act on endocrine cells to release hormones
  into the blood.
• These hormones have an effect on target cells and
  elicit a response.

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3. A Simple Neuroendocrine Pathway

• For example
• Neural and hormonal signals tell the hypothalamus
  to secrete prolactin releasing hormone.
• The hormone travels through the blood to the
  anterior pituitary which releases prolactin.
• Prolactin travels through the blood to the
  mammary glands stimulating milk production.

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Positive and Negative Feedback

• Recall,
• Positive feedback acts to reinforce the stimulus.
  It leads to a greater response.
• Negative feedback acts to reduce the response of
  the stimulus.

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Molecules Functioning as Hormones

• There are 3 major classes of molecules that
  function as hormones
• 1. Proteins/peptides-water soluble.
• 2. Amines-water soluble.
• 3. Steroids-not water soluble.

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Key Events

• There are 3 key events involved in signaling
• 1. Reception-is when the signal binds to the
  receptor protein in or on the target cell.
• Receptors can be inside or outside the cell.
• 2. Signal transduction-signal binds and triggers
  events within the cell (cascade events).
• 3. Response-changes a cells behavior.

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Signal Transduction

• Receptors for most water soluble proteins are
  embedded in the plasma membrane.
• Binding of a hormone initiates a signal
  transduction pathway.

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Signal Transduction

• The pathway is a series of changes where cellular
  proteins convert an extracellular chemical signal
  into an intracellular response.
• Examples
• Activation of an enzyme
• Uptake or secretion of a specific molecule
• Rearrangement of a cytoskeleton

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Signal Transduction

• The signals can activate proteins that can act to
  directly or indirectly regulate transcription of
  certain genes.
• Hormones can cause a variety of responses in
  target cells with different receptors.
• These responses are types of signal transductions.

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Water Soluble Hormones

• Most water soluble hormones have receptors
  embedded in the membrane.
• Surface receptor proteins activate proteins in
  the cytoplasm which then move into the nucleus
  and regulate transcription.

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Epinephrine Example-Water Soluble Hormone

• Liver cells and smooth muscle of blood vessels
  supplying skeletal muscle contain b-type
  epinephrine receptors.

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Epinephrine Example-Water Soluble Hormone

• Smooth muscle of intestinal blood vessels contain
  a-type receptors.
• The tissues respond differently to epinephrine.
• Increased blood flow and glucose to the skeletal
  muscles.
• Decreased blood flow to the digestive tract.

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Lipid Soluble Hormone

• Lipid soluble hormones have their receptors
  located inside of the cell. Either in the
  cytoplasm or the nucleus.
• Entrance of the signal and binding of the signal
  to the receptor initiates the signal transduction
  pathway.
• Binding to DNA stimulates transcription of genes.
• mRNA produced is translated into protein within
  the cytoplasm.

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Estrogen Example-Lipid Soluble Hormone

• Estrogen induces cells within the female birds
  reproductive system to make large amounts of
  ovalbumin.

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Paracrine Signaling

• Neighboring cells signal local regulators that
  convey signals between these neighboring cells.
• Neurotransmitters, cytokines, and growth factors
  are all examples of local regulators.

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Paracrine Signaling-Example

• Nitric oxide (NO).
• When blood O2 levels fall, endothelial cells in
  the blood vessel walls synthesize and release NO.
• NO activates an enzyme that relaxes neighboring
  smooth muscle.
• This results in the dilation of blood vessels and
  improves blood flow.

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Endocrine Control

• The hypothalamus integrates the vertebrates
  nervous and endocrine systems.
• It is found on the underside of the brain.
• It receives information from nerves throughout
  the body and brain.
• It initiates the appropriate endocrine signals
  for varying conditions.

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The Hypothalamus

• Contains 2 sets of neurosecretory cells.
• The secretions from these cells are stored in or
  regulate the activity of the pituitary gland.

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The Pituitary

• The pituitary gland has 2 parts.
• The anterior and the posterior.

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The Anterior Pituitary Gland

• It is regulated by hormones produced by
  neurosecretory cells in the hypothalamus.
• Some inhibit hormone release, others stimulate
  it.
• The adenohypophysis consists of endocrince cells
  that make and secrete at least 6 different
  hormones.
• Many of them target and stimulate endocrine
  glands.

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The Anterior Pituitary Gland

• FSH-stimulates production of ova and sperm.
• LH-stimulates ovaries and testes.
• TSH-stimulates the thyroid gland.
• ACTH-stimulates production and secretion of the
  hormones of the adrenal cortex.
• MSH-stimulates concentration of melanin in skin.
• Prolactin-stimulates mammary gland growth and
  milk synthesis.

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The Posterior Pituitary Gland

• The neurohypophysis is an extension of the
  hypothalamus.
• It stores and secretes 2 hormones ADH and
  oxytocin.
• ADH acts on the kidneys increasing H2O retention.
• Oxytocin signals uterine muscle contraction and
  mammary gland excretion of milk.

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The Thyroid Gland

• The thyroid produces 2 hormones.
• Triiodothyroxine (T3)
• Thyroxin (T4)
• In mammals, T4 is converted to T3 by target
  cells.
• T3 is mostly responsible for the cellular
  response.

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The Thyroid Gland

• The thyroid is crucial to development.
• It controls metamorphosis in frogs.
• It is required for normal functioning of
  bone-forming cells.
• It promotes branching of nerves in utero.
• It helps skeletal growth and mental development.
• It helps maintain muscle tone, digestion,
  reproductive functions, b.p., h.r.

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The Thyroid Gland

• The thyroid creates calcitonin.
• It works in conjunction with the parathyroid to
  maintain calcium homeostasis.

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Parathyroid Hormone

• Released by the parathyroid gland in response to
  low blood calcium levels.
• PTH induces the breakdown of osteoclasts.
• Ca2 is then released into the blood.
• PTH stimulates Ca2 uptake by the renal tubules.

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Parathyroid Hormone

• PTH also promotes the conversion of vitamin D
  into its active form.
• The active form of vitamin D acts on the
  intestines stimulating the uptake of Ca2 from
  food.
• When Ca2 gets above a certain setpoint, it
  promotes the release of calcitonin which opposes
  the effects of PTH lowering blood Ca2 levels.

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Pancreas

• The pancreas is both an endocrine and a exocrine
  gland.
• Exocrine-releases secretions into ducts.
• Endocrine-secretions diffuse into bloodstream.
• Islets of Langerhans are scattered throughout the
  exocrine portion of the pancreas.

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Pancreas

• Each islet contains a-cells and b-cells.
• a-cells produce glucagon.
• b-cells produce insulin.
• Insulin and glucagon oppose each other and
  regulate the concentration of glucose in the
  blood.

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Blood Glucose

• Glucagon gets released when blood glucose falls
  below a setpoint.
• Insulin gets released when blood glucose is
  elevated.
• Insulin stimulates most cells to take up glucose
  from the blood.
• It also acts to slow glycogen breakdown in the
  liver.

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Diabetes Mellitus

• Diabetes is an endocrine disorder caused by a
  deficiency in insulin or decreased response to
  insulin.
• There are 2 types
• Type I-insulin dependent.
• Type II-non-insulin dependent.

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Type I Diabetes

• Insulin dependent. Its an autoimmune disease
  resulting in the destruction of the bodys
  b-cells.
• The pancreas cant produce insulin and the person
  requires insulin injections.

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Type II Diabetes

• Non-insulin dependent.
• It is caused either by a deficiency in insulin,
  or usually by a reduced responsiveness by the
  cells to insulin.

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Adrenal Glands

• They are adjacent to the kidneys.
• They are made up of 2 different cell types.
• Adrenal cortex-the outer portion.
• Adrenal medulla-the inner portion.

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Adrenal Cortex

• Responds to endocrine signals.
• ACTH released from the anterior pituitary
  stimulates the release of corticosteriods.
• Glucocorticoids-cortisol involved in
  bioenergetics.
• Mineralcorticoids-aldosterone acts on salt
  balance.
• The cortex also releases sex hormones.

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Adrenal Medulla

• The medulla responds to endocrine signals.
• Produces the catecholamines epinephrine and
  norepinephrine.
• These are involved in the fight-or-flight
  response.