Feedback mechanisms, hormones and the endocrine system

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Feedback mechanisms, hormones and the endocrine
system

• Biology 12
• Mr. C

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What is a feedback mechanism

• Feedback is (generally) information about actions.

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• In cybernetics and control theory, feedback is a
  process whereby some proportion or in general,
  function, of the output signal of a system is
  passed (fed back) to the input. Often this is
  done intentionally, in order to control the
  dynamic behaviour of the system. Feedback is
  observed or used in various areas dealing with
  complex systems, such as engineering,
  architecture, economics, and biology.

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Drawing a feedback loop

• Lines are usually drawn, directed from input
  through the system and to output. The feedback is
  shown by another arrowed line, directed from
  output outside the system to an input, resulting
  in a loop on the diagram, called feedback loop.
  This notion is important for example, the
  feedback loop is a convenient place for a control
  device.

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In nature

• In biological systems such as organisms,
  ecosystems, or the biosphere, most parameters
  must stay under control within a narrow range
  around a certain optimal level under certain
  environmental conditions. The deviation of the
  optimal value of the controlled parameter can
  result from the changes in internal and external
  environments. A change of some of the
  environmental conditions may also require change
  of that range to change for the system to
  function. The value of the parameter to maintain
  is recorded by a reception system and conveyed to
  a regulation module via an information channel.

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Positive and negative feedback

• Biological systems contain many types of
  regulatory circuits, among which positive and
  negative feedbacks. Positive and negative don't
  imply consequences of the feedback have positive
  or negative final effect. The negative feedback
  loop tends to slow down a process, while the
  positive feedback loop tends to accelerate it.

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Useful vocab

• negative feedback The stopping of the synthesis
  of an enzyme by the accumulation of the products
  of the enzyme-mediated reaction.
• negative feedback control  Occurs when
  information produced by the feedback reverses the
  direction of the response regulates the
  secretion of most hormones.
• negative feedback loop A biochemical pathway
  where the products of the reaction inhibit
  production of the enzyme that controlled their
  formation.

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Negative feedback

• Feedback and regulation are self related. The
  negative feedback helps to maintain stability in
  a system in spite of external changes. It is
  related to homeostasis. Positive feedback
  amplifies possibilities of divergences
  (evolution, change of goals) it is the condition
  to change, evolution, growth it gives the system
  the ability to access new points of equilibrium

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An example of a simple negative feedback loop
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What if you get cold?
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What if you blood sugar changes?
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Integrating organs with feedback
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• For example, in an organism, most positive
  feedbacks provide for fast autoexcitation of
  elements of endocrine and nervous systems (in
  particular, in stress responses conditions) and
  play a key role in regulation of morphogenesis,
  growth, and development of organs, all processes
  which are in essence a rapid escape from the
  initial state.
• Homeostasis is especially visible in the nervous
  and endocrine systems when considered at organism
  level.

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Endocrine system
The endocrine system is a control system of
ductless glands that secrete chemical messengers
called hormones that circulate within the body
via the bloodstream to affect distant organs.
Hormones act as "messengers", and are carried by
the bloodstream to different cells in the body,
which interpret these messages and act on them.
The endocrine system does not include exocrine
glands such as salivary glands, sweat glands and
glands within the gastrointestinal tract.
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What is a hormone?

• hormone (hôr'mon')n. A substance, usually a
  peptide or steroid, produced by one tissue and
  conveyed by the bloodstream to another to effect
  physiological activity, such as growth or
  metabolism.

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How are hormones classified

• Hormones are grouped into three classes based on
  their structure
• steroids
• peptides
• amines

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Steriods

• Steroids are lipids derived from cholesterol.
  Testosterone is the male sex hormone. Estradiol,
  similar in structure to testosterone, is
  responsible for many female sex characteristics.
  Steroid hormones are secreted by the gonads,
  adrenal cortex, and placenta.

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Steroid structure
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Peptides and Amines

• Peptides are short chains of amino acids most
  hormones are peptides. They are secreted by the
  pituitary, parathyroid, heart, stomach, liver,
  and kidneys. Amines are derived from the amino
  acid tyrosine and are secreted from the thyroid
  and the adrenal medulla. Solubility of the
  various hormone classes varies.

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• The integration of body functions in humans and
  other higher organisms is carried out by the
  nervous system, the immune system, and the
  endocrine system.
• The endocrine system is composed of a number of
  tissues that secrete their products, called
  endocrine hormones, into the circulatory system
  from there they are disseminated throughout the
  body, regulating the function of distant tissues
  and maintaining homeostasis.
• In a separate but related system, exocrine
  tissues secrete their products into ducts and
  then to the outside of the body or to the
  intestinal tract.

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Endocrine Hormones

• Classically, endocrine hormones are considered to
  be derived from amino acids, peptides, or sterols
  and to act at sites distant from their tissue of
  origin.
• However, the latter definition has begun to blur
  as it is found that some secreted substances act
  at a distance (classical endocrines), close to
  the cells that secrete them (paracrines), or
  directly on the cell that secreted them
  (autocrines). Insulin-like growth factor-I
  (IGF-I), which behaves as an endocrine,
  paracrine, and autocrine, provides a prime
  example of this difficulty.

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What is the amount of hormones in the blood?

• Hormones are normally present in the plasma and
  interstitial tissue at concentrations in the
  range of 10-7M to 10-10M.
• Because of these very low physiological
  concentrations, sensitive protein receptors have
  evolved in target tissues to sense the presence
  of very weak signals.
• In addition, systemic feedback mechanisms have
  evolved to regulate the production of endocrine
  hormones.

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How do hormones travel in the blood!

• Once a hormone is secreted by an endocrine
  tissue, it generally binds to a specific plasma
  protein carrier, with the complex being
  disseminated to distant tissues.
• Plasma carrier proteins exist for all classes of
  endocrine hormones. Carrier proteins for peptide
  hormones prevent hormone destruction by plasma
  proteases.
• Carriers for steroid and thyroid hormones allow
  these very hydrophobic substances to be present
  in the plasma at concentrations several
  hundred-fold greater than their solubility in
  water would permit.
• Carriers for small, hydrophilic amino
  acid--derived hormones prevent their filtration
  through the renal glomerulus, greatly prolonging
  their circulating half-life.

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Nonsteroid hormones (water soluble) do not enter
the cell but bind to plasma membrane receptors,
generating a chemical signal (second messenger)
inside the target cell. Five different second
messenger chemicals, including cyclic AMP have
been identified. Second messengers activate
other intracellular chemicals to produce the
target cell response.
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Action of nonsteroid hormones
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Step two
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Step Three
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Action of steroid hormone

• The second mechanism involves steroid hormones,
  which pass through the plasma membrane and act in
  a two step process.

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• Steroid hormones bind, once inside the cell, to
  the nuclear membrane receptors, producing an
  activated hormone-receptor complex.

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• The activated hormone-receptor complex binds to
  DNA and activates specific genes, increasing
  production of proteins.

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How do tissue react to hormones?

• Tissues capable of responding to endocrines have
  2 properties in common they posses a receptor
  having very high affinity for hormone, and the
  receptor is coupled to a process that regulates
  metabolism of the target cells.
• Receptors for most amino acid--derived hormones
  and all peptide hormones are located on the
  plasma membrane. Activation of these receptors by
  hormones (the first messenger) leads to the
  intracellular production of a second messenger,
  such as cAMP, which is responsible for initiating
  the intracellular biological response.
• Steroid and thyroid hormones are hydrophobic and
  diffuse from their binding proteins in the
  plasma, across the plasma membrane to
  intracellularly localized receptors. The
  resultant complex of steroid and receptor bind to
  response elements of nuclear DNA, regulating the
  production of mRNA for specific proteins.

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What we have seen so far..

• Stomach and intestines
• Gastrin
• Secretin
• Cholecystokinin (CCK)
• Somatostatin
• Neuropeptide Y

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Getting a head start on hormones
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List of hormones and organs related to hormones

• Hypothalamus
• Thyrotropin-releasing hormone (TRH)
• Gonadotropin-releasing hormone (GnRH)
• Growth hormone-releasing hormone (GHRH)
• Corticotropin-releasing hormone (CRH)
• Somatostatin
• Dopamine

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Hypothalamus to Pituitary
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Pituitary gland

• Anterior lobe (adenohypophysis)
• GH (human growth hormone)
• PRL (prolactin)
• ACTH (adrenocorticotropic hormone)
• TSH (thyroid-stimulating hormone)
• FSH (follicle-stimulating hormone)
• LH (luteinizing hormone)
• Posterior lobe (neurohypophysis)
• Oxytocin
• ADH (antidiuretic hormone)

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Integration of blood and hormones
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A division of labour
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• Pineal gland
• Melatonin
• Thyroid gland
• Thyroxine (T4), a form of thyroid hormone
• Triiodothyronine (T3), a form of thyroid hormone
• Calcitonin
• Parathyroid gland
• Parathyroid hormone (PTH)
• Heart
• Atrial-natriuretic peptide (ANP)

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• Adrenal glands
• Adrenal cortex
• Glucocorticoids - cortisol
• Mineralocorticoids - aldosterone
• Androgens (including testosterone)
• Adrenal medulla
• Adrenaline (epinephrine)
• Noradrenaline (norepinephrine)

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Adrenal gland and kidney

• Kidney
• Renin
• Erythropoietin (EPO)
• Calcitriol

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• Liver
• Insulin-like growth factor
• Angiotensinogen
• Thrombopoietin
• http//users.rcn.com/jkimball.ma.ultranet/BiologyP
  ages/L/LiverHormones.html
• Islets of Langerhans in the pancreas
• Insulin
• Glucagon
• Somatostatin

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• Skin
• Calciferol (vitamin D3)
• Adipose tissue
• Leptin

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• In males only
• Testes
• Androgens (testosterone)

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• In females only
• Ovarian follicle
• Oestrogens
• Testosterone
• Corpus luteum
• Progesterone
• Placenta (when pregnant)
• Progesterone
• Human chorionic gonadotrophin (HCG)
• Human placental lactogen (HPL)

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Your challenge.

• Most hormones turn on and off a response
• You should be able to make feed back loops for
  regulating levels of major chemical groups in the
  body.
• What are feedback loops for regulating
• Oxygen and Carbon Dioxide
• Blood sugars ( you have seen it!)
• Blood salts
• Sexual and growth development

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More challenges

• Can you link feedback mechanisms to specific
  systems?
• Could you make a comparative table pairing
  hormones with organ systems?
• What disorders are associated with your adrenal
  gland, thyroid gland, pancreas, bones and blood
  sugar imbalance. (see on-line references)

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A possible end..or beginning
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List of on-line references

• http//www.dentistry.leeds.ac.uk/biochem/thcme/pep
  tide-hormones.html
• http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
  BioBookENDOCR.html
• http//www.endocrineweb.com/