Lecture 25 - Homeostasis and Hormones

• Homeostasis


  A relatively constant internal environment that supports cell functioning and life processes
• Set-point


  ‘Normal’ value of controlled variables. Each physiological condition has a particular set point or range around which it fluctuates. Each person has a unique set-point for each variable based on genetics and other factors.
• Normal range


  The restricted set of values that is optimally healthy and stable at a population level, this normal range permits optimal cell functioning
• Population reference range


  The set of values (high and low ends) for a regulated variable that are considered normal and/or healthy
• Population reference range


  Tends to be wider than any given individual's normal range
• Most individuals (~95%)

  Will have a set point that is within the population reference range
• If a variable moves outside an individual's "normal range"


  It may lead to illness, disease or symptoms of a disorder
• Negative feedback


  How we maintain homeostasis of regulated variables, keeps a regulated variable within its normal range, around its set-point
• Negative feedback control loop


  • Sensors - Monitor the variable and detect changes (deviation from set-point)
  • Control centre - Compares variable's changed value to its set-point. Sends signals to effectors if correction is required.
  • Effectors - Act to oppose the effect of the stimulus, thereby correcting the change and restoring the variable to its set-point.
• Positive feedback


  The amplification of regulated variables, not for homeostasis, but for processes that need to be driven to completion
• Positive feedback loop


  • Sensors - Monitor the variable and detect changes (deviation from set-point)
  • Control centre - Sends signals to effectors to amplify change.
  • Effectors - Action amplifies the change, to drive a process to completion
• Nervous system


  Neurons produce action potentials and release neurotransmitter at synapses (with neurons, muscle or glands)

  Neurotransmitter binds to chemically-gated ion channels on post-synaptic cell

  Extremely fast signalling
• Endocrine system


  Endocrine cells release hormones into the bloodstream to travel to target cells
• Nervous system


  Extremely fast signalling
• Endocrine system


  Relatively slower, but longer lasting action compared to the nervous system
• Endocrine glands


  • Hypothalamus
  • Parathyroid
  • Thyroid
  • Adrenal
  • Pituitary
  • Pancreas
• Endocrine system


  Consists of endocrine gland cells, hormones, the circulation/blood stream, and the target tissues
• Key variables maintained by the endocrine system


  • Blood sugar
  • Growth/repair
  • Basal metabolic rate
  • Blood calcium concentration
• Hormones


  Chemicals made from amino acids or cholesterol that are released by the endocrine gland cells, travel through the blood, and bind to receptors on either the target cell membrane or within the target cell
• Types of hormones


  • Water-soluble hormones (Peptides, Catecholamines)
  • Lipid (Fat) Soluble Hormones (Steroids, Thyroid Hormones)
• Water-soluble hormones


  • Stored until required (released by exocytosis)
  • Dissolved in the blood
  • Bind to cell-surface receptors
  • Activate secondary messenger systems
  • Response in milliseconds to minutes
• Lipid-soluble hormones


  • Steroids made from cholesterol as required (not stored)
  • Thyroid hormones made in thyroid cells, stored until required
  • Bound to carrier protein in blood
  • Diffuse across cell membrane, bind to intracellular receptors
  • Together with receptor, complex acts as a transcription factor for specific genes
  • Response takes hours to days
• Hormone mechanism of action


  1. Dissociates from carrier protein

  2. Diffuses across cell membrane

  3. Binds to intracellular receptor inside target cell's cytosol or nucleus

  4. Together with receptor, complex acts as a transcription factor for specific genes

  5. Target gene is activated

  6. New mRNA is generated and translated

  7. New protein mediates cell specific response
• The effects of a hormone stop when it unbinds from a receptor
• Once the effect of a hormone the constituents of hormone is used or excess hormone gets broken down, recycled, or excreted in sweat, urine or feces
• Water-soluble Hormones


  • Peptides (Most abundant)
  • Catecholamines (Amines)
• Lipid (Fat) Soluble Hormones


  • Steroids
  • Thyroid Hormones (Amines from amino acids)
• Water-soluble Hormones


  • Stored until required (released by exocytosis)
  • Unbound (No carrier protein), Dissolved in the blood
  • Cell surface, plasma membrane receptors
  • Through 2nd messengers, activates them for a big response!!
  • Milliseconds to minutes
• Lipid (Fat) Soluble Hormones


  • Steroids made from cholesterol as required (not stored)
  • Thyroid hormones made in thyroid cells, stored until required
  • Bound to carrier protein
  • Intracellular receptors in cytoplasm or nucleus
  • By altering gene transcription to make new proteins
  • Hours to days
• Water-soluble hormone storage


  Stored until required (released by exocytosis)
• Lipid soluble hormone storage 


  Steroids made from cholesterol as required (not stored) Thyroid hormones made in thyroid cells, stored until required
• Water soluble hormone transport 


  Unbound (No carrier protein), Dissolved in the blood
• Lipid soluble hormone transport


  Bound to carrier protein
• Water soluble hormone receptors 


  On the cell surface, plasma membrane receptors
• Lipid soluble hormone receptors


  Intracellular receptors in cytoplasm or nucleus 
• Water soluble hormone mechanism of action...

  Through 2nd messengers, activates them for a big response!!
• Lipid soluble hormone mechanism of action...

  By altering gene transcription to make new proteins
• Water soluble hormone speed of response ...

  Milliseconds to minutes 
• Lipid soluble hormone speed of response...

  Hours to days