LEC 29: ENDOCRINE SYSTEM

• Hormonal vs Neural Controls of Body Functioning

  • Hormonal (endocrine system)
  • Neural (nervous system)
• Hormonal (endocrine system)


  • Mode of Transmission: Releases hormones into bloodstream then to target cells
  • Speed of Action: slower
  • Duration of Action: last for longer durations depending on the half-life of the hormone
  • Target Cells: affect target cells throughout the body (circulates in bloodstream)
  • Feedback Mechanisms: negative feedback loops involving changes in hormone levels
• Neural (nervous system)


  • Mode of Transmission: electrical impulses along neurons to specific target cells via synapses
  • Speed of Action: faster
  • Duration of Action: more immediate but shorter-lived, last as long as electrical signal persists
  • Target Cells: target specific cells or groups of cells along neural pathways
  • Feedback Mechanisms: can have feedback loops, may also rely on reflex arcs for rapid responses to stimuli
• Hormones vs Paracrines vs Autocrines

  • Hormones: chemical messengers released into the bloodstream, act on distant target cells, ex. Cortisol, insulin, estrogen
  • Paracrine: local messenger cells act on neighbouring cells, don't circulate in the bloodstream, ex. Histamine, prostaglandins
  • Autocrine: local chemical messenger cells act on the same cells that secrete them, don't circulate in the bloodstream, ex. Interleukins, growth factors
• How Hormones are Classified Chemically

  • Amine Hormones: derived from amino acids, ex. Catecholamines, thyroid hormones, histamine, serotonin/melatonin
  • Peptide & Protein Hormones: chains of amino acid, peptide hormones are shorter, protein hormones are longer, ex. ADH, oxytocin, growth hormone, insulin
  • Steroid Hormones: derived from cholesterol, lipid-soluble molecules, ex. testosterone, estrogen, cortisol, aldosterone
  • Eicosanoids: derived from polyunsaturated fatty acids, prostaglandins which modulate inflammation and immune regulation
• Two Major Mechanisms by which Hormones Affect their Target Tissues

  • Water-Soluble Hormones: bind to receptors on the OUTSIDE surface of target cell membrane, activation of intracellular signalling pathways involves second messengers like cAMP or calcium, circulates freely in blood plasma, ex. Insulin, growth hormone
  • Lipid-Soluble Hormones: can diffuse through the plasma membrane, bind to receptors INSIDE target cell, acts as transcription factors, influencing gene expression, circulate bound to transport proteins, ex. Cortisol, thyroxine (T4)
• Direct Gene Activation (lipid soluble hormone)


  1. lipid-soluble hormone (ex. thyroid hormone) binds to intracellular receptor

  2. activates a region of DNA to make mRNA and initiation of protein synthesis
• Cyclic AMP (cAMP) Signalling Mechanisms (water soluble hormone)


  1. hormone (1st messenger) activates to its receptor which activates G protein which activates adenyle cyclase

  2. activated adenyle cyclase converts ATP into cAMP

  3. generates cAMP (2nd messenger) through the action of protein kinase enzymes

  4. cAMP 2nd messenger activates enzymes or causes membrane transport
• How Hormone Release is Regulated

  • Negative Feedback
  • Humoral Stimuli: hormone release is triggered by changes in blood levels of certain ions or nutrients, ex. Release of insulin in response to high blood glucose levels
  • Neural Stimuli: hormone release stimulated by neural input, ex. Release of adrenaline by the adrenal medulla in response to sympathetic nervous system activation
  • Hormonal Stimuli: Hormone release is triggered by the presence of the action of another hormone, ex. Release of thyroid-stimulating hormone by the pituitary gland in response to low thyroid hormone levels
• Factors that Influence the Activation of a Target Cell by a Hormone

  • Blood levels of the hormone
  • Number of receptors on the target cell
  • Affinity of the receptors for the hormone
  • low levels of hormone = Up-regulate = increase number of receptors
  • high levels of hormones = down-regulate = desensitizes target cell to prevent from overreacting
• Kinds of Interactions Between Different Hormones Acting on the Same Target Cell

  • Permissiveness: One hormone cannot exert its full effect without the presence of another hormone, ex. thyroid hormone is required for reproductive hormones to exert their effects
  • Synergism: Two or more hormones produce the same effect on a target cell, and their combined effects are amplified, ex. glucagon and epinephrine both increase blood glucose levels
  • Antagonism: One hormone opposes the action of another hormone, ex. insulin decreases blood glucose levels, glucagon increases blood glucose levels
• Half-life


  Length of time for a hormone to decrease to half its starting concentration in the blood