Endocrine System

• Nervous System


  Uses electrochemical impulses ("nerve impulses"), rapid delivery by axons, organ response is quick
• Endocrine System


  Uses chemical messengers ("hormones"), slower delivery by blood, response is prolonged
• Endocrine glands


  • Ductless glands that secrete their products (hormones) into interstitial fluid around secretory cells, diffuse into blood capillaries and then are carried to target cells throughout the body
• Types of endocrine gland organization

  • Discrete organ (e.g. pituitary, thyroid, parathyroid, adrenal glands)
  • Group of cells within another organ (e.g. hypothalamus, pancreas, ovary, testis)
  • Individual cells within another organ (e.g. enteroendocrine cells in GIT, kidney - erythropoietin)
• Hormone

  A molecule secreted by an endocrine gland that acts as a chemical messenger to regulate the functions of other cells in the body
• Types of hormones

  • Circulating hormones
  • Paracrine (act on neighboring cell)
  • Autocrine (act on same cell that secreted them)
• Target cell receptors

  Protein receptors that bind and recognize a given hormone, constantly replaced, 2000-100,000 receptors per target cell
• Down regulation


  Target cell decreases number of receptors due to increased hormone concentration (target cells become less sensitive to hormone)
• Up regulation


  Target cell increases number of receptors due to decreased hormone concentration (target cells become more sensitive to hormone)
• Changes hormones can cause in target cells

  • Stimulate protein synthesis
  • Change membrane properties
  • Activate or deactivate enzymes
  • Increase secretory activity
  • Stimulate mitosis
• Classes of hormones


  • Lipid-soluble (steroid, thyroid)
  • Water-soluble (amine, peptide, protein)
• Steroid hormones


  Derived from cholesterol (e.g. aldosterone, cortisol, testosterone, estrogen, progesterone)
• Thyroid hormones


  T3 and T4 (thyroxine), generally bind to a transport protein to circulate in blood
• Amine hormones


  e.g. epinephrine
• Peptide hormones


  1. 49 amino acids (e.g. ADH, oxytocin)
• Protein hormones


  50-200 amino acids (e.g. human GH, FSH, LH, TSH, ACTH, prolactin, parathyroid, calcitonin, insulin, glucagon)
• Factors affecting target cell activation by hormone-receptor interaction

  • Blood level of the hormone
  • Relative number of receptors for that hormone on or in the target cells
  • Affinity (strength) of the bond between the hormone & receptor
  • Influences exerted by other hormones
• Mechanism of action for lipid-soluble hormones

  Free hormone diffuses from blood, crosses plasma membrane, binds to receptor in nucleus or cytoplasm, complex binds to regulatory proteins to activate gene(s), directs synthesis of new proteins, new proteins alter cell's activity and cause hormone's responses
• Mechanism of action for water-soluble hormones

  Hormone diffuses from blood and binds to receptor on cell surface, activates G-protein, G-protein activates adenylate cyclase, adenylate cyclase converts ATP to cAMP (second messenger), cAMP activates protein kinases, activated protein kinases phosphorylate cellular proteins to produce responses
• Why steroid and thyroid hormones are effective when taken orally, but protein hormones like insulin need to be injected


  Steroid and thyroid hormones are not split apart during digestion and easily cross the intestinal lining because they are lipid-soluble, while protein hormones would be destroyed by digestive enzymes
• Stimuli that can trigger endocrine gland hormone release

  • Humoral (chemical changes in blood ion or nutrient levels)
  • Neural (nerve impulses)
  • Hormonal (tropic hormones from other endocrine glands)
• Most hormones are regulated by negative feedback systems
• Hypothalamus


  • Secretes 9 different neurohormones - 5 releasing hormones, 2 inhibitory hormones, oxytocin, antidiuretic hormone
• Hypothalamus regulation of anterior pituitary

  1. Secretes releasing and inhibiting hormones into hypophyseal portal system to reach and regulate anterior pituitary

  2. Secretes oxytocin and antidiuretic hormone into posterior pituitary where they are stored, action potentials trigger their secretion into vessels
• Pituitary gland


  • Attached to hypothalamus by infundibulum, has anterior lobe (glandular tissue releasing 6 hormones) and posterior lobe (neural tissue releasing 2 neurohormones)
• Hormones released by anterior pituitary

  • Growth hormone (GH)
  • Thyroid stimulating hormone (TSH)
  • Follicle stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Adrenocorticotropic hormone (ACTH)
  • Prolactin
• Pituitary Gland


  • Attaches to the hypothalamus by a stalk, the infundibulum
  • Has 2 lobes
• Anterior lobe


  • Glandular tissue and releases 6 hormones
• Posterior lobe


  • Neural tissue and releases 2 "neurohormones" (ADH & oxytocin)
• Hormones released by anterior lobe

  • Growth hormone (GH)
  • Thyroid stimulating hormone (TSH)
  • Follicle stimulating hormone (FSH)
  • Lutenizing hormone (LH)
  • Adrenocorticotropic hormone (ACTH)
  • Prolactin
• Tropic hormone


  Influences secretory activity of another gland
• Posterior Lobe


  • Not a "true" endocrine gland (storage area)
  • Hormones are made in hypothalamus and are stored in posterior lobe
  • Releases 2 hormones: Antidiuretic hormone (ADH) and Oxytocin
• Thyroid Gland


  • H-shaped gland just below the larynx
  • Composed of hollow spherical follicles containing a lumen filled with colloid
  • Follicle cells make the colloid which is thyroglobulin protein with attached iodines
  • Thyroid hormone is derived from the colloid
  • Parafollicular cells are found between the follicles and produce the hormone calcitonin
• Thyroid hormone


  Thyroxine (T4) - increases rate of metabolism, regulates growth & development
• Calcitonin


  Decreases blood calcium levels by inhibiting breakdown of bone and stimulating the deposition of calcium into bone matrix
• Parathyroid Gland


  • Four small knots of tissue embedded in posterior thyroid gland tissue
  • Produces parathyroid hormone (PTH) which acts to increase blood calcium levels
• Adrenal Gland


  • Pyramidal-shaped gland atop kidney
  • Has 2 structurally and functionally distinct regions - cortex and medulla
• Adrenal Cortex Zones


  • Zona glomerulosa (Mineralocorticoids)
  • Zona fasciculata (Glucocorticoids)
  • Zona reticularis (Gonadocorticoids)
• Mineralocorticoids


  Hormones that affect mineral homeostasis, main one is aldosterone which increases blood sodium and calcium, decreases blood potassium
• Glucocorticoids


  Involved with regulating metabolism and resisting stress, main one is cortisol which increases blood glucose, fatty acids and amino acids