ENDOCRINE SYSTEM

• Endocrine system


  Composed of any cell or tissue that produces hormones, which are chemical messengers that help control and coordinate many of the body's processes
• Intercellular communication


  • Direct communication (ions and molecules pass between neighboring cells through gap junctions)
  • Paracrine communication (chemical signals transfer information from cell to cell within a single tissue)
  • Autocrine communication (messages affect the same cell that secretes them)
• Endocrine communication


  Endocrine cells release hormones into the bloodstream, which transports them to different organs and tissues, altering their metabolic activities
• Target cells


  Specific cells in other tissues that have receptors to bind and read hormone messages
• Binding of a hormone to its target cell can change the type, amount, or activities of enzymes and proteins in the target cell, affecting its activity and metabolism in multiple tissues and organs
• Endocrine glands/organs


  • Hypothalamus
  • Pituitary gland
  • Pineal gland
  • Parathyroid glands
  • Thyroid
  • Adrenals
  • Pancreas
  • Heart
  • Fat
  • Digestive tract
  • Kidneys
  • Gonads (testes and ovaries)
• Hormone groups based on chemical structure

  • Amino acid derivatives (e.g. thyroid hormones, catecholamines, serotonin, melatonin)
  • Peptide hormones (e.g. glycoproteins, short chain polypeptides, small proteins)
  • Lipid derivatives (e.g. eicosanoids, steroids)
• Hormone transport


  Freely circulating or bound to carrier proteins
• Hormone inactivation


  Diffusion out of blood, binding to receptors on target cells, absorption/breakdown by liver/kidneys, enzymatic breakdown
• Hormone receptor


  Protein molecule that a particular hormone can bind to strongly; presence/absence determines cell sensitivity to hormone
• How hormones affect target cells

  1. Catecholamines and peptide hormones bind to extracellular receptors (first messenger)

  2. Steroid and thyroid hormones diffuse across cell membrane and bind to intracellular receptors
• Steroid and thyroid hormones


  Lipid soluble, able to diffuse across the lipid membrane of the cell and bind directly to receptors in the cell
• Catecholamines and peptide hormones


  Unable to get into the cell, have to bind to receptors on the outside
• Binding of catecholamines and peptide hormones


  1. Hormone binds to extracellular receptor (first messenger)

  2. Promotes release of second messenger in cell

  3. Second messenger acts as enzyme activator or inhibitor, changing metabolic reactions
• Second messengers


  Examples: cyclic AMP, cyclic GMP, calcium
• Increasing second messenger (cyclic AMP) level


  1. Activated G protein activates adenylate cyclase

  2. Adenylate cyclase converts ATP to cyclic AMP

  3. Cyclic AMP functions as second messenger
• Steroid hormones


  • Can diffuse across plasma membrane, no need for second messengers or G proteins
  • Can alter rate of DNA transcription, create enzymes/proteins, affect activity and structure of target cells
• Thyroid hormones


  • Can diffuse across plasma membrane, no need for second messengers or G proteins
  • Bind to receptors in nucleus and mitochondria
  • Increase ATP production
• Pituitary gland


  • Also known as the hypophysis, hangs inferior to the hypothalamus, connected by the infundibulum
  • Releases 9 important peptide hormones
• Hypothalamus


  • Regulates function of pituitary gland, makes hormones ADH and oxytocin which are transported to posterior pituitary for release
  • Makes regulatory hormones that control activity of anterior pituitary
  • Contains center that directly controls adrenal glands
• Regions of anterior pituitary


  • Pars distalis
  • Pars intermedia
  • Pars tuberalis
• Median eminence


  Swelling near attachment of infundibulum, where hypothalamus neurons release regulatory hormones into interstitial fluids to enter bloodstream
• Hypothalamus regulatory mechanism


  1. Hypothalamus releases regulatory hormones (e.g. TRH, CRH, GnRH)

  2. Allows anterior pituitary to release hormones (e.g. TSH, ACTH, FSH, LH)

  3. Pituitary hormones then affect other endocrine glands/organs
• Hormones released by anterior pituitary

  • Thyroid stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Follicle stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Growth hormone (GH)
  • Prolactin
  • Melanocyte stimulating hormone (MSH)
  • Oxytocin
  • Antidiuretic hormone (ADH)
• Release of hormones


  1. TSH released by hypothalamus

  2. ACTH released by anterior pituitary

  3. FSH and LH released by anterior pituitary

  4. Hormones released by testes and ovaries
• TSH


  Thyroid stimulating hormone, affects the thyroid and allows production of thyroid hormones
• CRH


  Corticotropin releasing hormone, released by hypothalamus to allow anterior pituitary to release ACTH
• ACTH


  Adrenocorticotropic hormone, released by anterior pituitary and affects the adrenals
• GnRH


  Gonadotropin releasing hormone, released by hypothalamus and affects anterior pituitary to release FSH and LH
• Hormones released by anterior pituitary


  • TSH
  • ACTH
  • Prolactin
  • Growth hormone
  • FSH
  • LH
• TSH


  Causes secretion of thyroid hormones
• ACTH


  Causes secretion of glucocorticoids like cortisol and cortisone from the adrenals
• Prolactin


  Causes mammaries to produce milk
• Growth hormone


  Causes growth
• FSH


  Causes secretion of estrogen and follicle development
• LH


  In females, induces ovulation and stimulates secretion of estrogen and progesterone. In males, stimulates production of androgens.
• MSH


  Melanocyte stimulating hormone, stimulates melanin production
• ADH


  Antidiuretic hormone, causes kidneys to retain water and decrease urination
• Oxytocin


  Stimulates contraction of uterus during labor and promotes milk ejection
• Thyroid


  • Located inferior to thyroid cartilage (Adam's apple)
  • Consists of follicles lined by cuboidal epithelium and containing colloid
  • Contains C cells that secrete calcitonin