5.4.1

• Signalling using hormones
• Types of hormones

  • protein or peptide hormones, and derivative of amino acids (e.g. adrenalin, insulin + glucagon)

  proteins are not soluble in phospholipid membrane + do not enter cell

  → protein hormones need to bind to the plasma membrane + release a second messenger inside cell

  • steroid hormones (e.g. oestrogen + testosterone)

  steroid hormones can pass through the membrane + enter cell + nucleus, to have a direct effect of DNA in nucleus
• Endocrine glands

  Where hormones are released directly into blood

  • they are ductless glands - consists of groups of cells that manufacture + release that hormone directly into blood in capillaries running through gland
  • consists of groups of cells surrounding a small duct that secrete their products into duct, which leads to site where secretion is required

  e.g. salivary gland secrete saliva into duct, which carries saliva to mouth

  → no visible ducts but groups of cells with associated capillaries
• endocrine - releases hormones
• exocrine - doesn’t release hormones
• Detecting the signal

  hormones only have effect on a specific type of tissue

  • cells may be grouped together in a target tissue such as the epithelium of collecting ducts

  → may be more widely dispersed in a number of tissues such as the receptors for adrenaline found in CNS + tissues innervated by peripheral nervous system including the heart, smooth muscle + skeletal muscle
• Detecting the signal 2

  • for non-steroidal hormones, the target cells must possess a specific receptor on their plasma membrane that is complementary in shape to shape of signalling molecule (hormone)
  • not all cells in body possess such a receptor as each hormone is different

  → a hormone can be carried in blood without affecting cells that possess the correct receptor will respond to hormone
• Two types of adrenergic receptor

  • alpha receptor are excitatory in smooth muscle + gland cells but cause relaxation of intestinal smooth muscles
  • beta receptors produce an inhibitory response, although in heart muscle the effect is excitatory

  → beta blockers are drugs that inhibit response of these receptor to adrenalin in control of certain heart conditions
• First and second messenger

  first messenger - hormone that transmits a signal around body

  second messenger - molecule that transmits the signal inside cell
• First and second messenger 2

  • non-steroidal hormones act via a G protein in membrane + initiate an effect inside the cell

  → they usually cause the release of another signalling molecule in cell; stimulates a change in activity of cell

  • many non-steroid hormones act via a G protein in membrane activated when hormone binds to receptor

  → G protein in turn activates an effector molecule - usually an enzyme that converts an inactive molecule into active second messenger
• First and second messenger 2

  • in many cells the effector molecule is the enzyme adenyl cyclase, which converts ATP to cAMP (cyclic AMP)

  → this second messenger may act directly on another protein (such as ion channel) or imitate a cascade of enzyme-controlled reactions that alter the activity of cell