Hormonal Communication

• Endocrine system: a communication system using hormones as signalling molecules
• Hormones: molecules (proteins or steroids) released by endocrine glands directly into blood. Act as messengers that carry signal from endocrine gland to a specific target organ or tissue
• Target cells: for non-steroid hormones, cells that possess a specific receptor on their plasma membrane, the shape of receptor is complementary to shape of hormone molecule. Many similar cells together form a target tissue
• Endocrine system uses blood circulatory system to transport its signals - the signals released by this system are hormones and as blood system transports materials around the body, hormones are transported throughout the body.
• Two types of hormone that work in different wats:

  • Protein and peptide hormones and derivatives of amino acids (e.g adrenaline, insulin and glucagon); proteins not soluble in phospholipid membrane and so don't enter cell, rather binding to the cell surface membrane and releasing a messenger inside the cell
  • Steroid hormones (e.g oestrogen and testosterone); can pass through membrane and enter cell nucleus to have direct effect on DNA in nucleus
• Endocrine glands release hormones directly into glands and are ductless, consisting rather of a group of cells that manufacture and release the hormone directly into the blood in capillaries running through the gland.

  
• Endocrine glands have groups of cells with associated capillaries and no visible ducts
• What type of cells receive endocrine signals?

  Target cells
• What is the role of target cells in relation to hormones?

  They respond to endocrine signals due to the presence of specific receptors.
• What are the characteristics of target tissues?

  • May consist of grouped target cells (e.g., collecting duct's epithelium)
  • May be widely dispersed across various tissues (e.g., adrenaline receptors in CNS and peripheral nervous system)
• Where are adrenaline receptors found?

  In the central nervous system and tissues innervated by the peripheral nervous system.
• What is required for non-steroid hormones to affect target cells?

  Target cells must have specific receptors on their plasma membrane that are complementary in shape to the hormone.
• How does the specificity of hormone receptors affect the response of cells to hormones?

  Only specific target cells with the correct receptor can respond to the hormone, preventing non-target cells from being affected.
• What happens if all cells in the body have a specific receptor for a hormone?

  All cells can respond to the hormone signal.
• Why can hormones be carried in the blood without affecting all cells?

  Because each hormone is different and can only bind to specific receptors on target cells.
• What is the significance of having specific receptors for hormones on target cells?

  It allows for precise regulation of physiological processes by ensuring that only certain cells respond to specific hormones.
• What are non-steroid hormones known as?

  First messengers
• What is the role of first messengers in cellular signaling?

  They bind to cell surface membranes to initiate an effect inside the cell
• What do non-steroid hormones typically cause the release of inside the cell?

  A second messenger
• What is the function of the second messenger in the cell?

  It usually stimulates a change in the activity of the cell
• How do many non-steroid hormones act within the cell?

  They act via a G protein in the membrane
• What is activated when a hormone binds to its receptor?

  A G protein
• What does the G protein activate?

  An effector molecule, usually an enzyme
• What is the name of the effector molecule in many cells?

  Adenyl cyclase
• What does adenyl cyclase convert ATP into?

  Cyclic AMP (cAMP)
• What is the role of cyclic AMP (cAMP) in the cell?

  It acts as a second messenger
• How can cAMP affect cellular activity?

  It can act directly on another protein or initiate a cascade of enzyme-controlled reactions
• What is the significance of the cascade of enzyme-controlled reactions initiated by cAMP?

  It alters the activity of the cell
• cAMP is the second messenger

  
• Adrenal cortex: outer layer of adrenal gland
• Adrenal gland: one of a pair of glands lying above the kidneys which releases adrenaline and a number of other hormones known as corticoids or corticosteroids such as aldosterone
• Adrenaline: a hormone released from adrenal glands which stimulates the body to prepare for flight or fight
• Adrenal medulla: inner layer of adrenal gland
• Adrenal glands are found lying anterior to kidneys and each gland is divided into outer adrenal cortex and inner adrenal medulla. Both regions have a good blood vessel supply and produce hormones secreted directly into the blood vessels.
• Adrenal cortex is the outer capsule of the adrenal gland and surrounds three distinct layers of cells:

  • Zona glomerulosa - outermost layer; secretes mineralocorticoids such as aldosterone
  • Zona fasciculata - middle layer; secretes glucocorticoids such as cortisol
  • Zona reticularis - innermost layer; thought to secrete precursor molecules used to make sex hormones
• Adrenal medulla found at center of adrenal gland and secretes adrenaline and noradrenaline
• Adrenal cortex uses cholesterol to produce a range of hormones which are steroid based able to enter cells directly by dissolving into cell surface membrane. Steroid hormones enter nucleus. They have a direct effect on DNA and cause protein synthesis.
• Action of steroid hormones:

  1. Steroid hormone passes through cell membrane of target cell
  2. Then binds with specific receptor with complementary shape in cytoplasm
  3. Receptor-steroid hormone complex enters nucleus of target cell and binds to another specific receptor on chromosomal material
  4. Binding stimulates the production of mRNA molecules. These code for the production of proteins.
• What are mineralocorticoids and give an example?

  Mineralocorticoids are hormones that help control sodium and potassium concentrations in the blood; an example is aldosterone.
• How do mineralocorticoids contribute to blood pressure maintenance?

  They help control concentrations of sodium and potassium in the blood, which contributes to blood pressure maintenance.