A&P: CH.16 [Endocrine System]

• endocrine system: interacts w/ nervous system to coordinate and integrate activity of body cells
• hormones are chemical messengers secreted by cells into extracellular fluids
• binding of a hormone to cellular receptors initiates responses that typically occur after lag period of seconds or even days but lasts super long
• hormones ultimately target most cells of the body, producing widespread and diverse effects
• major processes hormones control and integrate

  • reproduction
  • growth and development
  • maintenance of electrolyte, water, and nutrient balance of blood
  • regulation of cellular metabolism and energy balance
  • mobilization of body defenses
• endocrinology: study of hormones and endocrine organs
• endocrine glands: ductless glands that release hormones into surrounding tissue fluid and have rich vascular and lymphatic drainage that receives hormones
• endocrine glands include pituitary, thyroid, parathyroid, adrenal, and pineal glands
• pancreas, gonads (ovaries and testes), placenta, adipose tissue, thymus. intestines, kidneys and heart contain endocrine tissues
• autocrines: chemicals that exert effects on same cells that secrete them (i.e. prostaglandins released by smooth muscle to contract smooth muscle)
• paracrines: chemicals that affect nearby cells (i.e. growth factors secreted by fibroblasts to stimulate mitosis)
• amino acid based hormones: simple amino acid derivatives, peptides, proteins; most common hormone base
• steroids: synthesized from cholesterol; only gonadal and adrenocortical are steroid based
• eicosanoids: nearly all cell membranes release prostaglandins and leukotrienes; derived from arachidonic acid
• leukotrienes are signaling chemicals that mediate inflammation and some allergic reactions
• target cells: hormone influences activity of only those tissue cells that have the receptors for it
• hormones produce:

  • alteration of plasma membrane by opening or closing ion channels
  • stimulates synthesis of enzymes
  • activates or deactivates enzymes
  • induces secretory activity
  • stimulates mitosis
• water-soluble hormones (all amino-acid based hormones except thyroid hormones) act on receptors in plasma membrane; coupled to act on one or more intracellular second messengers which mediate target cell response
• lipid-soluble hormones (steroid and thyroid hormones) act on receptors inside cell, directly activating genes
• in order for target cell to respond to hormone, cell must have specific receptor proteins on plasma membrane or interior for it to bind
• hormones are molecular triggers rather than informational molecules
• target cell activation depends on

  1. blood levels of hormones
  2. relative number of receptors for hormone on a target cell
  3. strength of binding between hormone and receptor
• up-regulation: persistently low levels of a hormone cause target cells form additional receptors for a hormone
• down-regulation: prolonged exposure to high concentrations leads to desensitization of receptors, decreased response
• hormones influence number of their own receptors but also number of receptors that respond to other hormones
• synthesis and release of most hormones are regulated by some type of negative feedback mechanism
• humoral stimuli: critical ions and nutrient levels act as stimuli; simplest endocrine control
• neural stimulus: nerve fibers stimulate hormone release; "fight or flight" via norepinephrine and epinephrine
• hormonal stimulus: hormone release caused by another hormone; rhythmic hormonal release (tropic hormone)
• nervous system makes certain adjustments to maintain homeostasis by overriding normal endocrine controls
• hormones circulate blood by free flow or protein carrier bound
• lipid-soluble hormones like steroids and thyroid hormone travel in blood stream attached to plasma proteins; most others w/out carriers
• concentration of circulating hormone in blood at any time reflects

  1. rate of release
  2. speed of inactivation and removal (usually by kidneys or liver)
• half-life: length of time for hormone's blood levels to decrease by half
• permissiveness: one hormone can not exert its full effects without another hormone being present (thyroid hormone needed for reproductive structures)
• synergism: more than one hormone produce same effects at target cell and combined effects amplified
• antagonism: one or more hormones oppose action of another hormone
• hypothalamus known as a neuroendocrine gland as it has neural functions as well as production of release hormones
• with the exception of thyroid hormone, amino-acid based hormones exert their signaling effects through intracellular second messengers generated when a hormone binds to a receptor in plasma membrane
• cyclic AMP signaling mechanism

  1. hormone (1st messenger) binds receptor
  2. receptor activates G protein
  3. G protein activates adenylate cyclase
  4. adenylate cyclase converts ATP to cAMP (2nd messenger)
  5. cAMP activates protein kinases