Endocrine system

• Endocrine system


  Composed of endocrine glands and specialized endocrine cells located throughout the body
• Endocrine glands and cells

  Secrete minute amounts of chemical messengers called hormones into the bloodstream, rather than into a duct
• Hormones


  Travel through the general blood circulation to target tissues or effectors
• Target tissues


  Have receptors for a specific hormone
• Hormones


  Produce a particular response in the target tissues
• Classes of chemical messengers

  • Autocrine
  • Paracrine
  • Neurotransmitter and neuromodulators
  • Hormones and neurohormones
• Autocrine


  Released by cells and have local effect on same cell type
• Autocrine example


  • Eicosanoids
• Paracrine


  Released by cells that affect other cell types in close proximity
• Paracrine example


  • Somatostatin
• Neurotransmitter and neuromodulators


  Secreted by nerve cells
• Neurotransmitter and neuromodulators example


  • Nervous system function
• Hormones and neurohormones


  Secreted into blood and bind to receptors on target tissues
• Hormones and neurohormones examples

  • Epinephrine and insulin
• Endocrine system functions


  • Regulation of metabolism
  • Control of food intake and digestion
  • Modulation of tissue development
  • Regulation of ion levels
  • Control of water balance
  • Regulation of cardiovascular function
  • Control of blood glucose and other nutrients
  • Control of reproductive functions
  • Stimulation of uterine contraction and milk release
  • Modulation of immune system function
• Types of hormones


  • Water-soluble hormones (proteins, peptides, amino acids)
  • Lipid-soluble hormones (steroids and eicosanoids)
• Water-soluble hormone examples


  • Growth hormone, antidiuretic, prolactin
• Lipid-soluble hormone examples


  • Luteinizing hormone, androgens
• Humoral stimuli


  Blood-borne chemicals that can directly stimulate the release of some hormones
• Neural control


  Hormone release can be under neural control, where neurons release a neurotransmitter into the synapse with the cells that produce the hormone
• Hormonal control


  Hormone release can be controlled by other hormones, where a hormone is secreted that stimulates the secretion of other hormones
• The same three types of stimuli (humoral, neural, and hormonal) can stimulate or inhibit hormone release
• Humoral inhibition of hormone release

  Generally involves the actions of companion hormones, where each performs an opposite function to maintain homeostasis
• Neural inhibition


  Neurons inhibit targets just as often as they stimulate targets, where an inhibitory neurotransmitter prevents the target endocrine gland from secreting its hormone
• Hormonal inhibition


  Some hormones are inhibitory hormones that reduce the release of the hormone being controlled
• Hormonal inhibition example


  • Thyroid hormones can control their own blood levels by inhibiting their anterior pituitary tropic hormone
• Negative feedback


  Most hormones are regulated by a mechanism where the hormone's secretion is inhibited by the hormone itself once blood levels have reached a certain point
• Positive feedback


  Some hormones are regulated by a mechanism where the hormone's secretion is stimulated, as exemplified by tropic hormone action
• Hormone receptors


  A hormone can stimulate only the cells that have the receptor for that hormone
• Receptor site


  The portion of each receptor molecule where a hormone binds
• Receptor specificity


  The receptor site has specificity, allowing only one hormone to bind to it
• Lipid-soluble hormone receptors


  Bind to nuclear receptors due to their lipid solubility and small molecular size allowing to easily pass through the cell membrane
• Water-soluble hormone receptors


  Bind to membrane-bound receptors as they are polar molecules and cannot pass through the cell membrane
• Nuclear receptor action


  Lipid-soluble hormones stimulate protein synthesis by binding to their receptors, with the complex now binding to hormone-response elements on DNA to regulate the transcription of specific mRNA and protein synthesis
• Membrane-bound receptor action


  Activate G proteins or intracellular enzymes, which elicit specific responses in cells, including the production of second messenger molecules
• 1. protein activation


  G proteins, after several sequential actions, interact with adenylate cyclase to convert ATP to cAMP, which binds to protein kinases and activates them
• Signal amplification


  Hormones that stimulate the synthesis of second messengers act quickly and have an amplification effect, where a single hormone activates many second messengers, each of which activates enzymes that produce an enormous amount of final product
• Pituitary gland


  Small gland in the brain, controlled by the hypothalamus, divided into anterior and posterior regions, and secretes at least 6 hormones
• Hypothalamic control of the anterior pituitary

  Neurons of the hypothalamus secrete releasing hormones that stimulate the production and secretion of a specific hormone by the anterior pituitary
• Hormones that stimulate the synthesis of second messengers


  • Act quickly and have an amplification effect