endocrine

• Endocrine system


  • Acts with nervous system to coordinate and integrate activity of body cells
  • Influences metabolic activities via hormones transported in blood
  • Responses slower but longer lasting than nervous system responses
• Endocrinology


  Study of hormones and endocrine organs
• Comparison of Nervous and Endocrine Systems

  • Nervous system
  • Endocrine system
• Endocrine system


  • Controls and integrates: Reproduction
  • Growth and development
  • Maintenance of electrolyte, water, and nutrient balance of blood
  • Regulation of cellular metabolism and energy balance
  • Mobilization of body defenses
• Exocrine glands


  • Produce nonhormonal substances (examples: sweat, saliva)
  • Have ducts to carry secretion to membrane surface
• Endocrine glands


  • Produce hormones
  • Lack ducts
• Endocrine glands


  • Pituitary
  • Thyroid
  • Parathyroid
  • Adrenal
  • Pineal
• Hypothalamus


  Neuroendocrine organ
• Other tissues and organs that produce hormones

  • Pancreas
  • Gonads
  • Placenta
  • Adipose cells
  • Thymus
  • Cells in walls of small intestine, stomach, kidneys, and heart
• Chemical messengers of endocrine system: Hormones, Autocrines, Paracrines
• Hormones


  Long-distance chemical signals; travel in blood or lymph
• Autocrines


  Chemicals that exert effects on same cells that secrete them
• Paracrines


  Locally acting chemicals that affect cells other than those that secrete them
• Two main classes of hormones

  • Amino acid-based hormones
  • Steroids
• Amino acid-based hormones


  Amino acid derivatives, peptides, and proteins
• Steroids


  • Synthesized from cholesterol
  • Gonadal and adrenocortical hormones
• Eicosanoids are considered a hormone by some scientists, but most classify it as a paracrine
• Target cells


  Tissues with receptors for a specific hormone
• Hormone action on target cells

  • Alter plasma membrane permeability and/or membrane potential by opening or closing ion channels
  • Stimulate synthesis of enzymes or other proteins
  • Activate or deactivate enzymes
  • Induce secretory activity
  • Stimulate mitosis
• Water-soluble hormones


  • Act on plasma membrane receptors
  • Act via G protein second messengers
  • Cannot enter cell
• Lipid-soluble hormones


  • Act on intracellular receptors that directly activate genes
  • Can enter cell
• Second-messenger systems


  • Cyclic AMP
  • PIP2-calcium
• Cyclic AMP signaling mechanism

  1. Hormone (1st messenger) binds to receptor

  2. Receptor activates a G protein

  3. G protein activates or inhibits effector enzyme adenylate cyclase

  4. Adenylate cyclase then converts ATP to cAMP (2nd messenger)

  5. cAMP activates protein kinases that phosphorylate (add a phosphate) other proteins
• • Phosphorylated proteins are then either activated or inactivated
  • cAMP is rapidly degraded by enzyme phosphodiesterase, stopping cascade
  • Cascades have huge amplification effect
• PIP2-calcium signaling mechanism


  1. Hormone-activated G protein activates phospholipase C

  2. Activated phospholipase C splits membrane protein PIP2 into two second messengers: Diacylglycerol (DAG) activates protein kinases, Inositol trisphosphate (IP3) causes Ca2+ release from intracellular storage sites

  3. Calcium ions act as another second messenger, altering enzyme activity and channels, or binding to regulatory protein calmodulin

  4. Calcium-bound calmodulin activates enzymes that amplify cellular response
• Other signaling mechanisms


  • cGMP (cyclic guanosine monophosphate) is second messenger for selected hormones
  • Insulin receptor is a tyrosine kinase enzyme that autophosphorylates upon insulin binding
  • Activated tyrosine kinases provide docking sites for relay proteins that trigger cell responses
• Direct gene activation mechanism of lipid-soluble hormones

  1. Steroid hormone diffuses through plasma membrane and binds intracellular receptor

  2. Receptor-hormone complex enters nucleus

  3. Receptor-hormone complex binds specific DNA region

  4. Binding initiates transcription of the gene to mRNA

  5. mRNA directs protein synthesis
• Blood levels of hormones

  • Controlled by negative feedback systems
  • Increased hormone effects on target organs can inhibit further hormone release
  • Levels vary only within narrow, desirable range
• Hormone release


  Triggered by: Endocrine gland stimuli, Nervous system modulation
• The steroid hormone diffuses through the plasma membrane and binds an intracellular receptor.
• The receptor-hormone complex enters the nucleus.
• The receptor-hormone complex binds a specific DNA region.
• Binding initiates transcription of the gene to mRNA.
• The mRNA directs protein synthesis.
• Blood levels of hormones

  Controlled by negative feedback systems
• Increased hormone effects on target organs

  Can inhibit further hormone release
• Levels vary only within narrow, desirable range
• Hormone release is triggered by

  • Endocrine gland stimuli
  • Nervous system modulation
• Endocrine gland stimuli


  • Humoral stimuli
  • Neural stimuli
  • Hormonal stimuli
• Humoral stimuli


  Changing blood levels of ions and nutrients directly stimulate secretion of hormones