topic 4

• cell signaling- communication between cells
• signaling cell sends a signal and the target cell receives the signal and responds to it
• direct and indirect cell signaling types
• direct cell signaling- signal passed directly from one cell to another. signaling cell and target cell are connected by gap junctions
• indirect cell signaling- signaling cell releases chemical messenger, chemical messenger binds to a receptor on the target cell, activation of signal transduction pathway
• indirect cell signaling over short distance- paracrine (diffuses to a nearby cell) and autocrine (diffuses back to signaling cell, signaling cell and target cells are the same)
• indirect cell signaling over long distance- endocrine system (chemical messenger could be a hormone transported by circulatory system) and nervous system (electrical signals traveling along a neuron and neurotransmitter is released)
• gap junctions- specialized protein complexes create an aqueous pore between adjacent cells. Ions move through here, membrane potential changes, can open and close, chemical messengers travel through gap junction (cAMP)
• Cell signaling involves communication between cells
• Signaling cell sends a signal (usually a chemical)
• Target cell receives the signal and responds to it
• Types of Cell Signaling:

  • Direct:
  • Signaling cell and target cell connected by gap junctions
  • Signal passed directly from one cell to another
  • Indirect:
  • Signaling cell releases chemical messenger
  • Chemical messenger carried in extracellular fluid
  • Chemical messenger binds to a receptor on target cell
  • Activation of signal transduction pathway
  • Response in target cell
• Indirect Signaling Over Short Distance:

  • Paracrine:
  • Chemical messenger diffuses to nearby cell
  • Autocrine:
  • Chemical message diffuses back to signaling cell
• Indirect Signaling Over Long Distance:

  • Endocrine System:
  • Chemical messenger (hormone) transported by circulatory system
  • Nervous System:
  • Electrical signal travels along a neuron and chemical messenger (neurotransmitter) is released
• Direct Signaling:

  • Gap junctions:
  • Specialized protein complexes create an aqueous pore between adjacent cells
  • Movement of ions between cells
  • Changes in membrane potential
  • Chemical messengers can travel through the gap junction
  • Example: cAMP
  • Opening and closing of gap junction can be regulated
• Indirect Signaling:

  • Three steps:
  • Release of chemical messenger from signaling cell (gland)
  • Transport of messenger through extracellular environment to target cell
  • Communication of signal to target cell
  • Systems for indirect signaling have similarities and differences
• Chemical Messengers:

  • Six classes of chemical messengers:
  • Peptides
  • Steroids
  • Amines
  • Gases
  • Structure of chemical messenger affects signaling mechanism
• Peptide/Protein Hormones:

  • 2 - 200 amino acids long
  • Synthesized on the rough ER
  • Stored in vesicles
  • Hydrophilic
  • Soluble in aqueous solutions
  • Bind to transmembrane receptors
  • Rapid effects on target cell
• Steroid Hormones:

  • Derived from cholesterol
  • Synthesized by smooth ER or mitochondria
  • Three classes: Mineralocorticoids, Glucocorticoides, Reproductive hormones
  • Hydrophobic
  • Can diffuse through plasma membrane
  • Transported to target cell by carrier proteins
  • Slow effects on target cell (gene transcription)
• Amine Hormones:

  • Chemicals that possess amine group
  • Most hydrophilic
  • Diverse effects
• Other Chemical Messengers:

  • Gases:
  • Most act as paracrines
  • Example: nitric oxide (NO), carbon monoxide
• Ligand - Receptor Interactions:

  • Specific interactions
  • Ligand mimics:
  • Agonists - activate receptors
  • Antagonists - block receptors
• Types of Receptors:

  • Intracellular:
  • Bind to hydrophobic ligands
  • Transmembrane receptors:
  • Located in cell plasma membrane
  • Interact with hydrophilic messengers
• Intracellular Receptors:

  • Ligand diffuses across cell membrane
  • Binds to receptor in cytoplasm or nucleus
  • Regulates the transcription of target genes
• Ligand-Gated Ion Channels:

  • Ligand binds to transmembrane receptor
  • Receptor changes shape opening a channel
  • Ions diffuse across membrane
  • Movement of ions changes membrane potential
• Receptor Enzymes:

  • Ligand binds to transmembrane receptor
  • Catalytic domain of receptor starts a phosphorylation cascade
  • Phosphorylation of specific intracellular proteins
• 1. Protein-Coupled Receptors:

  • Ligand binds to transmembrane receptor
  • Receptor interacts with intracellular G-proteins
  • Subunits of G-protein dissociate
  • Some subunits activate ion channels
  • Some subunits activate amplifier enzymes
• Endocrine Regulation:

  • Hypothalamus integrates nervous and endocrine regulation
  • Regulates activity of pituitary gland
  • Produces neurohormones released by posterior lobe of pituitary
• Pituitary Hormones:

  • Pituitary gland secretes many hormones
  • Two distinct anatomic sections: Anterior pituitary (adenohypophysis) and Posterior pituitary (neurohypophysis)
• Homeostatic Imbalances of the Pituitary:

  • Growth Hormone Hypersecretion:
  • In youths
  • In adults
  • Growth Hormone Hyposecretion
• Posterior Pituitary:

  • Extension of the hypothalamus
  • Neurons that originate in hypothalamus terminate in posterior pituitary
  • oxytocin and vasopressin (ADH) synthesized in hypothalamus and travel in vesicles down axons to posterior pituitary
• Constitutive Activation of Aquaporin Genes in the Collecting Duct by ADH:

  • Antidiuretic Hormone
  • Receptor
  • ADH G-Protein
  • Adenylyl Cyclase
  • Aquaporin Channels in Vesicular membranes
  • ATP cAMP
  • PKA