Chapter 12 NEURONS

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Background from Introductory Biology Courses
  1. Hole's Human Anatomy and Physiology
    1. Nervous system
      1. Action Potential
      2. Nerve impulse
      3. Voltage gated channels
  2. On-Line Biology Book by M. J. Farabee 
    1. Nervous system
  3. Kimball's Biology Pages
    1. Neurons from Kimball's Biology Pages
    2. Excitable cells
  4. Action Potential videos and Quizes from McGraw-Hill
  5. Physioviva Educational Videos
  6. How Neurons Communicate Open Stax College

  1. Hill, Wyse, and Anderson
    1. Chapter Outline
    2. Chapter Summary
    3. Flashcards

  1. Neurons
    1. Specialized cells of the nervous system
    2. Nerves are bundles of neuron axons
    3. Neurons with their support cells (glial cells) make up nervous systems
  2. Control and Integration
    1. Nervous system
    2. Endocrine system
  3. Figure 12.1 Neuronal and hormonal signaling both convey information over long distances
  4. Vertebrate Nervous Systems
    1. Receptors
    2. Sensory neurons of the peripheral nervous system
    3. Central nervous system (brain and spinal cord)
    4. Motor neurons of the PNS
    5. Effector cells (muscle or gland cells)
  5. Simple Nerve Circuit: Reflex
    1. Sensory neuron:
    2. Interneurons:
    3. Motor neurons:
  6. Figure 12.2 Neurons have four functional regions that typically correspond to their four major structural regions  
    1. Dendrites
    2. Neuronal Cell body (Soma)
      1. axon hillock - site where action potentials originate
    3. Axon
    4. Presynaptic axon terminals

    Figure adapted from Wikipedia

  7. Vertebrate Neurons
  8. Glial cells (Fig 12.5)
    1. Supporting or Accessory Cells in Nervous System
    2. Schwann cells form myelin sheaths
    3. Roles of Support Cells
  9. HOW NERVE CELLS FUNCTION
    1. Neurons are Excitable Cells cells that can change membrane potentials (neurons, also muscle)
    2. Resting potential the unexcited state of excitable cells voltage differences across the plasma membrane
    3. Membrane potentials were first demonstrated using the giant axons of a squid (1mm diameter).
    4. Figure 12.7 Recording the resting membrane potential of a squid giant axon
  10. Membrane Resting Potential
  11. Figure 12.9c Graded potentials decrease exponentially with distance
    1. Occur in dendrites / cell body
    2. Small, localized changes in membrane potential
    3. travel only a short distance (few mm)
    4. decrease exponentially in strength as they spread out from the point of origin
    5. Hyperpolarization
    6. Depolarization
  12. Selective permeability of a membrane gives rise to a membrane potential (Fig 12.10)
    1. A hypothetical cell with a membrane that is permeable to potassium ions and impermeable to certain anions (A-).
    2. The resulting charge separation produces a net charge at the membrane surface.
  13. Fig 12.11
    1. The membrane potential
    2. results from relatively few charges sitting on the membrane in a small patch
  14. Figure 12.12a Concentration of major ions in intracellular and extracellular fluids
    1. All cells have low Na and Cl, high K and non-permeating anions (A)
  15. The Nernst Equation
    1. Example of Gibbs-Donnan equilibrium (Nernst Equation and Calculations of Membrane Potentials )
    2. If  z =1, [K] out = 8 mM, [K] in = 140 mM
    3. E = 61 *( -1.24)
    4. then E = -76 mV
    Nernst equation
    Goldman Equation
    1. actual potential should be -60 to -70 mV
    2. Goldman equation takes into account smaller permeabilities of Na+ and Cl-, as well as K+
    3. This equation predicts potential exactly.
     
    EXAMPLE of Goldman as applied to two ions only: Na+ and K+ (see page 308 of text, below Fig. 12.13)
    1. [K] out = 20 mM 
    2. [Na] out = 440 mM
    3. [K] in = 400 mM 
    4. [Na] in = 44 mM
    5. if Potassium is 10 times more permeable than Sodium
    6. then PK=10 and  PNa=1
    7.  V = 58 log ((10 * 20 + 1*440) / 10*400 + 1*44))
    8. V = 58 log (640/4044)
    9. V = -46 mV

     

    Nernst/Goldman Equation Simulator

     

     
  16. Membrane Proteins Involved in Electrical Signals
    1. Non-gated ion channels (leak channels)
    2. Gated Ion channels
    3. Ion pumps
  17. Non-gated ion channels (leak channels)
    1. always open
    2. specific for a particular ion
    3. K+ leak channels on neurons
  18. Gated ion channels
    1. Voltage-gated Channels of Axons
  19. Fig 12.20 The molecular structure of voltage-gated Na+ channels
  20. Ion pumps help maintain the concentration of major ions in intracellular and extracellular fluids
    1. Counteracts the tendency of Na to diffuse in and K to diffuse out.
    2. Sodium-Potassium Exchange Pump from McGraw Hill
    3. Figure 12.12c Ion pumps help maintain the concentration of major ions in intracellular and extracellular fluids
  21. The sodium and potassium gradients for a resting membrane
  22. Summing up the resting potential
  23. Figure 12.14 General features of action potentials  
    1. Action Potential (AP)
      1. The nerve impulse video from McGraw Hill
      2. Sodium channels from Physioviva
    2. large positive change (depolarization) in Membrane potential
    3. Three phases of AP
      1. Depolarization:
      2. repolarization,
      3. hyperpolarization
    4. Figure 12.15 Membrane permeability changes that produce an action potential
    5. Fig. 12.15a Resting Phase
      1. Both voltage-gated ion channels (Na+; K+)are closed
      2. Na-K pumps are maintaining resting potential
    6. Action Potentials
      1. Triggered by the net graded potential at the axon hillock
      2. Depolarization must reach threshold potential to fire
      3. All or nothing response, not graded
    7. Threshold potential
    8. Fig 12.15b Action Potential:Rising phase
      1. DEPOLARIZATION
      2. Voltage-gated Na+ channels open rapidly
      3. K+ voltage-gated channels stay closed
    9. Fig 12.15c Action potential: Falling Phase
      1. Repolarization
      2. gated K channels open rapidly
      3. K + exits and cell gets less positive
      4. Na+ channels closed and inactivated
    10. Figure 12.15d: Undershoot Phase
      1. K+ gated channels still open allowing K+ to exit
      2. Na-K pumps decrease Na+
      3. Cell hyperpolarizes
    11. Figure 12.15d Recovery Phase
      1. Both gated channels closed
      2. Na-K pumps restoring resting potential
    12. Refractory periods
      1. Absolute Refractory Period
        1. Na+ channels closed and inactivated
      2. Relative Refractory Period

    Image from Wikipedia

  24. NERVE IMPULSES
  25. Nerve Impulses
  26. Propagation of the Action Potential
  27.  video from McGraw Hill
    1. Na+ channels cause the electrical signal to pass down an axon, by causing depolarization that opens more channels father down
    2. Figure 12.25 Inactivation of voltage-gated Na+ channels prevents reverse propagation of an action potential
    3. Na+ moving into one segment of the neuron quickly moves laterally inside the cell
    4. Depolarizes adjacent segment to threshold
  28. Conduction of action potentials
  29. Figure 12.25 Conduction velocity
    1. speed at which the action potential travels down the length of an axon dictates speed of response
    2. The velocity of nerve-impulse conduction increases with increasing axon diameter in both myelinated and unmyelinated axons
    3. AP Velocity is directly related to axon diameter
  30. Conduction Velocity in Invertebrates
  31. Vertebrate Myelinated Fibers
  32. Figure 12.27 Myelinated axons speed the propagation of an action potential
  33. Nodes of Ranvier
    1. uncovered areas at regular intervals of the axon
    2. contain lots of Na+ channels
  34. Saltatory conduction
  35. Speed of Conduction
  36. How does myelination work ?
  37. Saltatory conduction in a myelinated axon

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