Biomedical Core

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Author:
faulkner116
ID:
192001
Filename:
Biomedical Core
Updated:
2013-01-14 16:33:29
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Module11
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Description:
Objective 17 -19
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  1. The action potential is used by
    neurons that need to send information over long distances.

    • Example:
    • The neuron that carries touch information from the great toe (hallux) to the medulla can reach lengths of 2 meters in humans, or 6 meters in a giraffe.
  2. The action potential results from the
    opening and closing of voltage-gated channels
  3. The axonal membrane has voltage-gated sodium channels with a special property:
    when the voltage becomes less negative, the channels open.

    Because the driving force (concentration + electrical forces) on sodium is pushing it into the cell, sodium ions rush in and make the cell membrane less negative inside. This opens up more voltage-gated sodium channels, and the neuron becomes less negative, and so on.
  4. Action Potential:

    1. Resting State
    All voltage-gated Na+ and K+ channels are closed. The axon plasma membrane potential: small buildup of negative charges along inside surface of membrane and an equal buildup of positive charges along outside surface of membrane.
  5. Stimulus causes
    depolarization to threshold.
  6. Threshold is
    the point at which depolarization will trigger an action potential.
  7. The self-regenerating process has a "tipping-point":
    at some point, enough voltage-gated channels open that the voltage changes that result will open up all voltage-gated sodium channels.

    Tipping point = threshold
  8. Action Potential:

    2. Depolarizing Phase
    When membrane potential of axon reaches threshold, the Na+ channel activation gates open. As Na+ ions move through these channels into the neuron, a buildup of positive charges forms along inside surface of membrane and the membrane becomes depolarized.
  9. Action Potential: 

    3. Repolarizing phase begins:
    Na+ channel inactivation gates close and K+ channels open. The membrane starts to become repolarized as some K+ ions leave the neuron and a few negative charges begin to buildup along the inside surface of the membrane.
  10. Action Potential:

    4. Repolarizing Phase continues:
    K+ outflow continues. As more K+ ions leave the neuron, more negative charges build up along inside surface of the membrane. K+ outflow eventually restores resting membrane potential. Na+ channel inactivation gates open. Return to resting state when K+ gates close.
  11. Steps in the Action Potential:
    -Resting potential

    -Threshold

    • -Depolariztion
    • *Na+ channels open
    • PEAK
    • *Na+ channels close (inactivate)
    • *K+ channels open

    -Repolarization

    -After-hyperpolarization (also called hyperpolarization)

    Return to resting potential
  12. When firing an action potential is impossible, we call this the
    Absolute refractory period.
  13. When it is merely difficult to fire an action potential, because not all of the voltage-gated channels have reset, we call this the
    Relative refractory period: it is not impossible, but relatively difficult to fire a new spike.
  14. The axonal membrane is said to be refractory, meaning
    resistant to change. As more and more voltage-gated channels "reset" to their resting state, it becomes possible to fire a new action potential.
  15. Action Potential begins at the
    Trigger Zone
  16. Trigger Zone
    -Near the axon hillock

    -Voltage-gated Na+ and K+ channels begin here and continue along the axon

    -No voltage-gated channels in cell body or dendrites

    -Passive spread of current in both directions from trigger zone
  17. Propagation of the Action Potential:
    *Action potential begins at the trigger zone

    *Some differences between myelinated and unmyelinated axons
  18. In virtually all neurons, there are no voltage-gated channels in the
    dendrites and cell body. No action potentials can occur here.
  19. Electric Current Flow:
    *Think of a nerve axon as a sort of cable

    • *Ions flow in one direction or another to cause a flow of electric current.
    •    -remember I=V/R

    • *g is conductance
    • *current equals voltage times    conductance
    • *bigger "cable"= more conductance= more current
    • *bigger voltage=more electrical potential= more current
  20. Moving further down the axon, however, the ripples encounter a fresh patch of axonal membrane where the voltage-gated channels have not yet been activated. These "fresh" voltage-gated channels are activated, and
    a new action potential is triggered at this point.
  21. Propagation of the Action Potential: Continuous Conduction in Unmyelinated Axons.....

    The action potential is forced to move in only one direction. Back toward the trigger zone, the voltage-gated channels are still in their
    absolute refractory phase.
  22. The only "fresh" active membrane is found toward the _____ ________, so the action potential ______ that direction.
    axon terminal; moves
  23. Saltatory conduction in myelinated axons.

    Myelin sheath is interrupted at
    nodes of Ranvier
  24. Propagation of the Action Potential:
    Saltatory Conduction in Myelinated Axons...

    A mechanism operates when the axon is ensheathed in an insulating layer of ______. The insulation keeps ______ of ions from depleting the "wave" of voltage _____, and the neuronal axon is tuned so that the height of the depolarizing "wave" is just enough to reach ________ at the next ____ of _______.
    myelin; leakage; change; threshold; node; Ranvier
  25. Propagation of the Action Potential:
    Saltatory Conduction in Myelinated Axons....

    Voltage-gated sodium and potassium channels are only found at the nodes of _______. There's no point in having them under the blanket of myelin. They are all clustered at high density in the ___ ______.
    Ranvier; node region
  26. The action potential is only triggered at _____. It therefore appears to ____ from node to ____, a phenomenon called ________ _________.
    nodes; jump; node; saltatory conduction
  27. Distribution of Ions Across the Neuronal Membrane is Altered by the Action Potential:

    Give the approximate concentration for inside & outside for the given ion.

    Sodium (Na+)
    Inside -10

    Outside - 140
  28. Distribution of Ions Across the Neuronal Membrane is Altered by the Action Potential:

    Give the approximate concentration for inside & outside for the given ion.

    Potassium (K+)
    Inside - 140

    Outside - 4
  29. Distribution of Ions Across the Neuronal Membrane is Altered by the Action Potential:

    Give the approximate concentration for inside & outside for the given ion.

    Chloride (Cl-)
    Inside - 20

    Outside - 103
  30. Distribution of Ions Across the Neuronal Membrane is Altered by the Action Potential:

    Give the approximate concentration for inside & outside for the given ion.

    Calcium (Ca++)
    Inside - zero

    Outside - 5

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