ch 11

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ch 11
2011-04-13 11:22:15
nervous system

nervous system notes
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  1. myelin sheath
    • only on axons
    • make up white matter
    • dev during late fetal period thru 1st yr
  2. PNS myelin sheath cells
    schwann cells
  3. CNS myelin sheath cells
  4. multipolar neurons
    several dendrites - one axon

    most neurons in CNS
  5. bipolar neuron
    1 dendrite - one axon
  6. unipolar neuron
    1 process (axon)

    sensory root of spinal nerves
  7. functional classifications of neurons
    • sensory
    • motoir
    • association
  8. neurons are excitable
    inherent ability to generate and conduct electrical impulses
  9. membrane potential
    exists when a barrier prevents movement of opposite charges toward each other
  10. polarized
    all cells have slightly different charges
  11. voltage
    a form of potential energy measured in volts (or millivolts-mV)

    a difference in charge between two points
  12. current
    movement of charged particles
  13. resistance
    possessed by the membrane as it restricts the movement of charges
  14. ions
    • cant diffuse across cell membranes
    • must pass through channels
    • channels are located at particular sites on membrane
    • ion channels are selective
  15. Passive channels
    • leakage, non-gated
    • always open
  16. Na channel
    allows Na in
  17. K channel
    allows K out
  18. resiting neuron
    high concentration of K inside and high concentration of Na outside cell
  19. Gated (active) channels
    • *may be open (activated)
    • * closed but able to open
    • * closed but unable to open (inactivated)
  20. chemically gated
    open when the appropriate chemical (neurotransmitter) binds
  21. voltage gated
    open in response and close in response to changes in membrane potential
  22. maintenance of resting potential
    • around -70 mV
    • varies from -40mV to 90mV
  23. maintenance of RP
    • passage of ions through leakage channels
    • * permeability of membrane for K leaking out 75X greater than Na leaking in
    • * activity of Na-K pump
  24. Na-K pump
    • membrane proteins that actively transport ions
    • (3) Na exported
    • (2) K imported
    • stabilizes the resting potential
  25. neurons use changes in their membrane potential as communication signals
  26. change
    produced by changes in membrane permiability to any ion
  27. signals
    graded potentials (incoming over short distances)
  28. action potentials
    • long distance signals of axons
    • same as nerve impulses
  29. depolarization
    reduction in membrane potential (becomes less negative)
  30. hyperpolarization
    membrane potential increases (becomes more negative)
  31. graded potentials
    • short lived, localized changes in membrane potential;
    • triggered by stimuli in neurons' environment that cause gated channels to open
  32. current in graded potentials
    • dissipates quickly
    • die out w/ increasing distance from site of origin
  33. action potential
    • nerve impulse
    • incolves 3 consecutive but overlapping changes in membrane potential
  34. 1) resting (non-conducting) state:
    voltage gated channels closed;only leakage channels open
  35. Each Na channel has 2 gates
    activation gate: closed at rest, open w/ depolarization

    inactivation gate: blocks channel once opened
  36. 2) depolarizing phase
    *huge increase in permiability to Na (about 1000x more permeable to resting)

    *axon membrane depolarized, Na rushes in, more depolarization

    *becomes self-generating when it reaches a critical level threshold (-55mV to -50mV)
  37. positive feedback
    more Na enters, greater depolarization, opens more Na channels

    overshoots to +30mV
  38. 3) repolarizing phase
    • decreased Na permeability (inactivation gates closing)
    • increased K permeability (k channels open; it leaves cell)

    *decrease of Na going in; Increase of K going out= repolarization= internal negativity restored
  39. 4) hyperpolarization
    K permeability continues fro a bit (more than needed to restore resting state; excess K efflux = afterhyperpolarization)

    • * Na channels are resting
    • * electrical condition back to resting; but not ion distribution
    • * ion distribution restores quickly by Na-K pumps after repolarization
  40. propagation
    transmission of action potential; begins at one end and is conducted to axon terminal (domino effect)
  41. threshold
    usually depolarization of 15 to 20 mV from resting required to generate an ap
  42. absolute refractory period
    neuron cannot respond to another stimulus (from opening of Na channels to their resetting)
  43. relative refractory period
    follows absolute; exceptionaly strong stimulus could reopen Na channels

    (most Na channels reset; some K channels still open; repolarization occuring)
  44. conduction velocity
    • axon diameter
    • degree of myelinzation
  45. continuous conduction
    depolarization the entire length of the axon
  46. myelinated
    30x faster
  47. saltatory conduction
    concentration of Na channels @ nodes
  48. group A
    • large diameter heavily myelinated
    • 150 m/sec
  49. group b
    • lightly myelinated intermmediate diameter
    • 15 m/sec
  50. group c
    • smallest diameter
    • 1 m/sec
  51. synapses-
    functions between neurons sites of information transfer
  52. presynaptic neuron
    conducts impulses towards synapse sender
  53. post synaptic neuron
    • conducts impulses away from synapse
    • reciever
  54. electrical synapse
    • uncommon
    • connected by protein channels to adjacent neurons
    • ions flow directly from one to another
    • rapid synchronized communication
  55. chemical synapse
    release and reception of neurotransmitters; 2 parts
  56. chemical synapse parts
    axon terminal- lots of synaptic vessicles that contain neurotransmitters
  57. receptor region
    receptors that bind to neurotransmitters
  58. information transfer at chemical synapse
    • 1. calcium channels open at presynaptic axon terminal
    • 2. neurotransmitter released
    • 3. neurotransmitter binds to synaptic receptors
    • 4. ion channels open in post synaptic membrane
    • 5. neurotransmitter effects are terminated
  59. 3 ways NT effects are terminated
    • degrade NT enzymatically in cleft
    • reuptake NT and store or degrade
    • diffusion away from synapse
  60. post synaptic potential and synaptic integration
    • neurotransmitters bind to receptors and open ion channels
    • * mediate graded potentials
    • * either excitatory or inhibitory
  61. timing
    fast timing of bursts of NT release by one or more presynaptic neurons
  62. spatial
    large number of neuron terminals from one neurons simultaneously stimulate a postsynaptic neuron
  63. ACH
    best known, action varies with type of receptor
  64. biogenic amines
    norepinephrine, dopamine, seratonin, histamine
  65. purines
    ATP, adenosine
  66. amino acids
    GABA, glutamate, glycine
  67. peptides
    endorphines, substance P, somatostatin
  68. dissolved gasses
    nitric oxide, carbon monoxide
  69. botulinus toxin
    inhibits ACH release, paralyzing voluntary muscle
  70. curare
    prevents ACh binding to receptors, paralyzes muscle, arrow poision
  71. nerve gas and malathion
    prevent ach breakdown by ache, prolonged muscle spasm
  72. barbituates
    decreased rate of ACH release, acts as CNS depressant