Nerve Impulses

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Nerve Impulses
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Types of Ion Channels

• Chemically gated
• Open when a chemical binds
• Voltage gated
• Open or close when there is a change in
  electrical charges
• Mechanically gated
• Open when a receptor changes shape
• Nongated
• Always open ions leak through
• ALL follow concentration gradient!
• ALL are ion specific!

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Overview of a Nerve Impulse

• Resting potential neuron is not stimulated at
  threshold level
• Action potential neuron responds to stimulus,
  sends message along axon
• Depolarization
• Repolarization
• Hyperpolarization
• Refractory period neuron is less sensitive to
  stimuli
• Potential difference in charge (measured in
  volts)

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Resting Potential

• Inside cell high K, low Na
• Outside cell low K, high Na
• Na/K pump maintains concentration gradient
  (active transport uses ATP)
• Voltage gated Na K ion channels closed
• Nongated Na K ion channels open
• K diffuses out 75x faster than Na diffuses in
• Polarized cell membrane
• Inside cell membrane negative charge
• Outside cell membrane positive charge

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Why is Membrane Polarized at Rest?

• Na/K pump transports 3 Na out and 2 K in
• K diffuses out faster membrane is more
  permeable
• Na is attracted to cell because of its
  concentration gradient
• Membrane is impermeable to large negatively
  charged ions

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Resting Potential
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Action Potential Nerve Impulse

• Occurs in excitable membranes neurons and
  muscles
• Critical level must be reached (threshold)
  before impulse is sent
• Only travels along the axon
• Lasts a few milliseconds
• 3 steps
• Depolarization
• Repolarization
• Hyperpolarization

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Depolarization

• Stimulus causes voltage gated Na ion channels to
  open
• Na ions rush into cell
• Depolarized cell membrane
• Inside cell membrane positive charge
• Outside cell membrane negative charge
• If enough stimuli, threshold is reached and an
  action potential is generated stimulating
  adjacent Na ion channels to open along axon
• Positive feedback mechanism
• All-or-none response

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Depolarization
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Repolarization

• Voltage gated Na ion channels close
• Voltage gated K ion channels open
• K ions rush out of cell
• Restores polarization of cell membrane
• Inside cell membrane negative charge
• Outside cell membrane positive charge
• Ion distribution is different than at resting
  potential
• Inside cell low K, high Na
• Outside cell high K, low Na

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Repolarization
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Hyperpolarization

• Voltage gated K ion channels finally close
• Cell membrane even more polarized
• Na/K pump corrects ion distribution
• Inside cell high K, low Na
• Outside cell low K, high Na

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Refractory Period

• Brief period of time following an action
  potential when the threshold level is increased
• Takes more stimuli to generate an action
  potential less sensitive

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Animations

• http//www.blackwellpublishing.com/matthews/channe
  l.html
• http//outreach.mcb.harvard.edu/animations/actionp
  otential.swf