Action Potential

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Action Potential

• How neurons send an electrical message

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How Neurons Communicate

• Action Potential is the electrical process that
  neurons use to communicate with each other
• Action Potentials are based on movements of ions
  (charged particles) between the outside and
  inside of the axon
• Action Potential is an All or Nothing Process
  (like a gun firing)

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Threshold Triggering Action Potential

• Threshold is something in the environment (light,
  pressure, heat, chemicals or another neuron) that
  tell the neuron to fire and send an electrical
  signal (action potential)
• How do you feel something that is intense?
• More neurons fire, the intensity of their
  electric impulse always stays the same.
• Thresholds can tell an neuron to fire (excitory)
  or to stop firing action potential (inhibitory)
• Lou Gehrigs Disease - too many inhibitory
  stimuli cause the muscles to freeze up.
• Parkinsons Disease - too many excitatory stimuli
  cause the muscles to move without control.

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Steps to Action Potential
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Step 1 Threshold is Reached

• Axon at Resting Potential - fluid inside the axon
  is mostly negatively charged with positive on the
  outside (polarized)
• An impulse is triggered in the neurons dendrite
  when stimulated by pressure, heat, light or a
  chemical messenger from another neuron (stimulus
  threshold).

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Resting Potential

• At rest, the inside of the cell is at -70
  microvolts
• With inputs from dendrites inside becomes more
  positive
• If resting potential rises above threshold, an
  action potential starts to travel from cell body
  down the axon
• Figure shows resting axon being approached by an
  AP

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Step 2 Action Potential Begins

• When neuron fires, its axon membrane is
  selectively permeable (has gates that open).
• Gates in the axon called ion channels open
  allowing positive sodium ions to enter the axon
  giving it a brief positive electrical charge the
  axon (depolarized).
• The brief positive charge is action potential.

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Depolarization Ahead of AP

• AP opens cell membrane to allow sodium (Na) in
• Inside of cell rapidly becomes more positive than
  outside
• This depolarization travels down the axon as
  leading edge of the AP

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Step 3 Refractory Period

• As the next gates open allowing positive sodium
  ions in, the previous gates close and begin to
  pump the positively charged sodium ions out of
  the axon. (repolarized).
• This step is called the refractory period and the
  axon cannot fire again until it returns to
  resting potential (negative polarized state
  inside the axon).
• The entire process is like falling dominoes all
  the way down the axon except these dominoes can
  set themselves back up as soon as the fall over.
• Q Why do you think the axon has to set itself
  back to a resting state so quickly (3
  milliseconds)?
• A So the neuron can fire again and send another
  message immediately after the last one.

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Repolarization follows

• After depolarization potassium (K) moves out
  restoring the inside to a negative voltage
• This is called repolarization
• The rapid depolarization and repolarization
  produce a pattern called a spike discharge

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Action Potential Within a Neuron
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DAILY

• DOUBLE

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How can a toilet represent Action Potential?

• Full Toilet Resting Potential
• Push Flush Lever Threshold Stimulus triggering
  Action Potential.
• Toilet Refilling/Cant Flush Repolarization/Refr
  actory Period
• Sewer Pipes One-way communication like action
  potential only goes from dendrite end to axon
  terminal end.