The Neuron

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The Neuron Action Potential

• The basic building block of our nervous system
  and how it sends messages.

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Cell Body Nucleus
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The Cell Body
Contains the cells nucleus

• round, centrally located structure
• contains DNA
• controls protein manufacturing
• directs metabolism
• no role in neural signaling

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Dendrites
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Dendrites

• Information collectors or receivers
• Receive inputs or signals from neighboring
  neurons
• Inputs may number in thousands
• If enough inputs the cells AXON may generate an
  electrical output

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Dendritic Growth

• Mature neurons generally cant divide
• But new dendrites can grow
• Provides room for more connections to other
  neurons
• New connections are basis for learning
• Studies show people with higher education have
  more dendritic connections than someone that is a
  high school dropout.

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Neural Networks
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Axon
Axon
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Axon

• Where all the action is
• Action Potential takes place electrical charge
  is sent down the axon.
• One axon per cell, 2 distinct parts
• tube-like structure
• branches at end (axon terminals) that branch out
  to dendrites of other cells

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Myelin Sheath Nodes of Ranvier
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Myelin Sheath

• White fatty casing on axon
• Acts as an electrical insulator
• Not present on all cells
• When present, increases the speed of neural
  signals down the axon allowing the action
  potential to jump to each Node of Ranvier -
  like a paved highway (see video below to compare
  mylenated axons vs. non-mylenated axons
• If this degenerates (dirt road), you have
  multiple sclerosis and cant control your muscles.

If time view this in a video click on the web
address below (it will use QuickTime) Mylenated
Axon
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Axon Terminal or Buttons
Axon Terminals
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Axon Terminal or Buttons

• This is where the electrical impulse triggers
  synaptic transmission to the dendrites of a
  receiving neuron.

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Glial Cells

• They are the janitors of the neuron.
• Support cells that provide neurons with
  structural support and nutrition.
• They also remove cell wastes and enhance the
  speed of the neuron

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Action Potential

• How neurons send an electrical message

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How Neurons Communicate

• Neurons communicate by means of an electrical
  signal called the Action Potential
• Action Potentials are based on movements of ions
  between the outside and inside of the axon
• When an Action Potential occurs, a molecular
  message is sent to neighboring neurons
• Action Potential is an All or Nothing Process
  (like a gun firing)

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Threshold Triggering Action Potential

• When a neuron is resting there is a balance of
  excitatory and inhibitory signals.
• When one of these exceeds the other stimulus
  threshold is reached triggering the neuron to
  transmit an electrical impulse down its axon
  (action potential)
• How do you feel something that is intense?
• More neurons fire, the intensity of their
  electric impulse always stays the same.
• 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).
• This minimal level of stimulation that causes the
  axon to fire is called Stimulus Threshold

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Resting Potential

• At rest, the inside of the cell is at -70
  microvolts
• With inputs to 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.
• Gates in the axon called ion channels open
  allowing positive sodium ions to enter the axon
  while potassium ions leave 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 and potassium ions back inside.
  (repolarized).
• This step is called the refractory period and the
  axon cannot fire again until it returns to
  resting potential (negative polarized state).
• The entire process is like falling dominoes all
  the way down the axon except these dominoes can
  set themselves back up as soon as they fall over.
• Why do you think the axon has to set itself back
  to a resting state so quickly (3 milliseconds)?
• 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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Finally, Hyperpolarization

• Repolarization leads to a voltage below the
  resting potential, called hyperpolarization
• Now neuron cannot produce a new action potential
• This is the refractory period

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Action Potential Within a Neuron
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Animated Action Potential

• To review this entire process click on the link
  below for a short video that helps explain this
  complex process http//www2.neuroscience.umn.edu
  /brainscience/cool_stuff.htm

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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.