ACTION POTENTIALS

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ACTION POTENTIALS

• 2008. 09.17
• Hong Yoo-chan

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Action Potentials

• Feature
• rapid, large alternations
• depolarization -70mV to 30 mV and repolarization
• propagate long distance
• Location
• nerve, muscle, endocrine, immune, reproductive
  cells
• called excitable membrane

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Voltage-Gated Ion Channels

• Repolarization
• both channels return to the closed state

• Depolarization
• sodium channels to rapidly open
• potassium channels to slowly open
• then undergo inactivation of sodium channels

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Action Potential Mechanism I

• 1. Resting state
• opening sodium channel
• depolarization start
• 2. Reaching Threshold potential
• trigger action potential
• rapid depolarization
• opening more fast voltage gated sodium channel
• trigger slow voltage gated potassium channel
• 3. Overshoot
• positive for cell inside
• negative for extracellular
• 4. Peak
• abrupt closuring of sodium channels
• opening of voltage gated potassium channel

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Action Potential Mechanism I (Cont.)

• 5. Rapid repolarization
• potassium channel gtgt sodium channel
• Hyperpolarization
• closing potassium channel
• 7. Resting state

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Action Potential Mechanism II

• All or none by positive feed back nature
• Action potentials either happen completely, or
  not at all
• Subthreshold stimuli produce subthreshold
  depolarizations and are not translated into
  action potentials
• Stronger threshold stimuli produce depolarizing
  currents that are translated into action
  potentials
• Threshold Voltage membrane is depolarized by
  about 15mV

cf) Local anesthetics blocks voltage gated Na
channel, preventing action potential.
Graded signal from peripheral cant be delivered.
Tetrodotoxin from ovary of puffer fish
blocks voltage gated Na, K channel.
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Refractive Periods

• Absolute refractory period
• Once an action potential has begun, a 2nd action
  potential cant be generated
• Relative refractory period
• Only for unusually strong stimuli
• Features
• Limits number of action potential up to 100 /s
• Contribute to separate each action potential
• Determine direction of action potential
  propagate

Threshold potential
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Action Potential Propagation

• Direction
• One-way propagation
• Refractive period prevent back propagation to the
  source
• Current flow but does not enough to initiate
  action potential

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Action Potential Propagation (Cont.)

• Velocity
• Velocity ? myelinated
• insulation ? ? leakage current?
• leakage ? ? graded potential speed ?
• of channel ? ? membrane pumping ?
• Cost effective transmission

• Velocity ? fiber diameter (D)
• D ? ? Resistance to local current ?
• Resistance ? ? ion flow speed ?
• Ion speed ? ? time to threshold ?

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Generation of Action Potentials

• External stimuli
• Receptor grade potential
• Synaptic grade potential

• Internal and spontaneous
• Pacemaker potential
• potential by activity of ion channels at the no
  stable, resting membrane
• neuron, muscle, cardiac muscle
• rhythmic behaviors breating, the hartbeat, and
  movement of stomach

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Summary

• The action potential ends as the sodium channels
  inactive and potassium channels open, restoring
  resting conditions.
• Depolarization of excitable membranes triggers an
  action potential only when the membrane potential
  exceeds a threshold potential.
• Regardless of the size of the stimulus, if the
  membrane reaches threshold, the action potential
  generated are all the same size.
• A membrane is refractory for a brief time
  following an action potential.
• Action potentials are propagated without any
  change in size from one site to another along a
  membrane.
• In myelinated nerve fibers, action potentials
  manifest saltatory conduction.
• Action potentials can be initiated by receptor at
  the ends of afferent neurons, at synapse, or in
  some cells, by pacemaker potentials.