The Action Potential

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The Action Potential
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• The Resting Potential of the Neuron
• results from difference in ion distribution
  inside and outside of cell (-70mV)

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• Forces Behind Resting Potential
• Selective Permeability- some molecules pass
  through membrane more freely than others ion
  channels
• Sodium-Potassium Pump- transports 3 Na
  out of, 2 K into cell
• Result
• Concentration Gradient
• Electrical Gradient

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• Forces Behind Resting Potential
• What would you do to generate electrical signal
  fast?
• RP takes work (Na/K Pump). Why do we bother?

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Molecular Basis of Action Potential

• Sodium channels open once threshold is reached,
  influx of sodium
• Potassium channels open at AP peak potassium
  flows out

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http//faculty.washington.edu/chudler/ap.html
http//www.blackwellscience.com/matthews/channel.
html
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• The All-or-None Law
• The size, amplitude, and velocity of an action
  potential are independent of the intensity of the
  stimulus that initiated it.
• How then is stimulus intensity coded?

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Figure 2.20  Saltatory conduction in a myelinated
axonAn action potential at the node triggers
flow of current to the next node, where the
membrane regenerates the action potential.
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The Action Potential

• The RefractoryPeriod
• Absolute Refractory Period
• Sodium gates are firmly closed
• Membrane cannot produce AP, regardless of the
  stimulation.
• Relative Refractory Period
• Sodium gates in usual state, but the potassium
  gates remain open.
• Stronger than normal stimulus needed for action
  potential.

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Propagation of the Action Potential

• From Axon Hillock
• To Terminal Buttons
• AP travels in one direction only. WHY?

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The Myelin Sheath and Saltatory Conduction

• Myelin Sheaths increase the speed of neural
  transmission
• Nodes of Ranvier-Short areas of the axon that
  are unmyelinated
• Saltatory Conduction-jumping action of actions
  potentials from node of Ranvier to node of Ranvier

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Signaling Without Action Potentials
Depolarizations and hyperpolarizations of
dendrites and cell bodies Small Local
neurons-produce graded potentials (membrane
potentials that vary in magnitude and do not
follow the all-or-none law)