Impulse Conduction in Neurons

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Impulse Conductionin Neurons
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Cell Membrane Potential

• In a resting neuron,
• a greater number of
• positive ions are located
  outside the cell,
  causing the
  membrane to be electrically
  charged
  (polarized).
• Polarization is maintained by the cells ability
  to control whether or not some ions are allowed
  to cross the cells membrane through protein
  channels.

• 70 mV
• resting
• membrane
• potential

axon
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• Events of Impulse Conduction
• A stimulus opens Na channels in the cell
  membrane allowing Na to flow into the cell.
  This causes the resting potential to rise
  (depolarization).
• If a stimulus causes enough of a change in the
  membrane potential, an action potential is
  achieved.
• The action potential triggers the opening of Na
  channels in adjacent areas of the axon, causing
  the nerve impulse to continue down the axon in a
  domino-like effect.
• K channels open, allowing K to flow out of the
  cell. The potential drops, returning to its
  starting point (repolarization).
• A single depolarization-repolarization cycle
  takes approximately .001 seconds.
• Link Neuron Action Potential

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The All-or-None Response

• The action potential acts according to an
  all-or-none principle It has to reach a
  threshold voltage before it will fire, and then
  it fires completely.

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• Unmyelinated neurons conduct impulses over the
  entire membrane surface.
• Myelinated neurons conduct impulses only at the
  nodes of Ranvier, allowing the impulse to move
  many times faster as the impulse jumps from one
  node to another (up to 150 m/s).
• Link Propagation of the Action Potential

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Synaptic Transmission

• When an impulse reaches the end of an axon,
  calcium ions diffuse into the axon terminal,
  causing synaptic vesicles to dock and fuse with
  the neurons membrane.
• Neurotransmitters are released into the synaptic
  cleft.
• Neurotransmitters attach to receptors on the
  postsynaptic membrane, causing ion channels
  within that membrane to open.
• Ions flow into the postsynaptic cell, eliciting a
  response.
• Neurotransmitters in the synaptic cleft are
  either broken down by enzymes or reabsorbed by
  the presynaptic neuron (reuptake).
• Link Synaptic Transmission

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• Synapse Cross-Section