Modeling Synaptic Transmission

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Modeling Synaptic Transmission

• Baktash Babadi
• Fall 2004,
• IPM, SCS, Tehran, Iran
• baktash_at_ipm.ir

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The Synapses
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What Happens in a Synapse?
Ca
Ca
Ca
Ca
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Stages of Modeling

• Dynamics of Calcium Ion
• Probability of Transmitter Release
• Kinetics of Receptor Binding
• Kinetics of Channel Opening

5
The General Framework
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Quantal Release

• Each Synapse has a number of independent release
  sites (N)
• Each site releases no or a single vesicle
• If the probability of release for each site is
  prel, the number of released vesicles obeys

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Modeling Transmitter Release

• Assumptions
• Upon release, the transmitter diffuses very
  rapidly in the synaptic cleft (nanoseconds)
• Given the narrowness of the synaptic cleft, the
  transmitter concentration will be high at the
  cleft after release
• The transmitter will be washed out quickly from
  the cleft by a number of mechanisms (pre-synaptic
  uptake, extra cellular enzymes, diffusion,)

• Thus the transmitter concentration in synaptic
  cleft after release can be modeled as a square
  pulse

T
t0 trel
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Synaptic Channel Gating (1)
ßS
aS
T
PS
1-PS
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Synaptic Channel Gating (2)
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Synaptic Channel Gating (3)

• Experimental fit to mossy fibers input to
  hippocampus CA3 region

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Synaptic Channel Gating (4)

• Single exponential Synapse (for fast synapses)

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Synaptic Channel Gating (5)

• Double exponential Synapse (for slower synapses)

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Synaptic Channel Gating (6)

• Alpha Synapse (for slower synapses)

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Synaptic Weight

• Synaptic current is generally modeled as

Constant
Between 0 and 1
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Inhibitory/Excitatory

• Post Synaptic Current (PSC)
• A synapse is excitatory if the post synaptic
  current is inward (influx of positive ions)
• A synapse is inhibitory if the post synaptic
  current is outward (influx of negative
  ions/efflux of positive ions)
• Post Synaptic Potential (PSP)
• A synapse is excitatory if it tends to depolarize
  the membrane (EPSP)
• A synapse is inhibitory if it tends to
  hyper-polarize the membrane (IPSP)

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Classification of Synapses

• Ionotropic
• Metabotropic

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The most common Synapses

• Glutamine ( Excitatory) (Asymmetric/Type I)
• AMPA Synapse (Fast)
• NMDA Synapse (Slow)
• GABA (- Inhibitory) (Symmetric/Type II)
• GABA A Synapse (Fast)
• GABA B Synapse (Slow)

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AMPA Synapse
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NMDA Synapse
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NMDA Synapse (2)
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GABA A Synapse
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GABA B Synapse