# Command: tsodyks_synapse

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Name:

    tsodyks_synapse - Synapse type with short term plasticity.

Description:

      This synapse model implements synaptic short-term depression and short-term
      facilitation according to [1]. In particular it solves Eqs (3) and (4) from
      this paper in an exact manner.

      Synaptic depression is motivated by depletion of vesicles in the readily
      releasable pool of synaptic vesicles (variable x in equation (3)). Synaptic
      facilitation comes about by a presynaptic increase of release probability,
      which is modeled by variable U in Eq (4).
      The original interpretation of variable y is the amount of glutamate
      concentration in the synaptic cleft. In [1] this variable is taken to be
      directly proportional to the synaptic current caused in the postsynaptic
      neuron (with the synaptic weight w as a proportionality constant). In order
      to reproduce the results of [1] and to use this model of synaptic plasticity
      in its original sense, the user therefore has to ensure the following
      conditions:

      1.) The postsynaptic neuron must be of type iaf_psc_exp or iaf_tum_2000,
      because these neuron models have a postsynaptic current which decays
      exponentially.

      2.) The time constant of each tsodyks_synapse targeting a particular neuron
      must be chosen equal to that neuron's synaptic time constant. In particular
      that means that all synapses targeting a particular neuron have the same
      parameter tau_psc.

      However, there are no technical restrictions using this model of synaptic
      plasticity also in conjunction with neuron models that have a different
      dynamics for their synaptic current or conductance. The effective synaptic
      weight, which will be transmitted to the postsynaptic neuron upon occurrence
      of a spike at time t is u(t)*x(t)*w, where u(t) and x(t) are defined in
      Eq (3) and (4), w is the synaptic weight specified upon connection.
      The interpretation is as follows: The quantity u(t)*x(t) is the release
      probability times the amount of releasable synaptic vesicles at time t of the
      presynaptic neuron's spike, so this equals the amount of transmitter expelled
      into the synaptic cleft.
      The amount of transmitter than relaxes back to 0 with time constant tau_psc
      of the synapse's variable y. Since the dynamics of y(t) is linear, the
      postsynaptic neuron can reconstruct from the amplitude of the synaptic
      impulse u(t)*x(t)*w the full shape of y(t). The postsynaptic neuron, however,
      might choose to have a synaptic current that is not necessarily identical to
      the concentration of transmitter y(t) in the synaptic cleft. It may realize
      an arbitrary postsynaptic effect depending on y(t).

Parameters:

      The following parameters can be set in the status dictionary:
      U	 double	- maximum probability of release [0,1]
      tau_psc   double	- time constant of synaptic current in ms
      tau_fac   double	- time constant for facilitation in ms
      tau_rec   double	- time constant for depression in ms
      x	 double	- initial fraction of synaptic vesicles in the readily
      releasable pool [0,1]
      y	 double	- initial fraction of synaptic vesicles in the synaptic
      cleft [0,1]

Transmits:

    SpikeEvent

References:

      [1] Tsodyks, Uziel, Markram (2000) Synchrony Generation in Recurrent Networks
      with Frequency-Dependent Synapses. Journal of Neuroscience, vol 20 RC50

Author:

    Moritz Helias

FirstVersion:

    March 2006

SeeAlso:

- synapsedict
- stdp_synapse
- static_synapse
- iaf_psc_exp
- iaf_tum_2000

Source:

    /opt/conda/conda-bld/nest_1512418852064/work/nest-2.12.0/models/tsodyks_connection.h

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