How to make the TM-model work
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1
University of Warsaw, Department of Physics and Biocenter Oulu,, Poland
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2
Nencki Institute of Experimental Biology, Poland
Short-term synaptic plasticity is an important phenomenon often observed in synaptic connections of the neocortex. It is yet not fully understood. Construction of computational models can elucidate the physiological background of these processes and enlighten the possible consequences of synaptic plasticity on properties of the synaptic connections and on the neural circuits in which they function.
One of the simplest models of short-term plasticity is the model proposed by Tsodyks and Markram (TM-model) [1],[2]. It can cover a variety of physiological processes that are responsible for depression and facilitation. The model equations can be solved analytically imposing some mathematical constraints. The TM-model characterizes the synaptic connection by its absolute amount of 'resources' that can be partitioned into three states 'effective', 'inactive' and 'recovered'. The transitions between these states are governed by time constants and a variable describing the utilization of synaptic resources (in case of the depression model it is set to a constant value). This variable corresponds the calcium concentration. The model assumes that one calcium molecule binds with one molecule of the 'resources'. The main problem concerning the TM-model of facilitation is that it can elevate the postsynaptic potential only twofold. However, it has been observed that this type of synaptic facilitation can enhance the PSP even a dozen of times.
We propose to modify the standard TM-model by considering that the molecule of resources binds with more than one molecule of calcium. Synaptotagmin, that is believed to be the "calcium sensor", binds with four molecules of calcium [3]. Therefore we require that the activation of resources happens only upon binding four molecules of calcium. The adjusted model of facilitation allows elevation of postsynaptic potential up to twenty times. We show that one can derive a measure of facilitation (in this case the two-spike PSP integral to single-spike PSP integral ratio [4]). Those measures were fitted to the experimental data from [4] yielding physiologically plausible values of the model parameters.
Acknowledgments: This work was founded by grant N N301 034336 of the Polish Ministry of Science and Higher Education.
References
1. M.V. Tsodyks & H. Markram "The Neural Code between Neocortical Pyramidal Neurons Depends on Neurotransmitter Release Probability". PNAS, 94, 719-723 (1997).
2. M. Tsodyks, K. Pawelzik & H. Markram. Neural networks with dynamic synapses. Neural Computation, 10, 821-835 (1998).
3. N. Brose , A.G.Petrenko, T.C.Sudhof & R.Jahn "Synaptotagmin: A calcium sensor on the synaptic vesicle surface". Science 256 (5059), 1021
4. A.M.Thomson "Activity dependent properties of synaptic transmission at two classes of connections made by rat neocortical pyramidal axon in vitro". Journal of Physiology 531, 807 (1997)
Conference:
Neuroinformatics 2009, Pilsen, Czechia, 6 Sep - 8 Sep, 2009.
Presentation Type:
Poster Presentation
Topic:
Computational neuroscience
Citation:
Jedrzejewska-Szmek
J,
Zygierewicz
J and
Michalski
A
(2019). How to make the TM-model work.
Front. Neuroinform.
Conference Abstract:
Neuroinformatics 2009.
doi: 10.3389/conf.neuro.11.2009.08.116
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Received:
25 May 2009;
Published Online:
09 May 2019.
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Correspondence:
Joanna Jedrzejewska-Szmek, University of Warsaw, Department of Physics and Biocenter Oulu,, Granz, Poland, asia@in.waw.pl