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Synaptic strength can be defined

»as the average amount of current or voltage excursion produced in the postsynaptic neuron by an action potential in the presynaptic neuron«

Synaptic strengths and their changes depend on many factors and mechanisms, from LTP and LTD to STDP and STF and STD. Possibly, also neuromodulators should belong to this list – but maybe they shouldn't.

Assuming that synaptic strength is a well-defined quantity even when the synapse is at rest, the strength of a synapse is a continuous or non-continuous function over time, that ideally can be plotted.

Now I wonder whether there have been recordings and/or simulations of the synaptic strength of a single synapse as such a function over time - and how the plot would look like at different time scales: long term trends superimposed by shorter and shorter fluctuations?

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What you are describing is exactly how long-term potentiation/LTP (or depression/LTD, for that matter) is measured electrophysiologically.

Here's an example from a 1988 review by Nicoll et al:

LTP example

Nicoll, R. A., Kauer, J. A., & Malenka, R. C. (1988). The current excitement in long term potentiation. Neuron, 1(2), 97-103.

The x-axis is time in minutes, the y-axis is a measure of synaptic strength: the slope of the local field potential response to simulation of a particular pathway. You could also measure an EPSP internally, measure voltage with a voltage-sensitive dye, etc.

At the point marked "tetanus", a long train of pulses is delivered that is known to cause LTP. Afterwards, you see an increase in synaptic strength that is maintained for many minutes.

It's also possible to perform these experiments over longer time scales, with different paradigms, in the presence of various drugs that influence LTP, etc. Different protocols will give different durations of LTP.

These sorts of plots are ubiquitous in the LTP/LTD research field, so if you read any papers in that area you are likely to come across many similar presentations.

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  • $\begingroup$ Thanks a lot, but what about larger time-scales, from days and months to years and decades? And what about smaller time-scales, down to seconds and milliseconds (why not)? Or will the concept of synaptic strength loose sense on the millisecond level? $\endgroup$ – Hans-Peter Stricker Feb 7 '20 at 23:49
  • $\begingroup$ @Hans-PeterStricker At a millisecond level you would be thinking about short-term plasticity rather than synaptic strength. At very long time scales the answer is far too much "it depends" to be contained in a stack exchange answer. $\endgroup$ – Bryan Krause♦ Feb 7 '20 at 23:54
  • $\begingroup$ But what is plasticity the plasticity of if not of synaptic strength? The other way around: What's the difference between plasticity and synaptic strength? $\endgroup$ – Hans-Peter Stricker Feb 8 '20 at 0:05
  • $\begingroup$ @Hans-PeterStricker I suppose in a sense you could consider it a measure of synaptic strength, but it isn't a mechanism typically employed in artificial networks, where "synaptic strength" is used as a concept. Read up on "short-term plasticity" and contrast it with what you know about long-term plasticity - they are really quite distinct biologically. $\endgroup$ – Bryan Krause♦ Feb 8 '20 at 0:08
  • $\begingroup$ May I note that by definition "synaptic strength" is a purely physical concept (it's all about voltages and currents) - the underlying biological mechanism don't really matter (in my understanding and in the context of my question). $\endgroup$ – Hans-Peter Stricker Feb 8 '20 at 0:18

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