MRG model & bipolar stimulation - asymmetric response?

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MRG model & bipolar stimulation - asymmetric response?

Post by Shamsie »


I have implemented the MRG model in Python and was able to reproduce some simple results. I use it mainly by applying a vector of extracellular potential along the axon (using e_extracellular).

However, I observed an unexpected outcome with bipolar stimulation and I cannot pinpoint its origin, whether it is an implementation problem, a misunderstanding or something else:

Let's say I apply an extracellular potential (see below) induced by a bipolar stimulation of of a given amplitude (1.3 mA), and the axon (200 mm long, 11.5 um diameter) does not fire.
However if I apply the exact same vector of extracellular potential induced by the same stimulation, but I inverse the polarity (see below), then the axon gets activated (an action potential propagates along the axon).

I expected a symmetric behavior, i.e. the same threshold amplitude for activation with both extracellular potential vectors. It turns out the difference in the amplitude required to activate the axon is about 0.2 mA, which I do not believe can be attributed to interpolation or other approximations.
Here are the graphs of the membrane potential with the initial and inverted bipolar stimulation:

and their corresponding GIF with membrane potential over time:

Here are more details about what I tried to highlight potential cues of where this difference comes from, but in vain:
I made sure the extracellular potential vectors have exactly symmetric peaks (I just duplicated half of the vector and inverted it before concatenating it with the first half), and that the axon is long enough (200 mm) compared to the internode length (1.3 mm) and the extracellular potential decay on the tails. I also tried to shift this bipolar shape of the extracellular potential along the axon, swept a range of axon node offsets, none of which influenced the difference.

I would be so grateful if anyone could give me any lead, clue, idea, anything that could help me understand why I see this difference, or how I could at least identify it as an error of implementation or as an intrinsic characteristic of the model, because I run out of ideas.

I thank you in advance,
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Re: MRG model & bipolar stimulation - asymmetric response?

Post by ted »

Some questions to help guide your approach to answering your own question--

Are there any asymmetries in the model axon itself, or in how the axon's state variables are initialized?
If not, is there a spatial asymmetry in the stimulus?
If not, is there an asymmetry in the spatial relationship between the model axon and the model stimulus?
If not, is there a temporal asymmetry in the stimulus?
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