Rm alteration for myelin modeling

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makrseke
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Joined: Mon Apr 03, 2017 6:43 am

Rm alteration for myelin modeling

Post by makrseke »

Dear all,

I was wondering: how can one incorporate the membrane resistance (Rm, [ohm-cm^2]) of a section into Neuron?
I am modeling myelin, and the Rm and Cm (membrane capacitance) are both changing in the myelinated sections, from the values of the non-myelinated axon. However, nowhere in Neuron is to be found a way to change this Rm. I am keeping the diameter of the myelinated section similar to the bare axon's diameter.

Could this be done via changing the axial resistivity (Ra, [ohm-cm]) and the constant lambda (λ)?
I found the equation: λ = (sqrt((Rm*d)/Ra))/2 (source: http://www.columbia.edu/cu/biology/cour ... cture5.pdf)
Hence: Ra = (Rm*diameter)/(4*(λ^2)).

Is this legit to use like that? Can one alter it via another property maybe?

Thank you very much!
Best,
ted
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Re: Rm alteration for myelin modeling

Post by ted »

makrseke wrote:how can one incorporate the membrane resistance (Rm, [ohm-cm^2]) of a section
Insert pas, then specify the equivalent value of g_pas. If no other channels are present in the section, set e_pas to the resting potential of the unexcited axon.
Could this be done via changing the axial resistivity (Ra, [ohm-cm]) and the constant lambda (λ)?
No.
makrseke
Posts: 16
Joined: Mon Apr 03, 2017 6:43 am

Re: Rm alteration for myelin modeling

Post by makrseke »

ted wrote:
Insert pas, then specify the equivalent value of g_pas. If no other channels are present in the section, set e_pas to the resting potential of the unexcited axon.
Dear Ted,

Thank you very much for your quick reply and suggestion! I had this idea before as well, but I wasn't sure on what to put to the e_pas since I didn't want it to be like an actual passive channel there. But yes, now it makes sense.

Best,
ted
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Re: Rm alteration for myelin modeling

Post by ted »

How to represent the properties of internodal membrane is a good question. It depends largely on the modeler's hypothesis. Some modelers, especially those who just want to do the "mathematician's analysis of CV vs. diameter," simply assume conductance is 0. Others (e.g. those who want to explore the consequences of demyelination) use a linear resistance, which is easily implemented in NEURON with pas. Some, who are concerned about energy consumption or issues related to abnormal axonal function that may be revealed by repetitive activation, put voltage-gated potassium channels in the internode, plus a potassium accumulation mechanism that WRITEs ko (extracellular [K]) in the narrow space that lies between the axon membrane and the myelin, and use extracellular to represent the electrical properties of the myelin sheath and the longitudinal resistance to current flow in that narrow gap between myelin and axon membrane.
makrseke
Posts: 16
Joined: Mon Apr 03, 2017 6:43 am

Re: Rm alteration for myelin modeling

Post by makrseke »

Those are very good-to-know points! Thanks again Ted!
Best,
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