"Extracellular stimulation and recording" - question

[Qroid_montreal]

I have a question about the Extracellular Stimulation and Recording post in the Hot Tips forum (viewtopic.php?f=28&t=168).

The post says that there are two approaches to extracellular stimulation. One is to introduce into the cable equation a current term that's equal to the second spatial derivative of the extracellular field. The second is to assume that the extracellular medium is linear, allowing someone to approximate the external field of multiple currents as

Code: Select all

 Vo(x,y,z) = SUMMA Z_j(x,y,z) I_j         Eq. 1
              j 

where Vo(x,y,z) is the external field at location x,y,z, Z_j is the transfer impedance to that location, and I_j is the jth current. Vo is then used to drive e_extracellular.

I'm used to implementing the first approach, and I want to contrast it with the second. The first approach calculates current; the second approach calculates extracellular field and feeds this into NEURON via e_extracellular. How does the second approach calculate membrane current though? How do I see the exact code that uses e_extracellular to calculate membrane current?

Thanks a lot.

[ted]

I'm used to implementing the first approach, and I want to contrast it with the second.

Has been done before by others. Altman & Plonsey published a critique of the activating function approach in 1990, I think--
Analysis of excitable cell activation: relative effects of external electrical stimuli
MEDICAL AND BIOLOGICAL ENGINEERING AND COMPUTING
Volume 28, Number 6, 574-580

How does the second approach calculate membrane current though?

It doesn't. It adds one or two more layers to the cable equation, then uses the extracellular potential as a forcing function that drives the system matrix.

How do I see the exact code that uses e_extracellular to calculate membrane current?

Download NEURON's source code and unzip it.