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

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

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.