# Looking for Ri in the Soma equations

Hello,
I am a PhD Student in France, and I worked on a neuron/synapse model that a Brian2 user has sent to me a few months ago and I wanted to determined the membrane voltage equations of the model I am using.
My issue is that my model looks like this :

# Set model’s morphology

``````morpho = br2.Soma(10*um)
d_l = 10*um

# Set its biophysical parameters
eqs = '''
Im = gL*(EL-v) : amp/meter**2
Is = gs1*(Es-v): amp (point current)
dgs1/dt = -gs1/taus : siemens
'''
``````

Then, when i change the value of Ri in the NeuronGroup function, the voltage has a new shape. However, I didn’t use it in my equations.

I thought that my equation would be :

dydt = (1/(Cm)).(gL.(vrest - y) + (gsmax . np.exp(-t/tauS)) . (Es - y))

But Ri seems to be somewhere in this equation.

I didn’t find it in the source files, or maybe I didn’t understand them.

Can you enlighten me on this point please ?

(If my Topic doesn’t follow the rules of this website, feel free to tell me, I will adapt it)

Hi @gmarthe Is the `morpho = br2.Soma(10*ujm)` the full morphology of the model, or does it later get additional compartments? This looks a bit odd, since it is a single-compartment model, but the equations are for `SpatialNeuron`, which is there to support multi-compartment models. When you say that you change the value of `Ri` in `NeuronGroup`, you rather mean `SpatialNeuron`, right?
If this is really a single-compartment neuron simulated with `SpatialNeuron`, then I am indeed surprised that changing `Ri` makes a difference here. Your equation looks correct to me for a single compartment model, only with multiple compartments the value of `Ri` should come into play for calculating the diffusion current (current between compartments). Could you give some more details about what you are doing when you see changes in voltage shape?