I have a question about the cytoplasmic resistance Ra defined in NEURON:

In my model (about dendritic spines), I want to express the neck resistance (Ohm, not Ohm.cm) as explicitly as possible. The relation between Rneck and Ra is the following:

Rneck = (Ra*nech_Length)/(pi*(neck_Diameter)^2)

My problem is that when I enter this formula and set Rneck as a variable, varing its value on the GUI just doesn't change anything. As if it wasn't linked to anything regarding the spine. Howerver, Ra (which I conserved in the model) does affect the voltages in the spine when I vary it.

Here are the formulae where I cahnged Ra to Rneck:

forall { // this block is set in the cell definition (topology, geometry and conductances)(independante hoc.file)

insert pas

g_pas = 1/RmSpec // mho/cm2 conductance

e_pas = -70 // mV reversal potential (will be modified by seting Vrest)

Ra = RaSpec

for i = 0, 3 { // because I work with 4 spines

neck

*{*

Rneck

Rneck

*= (RaSpec*NECK_LENGTH**)/(pi*(NECK_DIAM**)^2)*

Rn = RneckRn = Rneck

}

}

Then, in the simulation hoc.file I set this:

strdef space_constant_axon, space_constant_dendrite

proc ch_space_constant() {

forall{

Ra = RaSpec

g_pas = 1/RmSpec

}

for i = 0, 3 {

neck}

}

Then, in the simulation hoc.file I set this:

strdef space_constant_axon, space_constant_dendrite

proc ch_space_constant() {

forall{

Ra = RaSpec

g_pas = 1/RmSpec

}

for i = 0, 3 {

neck

*{*

Rn = RneckRn = Rneck

}

}

lamda_ax = sqrt((RmSpec * axon.diam * 0.0001)/(4 * axon.Ra)) * 10000 // µm

lamda_dend = sqrt((RmSpec * dend.diam * 0.0001)/(4 * dend.Ra)) * 10000 // µm

sprint(space_constant_axon, "axon space constant %.0f um", lamda_ax)

sprint(space_constant_dendrite, "dendrite space constant %.0f um", lamda_dend)

}

What I am expecting is to see the amplitude of EPSPs in the dendrite decrease (and increase in the spine head) when I increase the neck resistance. But it doesn't work (no change at all) and I don't really se what's wrong.

Could you advice, please.

Thanks in advance.}

}

lamda_ax = sqrt((RmSpec * axon.diam * 0.0001)/(4 * axon.Ra)) * 10000 // µm

lamda_dend = sqrt((RmSpec * dend.diam * 0.0001)/(4 * dend.Ra)) * 10000 // µm

sprint(space_constant_axon, "axon space constant %.0f um", lamda_ax)

sprint(space_constant_dendrite, "dendrite space constant %.0f um", lamda_dend)

}

What I am expecting is to see the amplitude of EPSPs in the dendrite decrease (and increase in the spine head) when I increase the neck resistance. But it doesn't work (no change at all) and I don't really se what's wrong.

Could you advice, please.

Thanks in advance.