question about i_pas (newbie)

[psun]

Hi,

I'm a Neuron newbie and am trying to build a model of a myelinated axon. My question is about the passive current i_pas -- would I be right in assuming that it is a passive current that flows longitudinally inside the cell?

Thanks,
--Pam

[ted]

No. From the Programmer's Reference:

pas

mechanisms

SYNTAX
insert pas
g_pas -- mho/cm2 conductance
e_pas -- mV reversal potential
i -- mA/cm2 non-specific current
DESCRIPTION
See $NEURONHOME/src/nrnoc/passive.mod

Passive membrane channel.

[psun]

Thanks. How would I determine the intracellular current then?

[ted]

Pick a pair of adjacent points and axial current is (voltage difference between the points)/(axial resistance between the points). The practical issues are
--what pair of points to choose
--how to find the voltage difference between them
--how to find the axial resistance between them

What pair of points to choose: a good choice would be the centers of a pair of adjacent segments in the same section. One of the points could be the 0 or 1 end of a section but only if that point is attached to some other section (because axial current is 0 at 0 or 1 ends that are free terminations).

Example: Suppose your model is
create axon
access axon
L = 1000
diam = 1
Ra = 160
insert hh
nseg = some_number // best to use an odd number
// 45 would be good for this example

Then segment centers (also called "internal nodes") are located at
L*(n+0.5)/nseg
where n = 0..nseg-1

Voltage gradient between the first two internal nodes is (v(0.5/nseg) - v(1.5/nseg))
Axial resistance between the first two internal nodes is ri(1.5/nseg)
(BE SURE TO READ ABOUT ri in the Programmer's Reference).
So axial current between the first two internal nodes is (v(0.5/nseg) - v(1.5/nseg))/ri(1.5/nseg)

[psun]

Fantastic, that makes sense. Thank you!