Minor capacitive current during IClamp simulation

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vogdb
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Joined: Sun Aug 13, 2017 8:51 am

Minor capacitive current during IClamp simulation

Post by vogdb » Fri Aug 18, 2017 5:38 am

Simple soma-only that does not have any current mechanism except the capacitive one.

Code: Select all

create soma
access soma

objref stim
soma stim = new IClamp(0.5)
stim.amp = 1
stim.del = 50
stim.dur = 200

tstop = 300
During the injection the capacitive current is constant and equals a minor value 6.3e-04 micro A (IClamp is 1 micro A). My question is why? Shouldn't the current stop as soon as the capacitor is charged fully?

ted
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Re: Minor capacitive current during IClamp simulation

Post by ted » Fri Aug 18, 2017 4:10 pm

I don't see the problem.
When the IClamp generates no current, soma.i_cap(0.5) is 0.
When the IClamp generates a nonzero current, soma.i_cap(0.5) is nonzero.
In both cases, the value of soma.i_cap(0.5) is what Kirchhoff's current law predicts that it must be.

By the way, keep in mind that IClamp.i is in nA, but i_cap is in units of mA/cm2 because it is the density of capacitive current over the soma's PI*diam*L surface area.
soma print area(0.5)
will report the value in square microns, and 1e8 square microns equals 1 square cm.

vogdb
Posts: 4
Joined: Sun Aug 13, 2017 8:51 am

Re: Minor capacitive current during IClamp simulation

Post by vogdb » Mon Aug 21, 2017 4:59 am

Sorry for the late response. I forgot to turn on notifications.
ted wrote:I don't see the problem.
When the IClamp generates a nonzero current, soma.i_cap(0.5) is nonzero.
In both cases, the value of soma.i_cap(0.5) is what Kirchhoff's current law predicts that it must be.
I agree totally but I don't understand still a single detail. Shouldn't be the plot of i_cap be the same as in the case when i_pas is used?
Image

Am I correct that in the case when there are no mechanisms in soma the circuit can be represented as a capacitor, an injected current and no battery Image

I'm missing something very important and fundamental.

ted
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Re: Minor capacitive current during IClamp simulation

Post by ted » Mon Aug 21, 2017 9:20 am

Code: Select all

.                   Icl Im     
.  +---current clamp->-+->-membrane---+
.  |                                  |
. ---                                ---
.  -                                  -

Im = Icl
c*dv/dt + g_pas*(v-e_pas) = Icl

If g_pas == 0
c*dv/dt = Icl

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