measuring the total current seen at the soma through SEvclam

[fabien tell]

Hello,

I apologize if my question has already been answered, I did find a clear answer, at least for me.

I build a simple neuron (a soma connected to 4 primary dendrites each connected to 2 secondary dendrites).
I inserted in each compartment a pas, a sustained IKdr and a transient IKA.

I try to record from the soma by using the SEcLAMP protocol. When I stepped to , let say, 0 mV I got the expected voltage jump, and at the soma I can read the amplitude of IKDR , I KA, iK (the sum of both) and even Imembrane if I insert "extracellular" in the soma.

Of course the voltage jump does also activate currents in the dendrites but with an attenuation of the step and then of the K+ currents.
My question is: When I record in the soma do I record the total membrane current (Ik) "seen" at the soma or only the Ik I inserted in the soma ?

When I remove the K channels in the soma , I do not see a "remote" current in the soma during a voltage step despite IK are activated in the dendrites.


Thanks a lot

[ted]

I did find a clear answer

Did not find a clear answer?

I try to record from the soma by using the SEcLAMP protocol.

"Protocol" is used to refer to one or more actions that one carries out. There is no SEcLAMP "protocol." However, there is an object class called SEClamp. An instance of the SEClamp class is a computational model of a single electrode voltage clamp.

When I record in the soma do I record the total membrane current (Ik) "seen" at the soma or only the Ik I inserted in the soma ?

Attaching an SEClamp to a model cell is exactly like attaching a real single electrode clamp to a real cell. Currents that spread from anywhere in the cell to the location of the SEClamp will be captured by the SEClamp.

When I remove the K channels in the soma , I do not see a "remote" current in the soma during a voltage step despite IK are activated in the dendrites.

You are asserting that
1. K channels are still present in the dendrites.
2. None of the current generated by K channels in the dendrites is captured by the SEClamp at the soma.
What evidence can you provide that these two statements are true?

[fabien tell]

Hello Ted,

Thanks for your prompt reply.
I think I made a mistake. If now I plot the Seclamp[0].i which I understand means the current measured by the electrode (for the newbies like myself: on a Current graph : plot what ? show object: SECLAMP[0] : i), instead of plotting Ik (plot what? section: soma: Ik) I get a realistic recording of the total current seen at the soma. In this configuration, I do see current spreading from dendrites. I had misunderstood the use of this object and the way to measure the current.
Am I right?

Thanks a lot for you patience

Fabien

[ted]

If now I plot the Seclamp[0].i which I understand means the current measured by the electrode

Your understanding is correct--every SEClamp has a variable called i that reports the current that the clamp injects into the cell. By the way, "case" is very important, so be careful about it--NEURON has an SEClamp class, but it has no class called Seclamp or SECLAMP.

for the newbies like myself: on a Current graph : plot what ? show object: SECLAMP[0] : i

The "Plot what" tool is quite useful, even for very experienced NEURON users. For one thing, it always remembers the names of important things, and their correct spelling, and for another, it reduces the amount of typing that I have to do (so it protects me from making typing errors).

instead of plotting Ik (plot what? section: soma: Ik)

If a section has a mechanism that WRITEs ik (notice that "ik" is lower case!), then that section will automatically have a variable called ik that reports the potassium current that passes through its membrane, and ik will be in "density units" (mA/cm2 to be precise). This will be the sum of all potassium currents that pass through the membrane. Example: if a section called dend has three different kinds of potassium channel mechanisms (say a delayed rectifier, and an A current, and a GABA-B inhibitory synapse that also WRITEs ik), then dend.ik(0.5) will report the sum of the potassium currents generated by these three different mechanisms that passes through the segment that contains the location that is halfway between dend's 0 and 1 ends. The potassium currents generated by the individual mechanisms will also be available to hoc, using names that depend on the details of the NMODL code that defines the properties of those mechanisms.

In this configuration, I do see current spreading from dendrites.

Exactly as it should be.
You might find it helpful to read chapters 5 and 6 of The NEURON Book, or at least this article
Hines, M.L. and Carnevale, N.T.
The NEURON simulation environment.
Neural Computation 9:1179-1209, 1997
which is available here http://www.neuron.yale.edu/neuron/stati ... /nctoc.htm

[fabien tell]

Ted,

Thanks a lot

[ted]

You're welcome, and thanks for using NEURON in your research!