DC Space contanst

[Meena]

Hi,

My understanding of the DC space constant is:

lambda= sqrt ( (d*Rm)/4*Ri))

Where Rm is membrane resistivity in ohm.cm^2
d is diameter in um (or cm really)
and Ri is ohm.cm

My question is what is Ri ?
My guess is that Ri should be calculated based on the neuron ( I am guessing that Ri is the input resistance).

I know that Ra is the cytoplasmic resistivity in ohm.cm

How is Rm usually obtained ? and how is Ri obtained?
Also, when i try typing "print soma.Ra" i get 35.4 ohm.cm which is the default value for cytoplasmic resistivity in NEURON.

However when I do a "print soma.Ri" I get some error

when I do a print R I get the value 8.3...what is "R"?

Please do advice. Thanks in advance.

[ted]

what is Ri ?

Cytoplasmic resistivity.
In NEURON, the keyword that means cytoplasmic resistivity is Ra. Look it up in the
Programmer's Reference. You'll see that it is called "axial resistivity" but that's the same
as what a biologist would call "cytoplasmic resistivity."
http://www.neuron.yale.edu/neuron/stati ... ry.html#Ra

How is Rm usually obtained ? and how is Ri obtained?

You'll have to read the experimental literature for this. Here are a couple of suggestions:
Spruston, N. and Johnston, D. Perforated patch-clamp analysis of the passive
membrane properties of three classes of hippocampal neurons. Journal of
Neurophysiology 67:508-529, 1992.
Thurbon, D., Lüscher, H.-R., Hofstetter, T., and Redman, S.J. Passive electrical
properties of ventral horn neurons in rat spinal cord slices. Journal of Neurophysiology
79:2485-2502, 1998.
Do not confine your reading to just these two articles. They may use methods described
in earlier sources--track those down too. You may also want to look at papers that
cite these two articles, to see other methodologies.

when I do a "print soma.Ri" I get some error

Correct. Sections do not have an Ri.

when I do a print R I get the value 8.3...what is "R"?

The gas constant in Joules / (Kelvin mole)--see http://en.wikipedia.org/wiki/Gas_constant

[ssothro]

Hi Ted,
I have a question on the previous post regarding the DC space constant (lambda= sqrt ( (d*Rm)/4*Ri)) ).

In an existing cell model, is there a way to calculate Rm? Can one utilize the extracellular mechansim, use i_membrane for transmembrane current and divide the resting mebrane potential by i_membrane?

Thanks,
Stamatis

[ted]

In an existing cell model, is there a way to calculate Rm?

Yes. Several. Before we discuss them, first let's dispose of this:

Can one utilize the extracellular mechansim, use i_membrane for transmembrane current and divide the resting mebrane potential by i_membrane?

No. Think about it. First, ask yourself this: when a cell is at rest, what is its
membrane current? Then ask yourself this: how would an experimentalist
measure the membrane resistance of a spherical cell?