surface specific membran capacity about 4.5 myFarad/cm2?

Edmund Müller edmund.mueller at freenet.de
Fri Feb 1 14:42:34 EST 2002


Hallo,

>Measuring capacity can be somewhat tricky.  Exactly what
>experimental technique are you using?  

Whole-cell recording in voltage clamp with holding -50 mV and a hyperpolarizing
step of -10 mV to -60 mV for 20 ms.

>What do your data look like?

Well, quite classical considering the large axon this cell has: The decaying
current can very well be approximated biexponential in the form

I=Ifast*exp(-t/taufast)+Islow*exp(-t/tauslow).

>How are you calculating time constant? How are you calculating
>capacitance?

The time constants I get directly from the above equations. I assume the faster
belonging to the soma, the slower to the axon. So for the soma I just take the
first part of the above equation, integrate over time and divide by 10 mV, the
step used. This should produce the capacity of the cell, shouldn't it?

>the neurites.  Fourth, because of the cable properties, the area of the
>neurites (certainly the region within, say, one space constant) must
>be considered in calculating the area of the cell. 

Even if I try to neglect that in the above manner? Well how big is the space
constant for rat hippocampal cells. Since I didn't measure I must guess. Any
formulas available like space constant= function (diameter axon, etc.)?

>Fifth, are you sure
>your cells are actually the size you say?  Where did you get the
>size information?  These are some of the factors you must consider.

By taking top view photos of it and approximating an elliptical shape of the
soma with 9 x 6 micrometers. For the third dimension which I cannot see I
guessed a rotation symmetric cell so the third dimension is 6 micrometers. 

I must admit that the cell has a large long axon.

Edmund




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