[Neuroscience] Series resistance and capacitance compensation in current clamp

Hyunchul Lee via neur-sci%40net.bio.net (by aurion from rocketmail.com)
Thu Feb 22 18:26:30 EST 2007


Thank you, Dr. Ferber- but let me clarify my question-

We're making current clamp recordings, and injecting square current 
pulses whilst in current clamp.
Our amplifier seems to "turn off" capacitance/series resistance 
compensation in current clamp mode, though I'm not so sure.
In our preparation, a strong inward rectifying current is expected to be 
seen in medium spiny neurons by applying hyperpolarizing pulses.
We've been applying 20 pA steps from 0 to -1nA (the maximum current 
injection possible for our amplifier), yet we do not see any inward 
rectification.
Inward rectification is a defining characteristic of medium spiny neurons.
These cells have been labelled iontophoretically with neurobiotin, and 
we see that they are indeed medium spiny neurons.
We've checked our intracellular and ACSF ionic concentrations against 
what others are using, and find no great difference.
The cell fires what appear to be normal action potentials with 
depolarizing pulses in current clamp.

I'm thinking that perhaps- if the access resistance is quite high, the 
injected current will tend to pass through the walls of the pipette 
rather than into the cell, thus charging the interior of the pipette and 
thus affecting the potential difference between the pipette and the 
ground.  This is just my thinking, but the HEKA manual seems to indicate 
that compensation dials are disabled when switching to current clamp 
"for the benefit of the user".  Is there anything we can do, short of 
getting a new amplifier?

Thanks,
Hyunchul

Dr. Michael Ferber wrote:
>> 1. Will this stuff up our membrane potential values during large current
>> steps (<1nA)?  In what way?
>>     
> In voltage clamp(!!!!) the problem is the ratio between RS and the membrane 
> resistance. Here you have a voltage-divider. Therefore the membrane potential 
> does not corespond to your settings and the mistake depends on the ratio of 
> RM and RS. Due to the fact that the RM depends on the number of open channels 
> the mistake is not linear and increases with increasing membrane current 
> (=decreasing membrane resistance).  If RS is low and RM is high everything is 
> fine, but if you have huge membrane currents the real membrane potential may 
> be far away from the value you set. 
>   
>> 2. Is there any legitimate way we can adjust our data post-hoc?
>>     
> I see no way.  
>   
>> 3. Will compensating make a difference in current clamp for our case?
>>     
> In this case you apply a constant current and the potential is floating 
> freely.... and if you compensate your electrode properly the very fast 
> components (RS, pipette resistance ....) should be cancelled anyway.
>
> Capacitance compensation makes your systen faster (simplyfied). This may be of 
> interest if you are interested in the kinetics of processes (i.e activation 
> of channels) .
>
> Regards
> Michael
>
>   



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