IUBio

[Neuroscience] Re: Electrophysiological equipment

r norman NotMyRealEmail at _comcast.net
Thu Oct 6 07:55:53 EST 2005


On 6 Oct 2005 04:14:19 -0700, "Hebb" <hebb at poczta.onet.pl> wrote:

>Hi, I`ve got presumably simple question.
>I`m using the stimulus isolation unit, but I`m not quite sure what`s
>going on in the solution after polarization of current is changing.
>Why stimulus artifact after this operation is always reversed, and the
>proper potential either reversing or just changing the shape?
>Would anybody be so kind to explain this to me, especially how the ion
>mechanism in the solution is look like?
>

You don't say what kind of electrodes you are using and what stimulus
parameters -- duration, amplitude, frequency.

Ordinarily you stimulate with very brief pulses of voltage or current.
If you are using long, sustained stimuli or DC, then things change
significantly.  Let me assume you are using brief pulses.

Ordinarily you set the stimulus isolation unit to produce "bipolar"
stimuli.  That, in electrical terms, means the stimulus is AC coupled,
with no direct current component.  That is, there is no net flow of
current through your preparation.  During the stimulus pulse, an
intense, brief current flows in one direction.  Then, immediately
afterwards, a smaller reverse current of longer duration flows in the
opposite direction to make the total zero.  If you look at the
stimulus pulse shape you see a distinct "droop" on the top of the
stimulus pulse -- it is not flat.  You see a distinct "negative
afterpotential"  (opposite direction)  that decays slowly, at least
much more slowly than the rise and fall of the stimulus pulse, itself.

 If you are using "monopolar" stimulation, then this situation changes
and you do get a net flow of current in one direction.  If you need
long, sustained stimuli or DC for some reason, you must use the
unipolar setting and then you definitely have unidirectional flow of
current.

The reason to use bipolar stimulation has to do with electrode
polarization.  Current flow in the wires is carried by electrons.
Current flow in the solution is carried by ions.  At the electrode
interface, there has to be a conversion.  There are two ways this can
happen.  First, there is a capacitative effect, where electrons
accumulate (or get depleted) on the metallic side producing an excess
negative (or positive) charge on what is essentially one plate of a
capacitor.  In the solution, which acts as the other plate, the
electric field attracts or repels ions producing the continuity of
current flow.  Nothing physical crosses the metal-solution interface.
This can only happen if there is no net flow of current -- that is, if
the isolation unit is set to bipolar.  This is the situation when
non-polarizable electrodes (stainless steel, platinum, etc) are used
and it happens to  a significant degree no matter what kind of
electrode is used.  

An alternative process can occur, a redox reaction at the electrode
interface.  This is what happens using Ag-AgCl electrodes.  At one
electrode, Ag is oxidized to Ag+, releasing an electron into the wire.
At the same time, a Cl- ion from the solution moves to the electrode
and combines with the Ag+ to form insoluble AgCl which deposits onto
the electrode surface.  At the opposite electrode, the reverse occurs.
AgCl is reduced, absorbing an electron from the wire to produce
metallic Ag and releasing a Cl- ion into the solution which can then
carry the current.  This reaction is the reason why chlorided silver
electrodes are commonly used.  

In either case, once the current is produced in the electrolyte
solution, it is carried by a combination of ions. Since in
extracellular fluid, NaCl is the primary compound  present, the
current is carried by a combination of Na+ moving in one direction and
Cl- moving in the other.  Inside the cell, the current is primarily
carried by K+ movement, although the insoluble anions also  carry some
of it.  Across the membrane, the current is a combination of
capacitative current (equal to C dV/dt) and to ionic current carried
by ions moving though specific ion channels.

The actual ion movement is quite small.  A current of  1 mA in a pulse
of 0.1 ms duration represents the movement of one tenth of a
microcoulomb or one picomole of ions.  One microliter of a 0.1 M
solution contains 100,000 picomoles of ions.  The ion movements are so
small so that there is no significant (or even detectable) change in
ion concentrations for any reasonable situation.

Of course, of you are using direct current or long sustained stimuli,
you must use polarizable electrodes capable of sustained the necessary
redox reaction to a significant degree and then you can get ion
changes in the immediate vicinity of the  electrode.  If that is what
you are doing, you have to learn a lot more about the electrochemistry
of electrodes and polarization to do it successfully.

Does this help?




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