[Neuroscience] Re: stimulation electrodes

r norman via neur-sci%40net.bio.net (by r_s_norman from comcast.net)
Fri Aug 5 14:16:38 EST 2011


On Fri, 5 Aug 2011 11:58:14 -0700 (PDT), jonesmat
<mathewjones from wisc.edu> wrote:

>On Jul 17, 10:57 am, daniela spitz <danielaspi... from googlemail.com>
>wrote:
>> Hi,
>>
>> I would like to know the difference between monopolar and bipolar
>> stimulating electrodes. I can imagine that the strength and actual area
>> being stimulated (in slices) could be changed with different sizes of
>> bipolar electrodes. But, to select between mono and bipolar electrodes is
>> bit puzzling for me. Anywhere I could get literature on this?
>>
>> thanks
>> daniela
>>
>> ps: sorry, if this question could be a basic thing for you, but for me, not.
>
>
>Hi,
>
>In reality, all electrodes have to be bipolar in some sense, because
>there always has to be a (+) and a (-) pole in order to pass any
>current. But when people say monopolar, they usually mean that one
>pole is the "electrode" which you stick into the tissue, and the other
>pole is a "ground" wire sitting far away in the bath (usually *not*
>the same as the ground for your recording amplifier). In contrast,
>bipolar usually means a pair of wires that are close together, like a
>fork, that you stick into the tissue next to each other. The idea is
>that this "should" help to restrict the electric field and make it
>more local so that you're not stimulating things you don't want to.
>Also, there are at least two common kinds of bipolar: 1) a pair of
>parallel wires, or 2) an inner wire as one pole, wrapped in an
>insulating material, which is then coated with a second conductive
>shell. These are called "concentric" electrodes.
>
>Which type you use depends on your specific needs. In general, if you
>want the most local stimulus, probably a bipolar electrode is better.
>One can also make very small, very local stimulating electrodes by
>using "theta" glass (i.e., double-barreled glass pipetes) that you
>pull down to ~1um with a pipette puller. Then fill them with ACSF, and
>run a wire down each barrel to connect to the stimulator.
>

There is another sense of monopolar vs. bipolar, depending on the
words you use to describe stimulation.  A "monopolar" stimulus
produces a voltage (or current) of one polarity, for example, you turn
on a positive potential for a brief time, then shut it off.  This
produces a "DC" current flow through the system.  A "bipolar" stimulus
uses capacitative coupling to connect the stimulator to the electrode.
Then the same positive potential produces an initial positive current
flow which drop off exponentially with time.  When you turn off the
stimulus, you get a negative "afterpotential" which similarly decays
exponentially.  This produces an equal flow of positive and negative
current through the electrode.

The difference is that a monopolar stimulus requires non-polarizable
electrodes.  That is the situation with a properly chlorided silver
wire immersed in a chloride containing extracellular (or
intraelectrode) solution.  With each stimulus pulse, one electrode
cumulatively oxides silver to AgCl and the other reduces AgCl to
metallic Ag.  If you have, say, platinum or stainless steel
electrodes, there is no redox reaction at the electrode-solution
interface, only capacitative coupling, so a bipolar stimulus is
necessary.  Usually very brief bipolar stimulation is appropriate for
triggering action potentials but monopolar or DC coupling is essential
for passing current to control membrane potential.



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