Reference Section • 61
Chapter 5
manipulation can alter this for a given set of experimental conditions; fortunately it is
relatively small. On the other hand, the combined resistance of the grounding pellet
and agar bridge are larger, but one can take precautions to minimize them. A 1 mm
diameter Ag/AgCl pellet in Ringer's solution has a resistance of 300-600 , depending
on how much of the surface is in contact with the saline. The larger the surface area in
contact with the saline, the smaller the resistance.
The resistance of an agar bridge depends on the length and diameter of the bridge, as
well as its contents (i.e. Ringer's Solution versus 3 M KCl). For a 1 cm long bridge:
1 mm diameter 2 mm diameter
Ringer's 10.2 k 2.6 k
3 M KCl 510 130
Therefore, to minimize R
b
, it would be best to eliminate the agar bridge and ground the
preparation directly with a Ag/AgCl pellet. The pellet should be as large as practical,
and the area of it in contact with the solution should be maximized. With this kind of
bath electrode, you should avoid perfusing the bath with solutions containing different
chloride activities. The DC offset of an Ag/AgCl pellet changes with chloride activity.
In order to minimize R
b
when using an agar bridge, it is best to fill the bridge with 3 M
KCl instead of Ringer's solution. When the agar bridge is filled with 3 M KCl, the sum
of all components of R
b
will be approximately 1-2 k. If leakage of KCl from the agar
bridge is a problem, it may be necessary to fill the agar bridge with Ringer. In this
case, R
b
will be several kilohms.
Use of a Bath Headstage
Another method for minimizing the effect of the voltage drop across Rb is to actively
control the bath potential, measured near the outside surface of the cell. This is
achieved using a virtual-ground circuit, the bath headstage.
The MultiClamp 700A is compatible with one of the following bath headstages from
Axon Instruments: VG-2-x0.1 and VG-2A-x100. These headstages attach to the
MultiClamp 700A via the rear-panel 15-pin D connector.
The basic design of both types of headstage is illustrated in Figure 4.1.
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