60
•
Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
Bath Headstage and Electrodes
The Bath Headstage is used when recording from cells with a large conductance, in
order to minimize errors due to current flow through the bath electrode. The VG-2
series Bath Headstage is optional hardware that can be used with the MultiClamp 700A
for this purpose.
In most experiments, the bathing solution is grounded by a solid grounding electrode
(such as an agar/KCl bridge) and all measurements are made relative to the system
ground (on the assumption that the bath is also at ground). This assumption may not
be true if the Cl- concentration or the temperature of the bathing solution is
significantly changed, if there is restricted access from the extracellular space to the
grounding point, or if the membrane current is sufficiently large as to cause a
significant voltage drop across the resistance of the grounding electrode. The latter
circumstance, which normally arises only when voltage clamping large cells with large
membrane currents, is the situation for which the bath headstage is intended.
In a simple voltage clamp setup, the voltage drop across the resistance of the bath
grounding electrode (R
b
) is indistinguishable from the membrane potential. That is,
the potential recorded by the microelectrode is the sum of the transmembrane potential
(V
m
) and the voltage drop across R
b
. Problems arise if the product of the clamp current
(I) and R
b
is significant. For example, for I = 5
µ
A and R
b
= 2 k
Ω
, the voltage error is
10 mV.
To minimize this problem with the MultiClamp 700A, the following two strategies can
be adopted.
R
b
Minimization
There are three main contributors to R
b
:
•
The cell access resistance from the membrane surface to the bath
•
The resistance of the grounding pellet
•
The resistance of the agar bridge (if used)
Typical values of the access resistance of a 1 mm diameter sphere in Ringer's solution
(such as an oocyte) are on the order of 150-200
Ω
. This is a given, and no amount of
