IX2-700 Dual Intracellular Preamplifier
8
Apply a train of current pulses to the electrode. Note that the Z test may not
be used for this as it disables the Bridge Balance control. This can either be from
an external source or from the Step Current Command. You will see a train of
pulses on the voltage trace of the oscilloscope. These pulses are due to the voltage
drop across the series resistance of the microelectrode. Rotate the bridge balance
control clockwise until the pulses on the amplifier output reach a minimum
amplitude. Rotation past the null point will reverse the polarity of the displayed
pulses. Adjust the bridge balance for the best null. The bridge is now balanced and
a stimulus from any source will give minimum voltage at the amplifier output. The
resistance of the electrode can be read on the ten turn dial. The bridge balance may
be locked into this position if desired.
An alternative method of balancing the bridge is as follows: Zero the voltage
trace using the offset control and note the position. Set the holding current to some
value other than zero and switch to either the + or - position. The oscilloscope trace
will depart from its previous position. Adjust the bridge balance control until the
scope trace returns to its initial position. Switch to the opposite polarity and
readjust the bridge balance. When the trace does not move when you switch
polarity, the bridge is balanced.
The main disadvantage with a bridge circuit arises from changes in the
microelectrode resistance. The circuit works on the assumption that the bridge is
balanced for the electrode resistance. Often the resistance of the electrode changes
slightly as you penetrate the cell. In some applications this will not be significant,
while in others it will. One method to attempt to correct for this change is to
balance the bridge after you have penetrated the cell. This method takes advantage
of the time it takes to charge the membrane capacitance. If the pulse duration used
to balance the bridge is sufficiently short, the potential drop recorded will be
predominantly result of the drop across the electrode. However, this does not
account for the local potential that is developed within the cell in the region of the
electrode. For a more complete discussion of this phenomena, see Peskoff and
Eisenberg (1973). Allowing for this generalization, the bridge can be balanced
within the cell (as above), but with very short pulses. Ideally these would be in the
nanosecond range, but 1 msec will usually work. Because both methods (inside
and outside cell) involve approximations, it is a good idea to try balancing the
bridge both ways to see if the results are similar. Another good routine practice is
to check the electrode resistance immediately upon leaving the cell to compare the
values. Data obtained when there is a large discrepancy between these values
should be discarded, as the true membrane potential will be unknown.
