Reference Section
•
63
Chapter 5
Bridge Balance
•
Used to subtract voltage drops across the microelectrode when in I-Clamp mode.
•
Bridge balance is activated by pressing the
button in the Bridge Balance
box in the I-Clamp pane or by checking the checkbox and using manual glider
control.
•
See also
Capacitance Neutralization.
In some experiments it may be desired to inject a current (I) into a cell in current-clamp
mode,
e.g.
to depolarize the cell and evoke action potentials. The flow of I through the
microelectrode produces a voltage drop across the electrode that depends on the
product of I and the microelectrode resistance (R
e
). This unwanted IR
e
voltage drop
adds to the recorded potential. The Bridge Balance control can be used to balance out
this voltage drop so that only the membrane potential is recorded. The term “Bridge”
refers to the original Wheatstone Bridge circuit used to balance the IR voltage drop and
is retained by tradition, even though operational amplifiers have replaced the original
circuitry.
The technique is illustrated schematically in Figure 4.5A. A differential amplifier is
used to subtract a scaled fraction of the current I from the voltage recorded at the back
of the microelectrode, V
p
. The scaling factor is the microelectrode resistance (R
e
).
The result of this subtraction is thus the true membrane potential, V
m
.
Figure 4.5B shows how bridge balance is done in practice. When the current is
stepped to a new value (top), there is a rapid voltage step on V
p
due to the ohmic
voltage drop across the microelectrode (middle). Following this instantaneous step,
there is a slower rise in V
p
largely due to the membrane time constant of the cell.
Correct adjustment of the bridge amplifier removes the instantaneous step, leaving the
corrected V
m
trace (bottom). Although this adjustment is done with a step current
injection, the correction remains valid for any arbitrary waveform of injected current,
provided the microelectrode maintains a constant resistance.
