Commander
Reference
The
technique
is
illustrated
schematically
in
Figure
5.5A.
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
49
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.
Figure
49.
Schematic
bridge
balance
circuit
and
adjustment
procedure.
117
