Compensation Procedures
EPC9 Manual
64
It is standard practice to take care of the fixed offsets by performing a reference
measurement at the beginning of an experiment. Thereby an adjustable amplifier
offset is set for zero pipette current. Thereafter the command potential of the
amplifier (displayed as
V-membrane
) will be equal in magnitude to the membrane
potential if no changes in offset potentials occur. The polarity of the command
potential will be that of the membrane for whole-cell and outside-out configurations
but will be inverted in the cell-attached and inside-out configurations. In cell-
attached configuration an additional offset is present due to the resting potential of
the cell under study. Liquid-junction potentials may appear or disappear during the
measurement when solution changes are performed or in the case that the pipette
solution is different from the bath solution (Barry & Lynch, 1991; Neher, 1992; Neher,
1995).
Conventionally, these problems are handled by applying the appropriate corrections
and sign inversions during offline analysis. The EPC9 software allows this to be done
at the time of data acquisition, considering the relevant
Recording Mode
, which may
or may not result in inversion of holding and commanded voltages. In addition, it
offers three features which interact in a way to provide for simple online handling of
offsets, such that
V-membrane
reads the correct membrane potential (correct both in
polarity and magnitude) at all times. These are:
V
0
: An adjustable hardware offset-voltage. The actual potential applied to the pipette
is the sum of
V-membrane
and
V
0
.
Auto-V
0
: An automatic operation which systematically varies
V
0
for zero pipette
current. Before doing so, the procedure sets
V-membrane
to an appropriate value (see
below).
LJ
: A software variable which can be set by the user. It represents the sum of all
“variable offsets” applicable at a given time (
Offset Sum
).
In order for the variable
LJ
to actually represent the various offsets, and to correct for
these, three conditions have to be met:
• The user has to calculate
LJ
correctly, according to the rules outlined below.
This value should then be entered in the
LJ
control.
• A
Auto-V
0
operation has to be performed at the start of an experiment. The
software will then set
V-membrane
to
-LJ
(for
Out Out
and
Whole Cell
mode) or
+LJ
(for
On Cell
and
In Out
modes) at the start of an
Auto-V
0
operation.
• During experiments,
V
0
has to be changed in parallel with user-induced
changes in
LJ
. This is done automatically by
Pulse
and
E9Screen
.
