Guide
to
Electrophysiological
Recording
Another
destabilizing
factor
is
the
non
‐
ideal
nature
of
the
membrane.
In
theory
the
membrane
is
simply
modeled
as
a
parallel
resistor
and
capacitor.
In
practice,
a
distributed
model
applies.
The
capacitance
elements
are
themselves
non
‐
ideal;
they
should
be
modeled
by
an
ideal
capacitor
with
a
series
resistance
component.
For
real
membranes,
the
phase
shift
at
high
frequencies
is
less
than
90°.
In
the
Axoclamp
900A,
a
phase
‐
shift
control
is
included
to
allow
the
user
to
empirically
add
some
phase
lag
to
the
circuit
to
build
the
total
high
‐
frequency
phase
shift
up
to
90°.
The
input
capacitance
of
the
voltage
‐
recording
microelectrode
(CHANNEL
1)
adds
another
frequency
‐
dependent
variable
into
the
system,
which
also
tends
to
decrease
the
stability.
The
effect
of
this
input
capacitance
is
usually
minimized
by
carefully
adjusting
the
capacitance
neutralization
control
to
maximize
the
bandwidth
of
CHANNEL
1.
Inter
‐
Electrode
Coupling
Capacitance
and
Shielding
When
the
voltage
‐
recording
microelectrode
and
current
‐
passing
microelectrode
are
applied
to
a
cell
there
is
considerable
coupling
capacitance
(C
x
)
between
them
due
to
their
proximity.
This
capacitive
coupling
introduces
a
low
impedance
feedback
pathway
at
high
frequencies
that
completely
bypasses
the
cell
membrane.
The
presence
of
this
capacitance
is
the
single
most
significant
destabilizing
influence
on
the
voltage
clamp,
and
for
this
reason
it
is
essential
that
it
be
reduced
as
far
as
is
practical.
In
point
of
fact,
coupling
capacitance
as
low
as
0.01
pF
can
destabilize
the
response
at
high
gain
settings.
There
are
three
ways
to
reduce
C
x
:
1.
Introduce
the
two
microelectrodes
into
the
preparation
at
a
wide
angle,
preferably
greater
than
90
°
.
Keep
the
tips
of
the
microelectrodes
as
far
apart
as
possible.
Generally
it
is
better
to
have
the
tip
of
the
current
microelectrode
nearer
the
center
of
the
cell,
so
that
field
potentials
do
not
affect
the
local
membrane
potential
unevenly
(see
Eliminating
the
Voltage
Error
Due
to
Coupling
Between
Intracellular
Microelectrodes
on
page
106).
105
