Bridge Balance - Intan Technologies CLAMP User Manual

Intan CLAMP System
Note that the electrode parasitic capacitance C
shown in the diagram below because it is assumed that this
element has already been cancelled by the fast transient
capacitance compensation circuitry prior to cell contact.
After breaking into a cell, the voltage clamp is typically set to
a value near the cell's expected resting potential (e.g., -70
mV) to prevent the activation of voltage-gated ion channels.
A small voltage step is applied around this potential (e.g., a
step from -70 mV to -60 mV) and the resulting current is
measured. The plot below shows a typical clamp voltage
profile and measured current waveform. The current takes
the form of a decaying exponential with a time constant τ:
i(t) = I ·exp(-t/τ) + I . (These exponential current peaks are
0 1
much wider and slower than the "fast transient" peaks
caused by uncompensated pipette capacitance)
Enabling Cell Parameters causes an exponential curve to
be fit to the data and the membrane parameters R
can be estimated, along with the series resistance R
Traditional patch clamp instruments sometimes use positive
feedback circuits to reduce the effects of the series
resistance R in voltage clamp mode. (The measured
S
current flowing through the electrode creates a voltage drop
across R , causing the voltage in the cell to be slightly
S
different from the clamp voltage.)
is not
P
and C
M M
.
S
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The Intan CLAMP system does not support series
resistance compensation in voltage clamp mode, but the
Plot VCell control (see Voltage Clamp Controls above) can
be used to estimate the true cell potential by subtracting the
voltage drop across the series resistance in real time. The
user may then adjust the voltage clamp parameters to
produce the desired voltage in the cell.

Bridge Balance

When applying current clamp pulses through real electrodes
connected to cells, the measured membrane potential is
distorted by an artifact caused by the voltage drop across
the electrode. Referring to the diagram below, we would like
to apply a current signal I
clamp
voltage V equal to the intracellular potential V
elec
However, the clamp current flows through a series
resistance R comprised of the pipette resistance R
S
access resistance R . This leads to a voltage offset ΔV that
A
is equal to I ·R .
clamp S
The plot above shows an example of this phenomenon for a
simple clamp current that starts at zero and pulses to I
Luckily, the intracellular potential V
the measured electrode voltage V
clamp current (at every instant in time) multiplied by R
which was previously measured using the Cell Parameters
and measure an electrode
.
cell
and the
P
.
stim
can be recovered from
cell
by subtracting the
elec
,
S
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