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10.4 Current Clamp Recording

manually). After an iteration or two, it should be possible to reduce the transient to only a few percent of its
original amplitude. However, if the cell has an unfavorable shape (for example, a long cylindrical cell or one with
long processes), the cell capacitance transient will not be a single exponential, and the cancellation will not be as
complete.

10.3.3

Series Resistance Compensation

Series resistance (Rs) compensation is important when the membrane capacitance is large or when the ionic currents
are large enough to introduce voltage errors. To use Rs-compensation, you first adjust the transient-cancellation
controls (including

C-fast

and

-fast

if necessary) to provide the best cancellation. Then you select

Rs Comp

turning up the

%-comp

control to provide the desired degree of compensation.

Note:

The ”R-series” control determines (along with the

%-comp

control) the amount of positive

feedback being applied for compensation. It should be adjusted with some care, since too high a setting
causes overcompensation (the

EPC 10 USB

will think that Rs is larger than it is); this can cause

oscillation and possible damage to the cell under observation.

How you should set the Rs-compensation controls depends on the approximate value of the uncompensated
membrane-charging time constant

, which you can calculate as the product of the

C-slow

and

R-series

set-

tings (for example, suppose

C-slow

is 20 pF and

R-series

is 10

Ω;

is then 20

∗

Ω = 200

µs

. If

smaller than about 500

µs

, you should use the 2

µs

setting of the Rs-compensation switch to provide the necessary

rapid compensation; however, the slower settings will provide compensation that is less prone to high-frequency
oscillations from maladjustment of the controls. How much compensation you can apply is also determined by

is larger than about 100

µs

, you can use any degree up to the maximum of 90% compensation without serious

overshoot or ringing in the voltage clamp response. For smaller values of

, the

%-comp

setting should be kept

below the point where ringing appears in the current trace. As in the case of patch recording, there is rarely need
to use the full bandwidth of the

Filter 1

in whole-cell recording. This is because typical membrane charging time

constants (even after Rs-compensation) are considerably longer than 16

µs

, which is the time constant correspond-

ing to a 10 kHz bandwidth. Thus, the current monitor signal is expected to contain no useful information beyond
this bandwidth. In whole cell recording, the voltage and current monitor signals follow the usual convention, with
outward currents being positive. This is because the pipette has electrical access to the cell interior.

10.4

Current Clamp Recording

To switch from voltage clamp to current clamp recording, just select the

C-Clamp

mode in the amplifier dialog.

This provides a recording of the cell membrane potential, which can be monitored at the

V-mon

output of the

EPC 10 USB

When you switch to the current clamp mode, the following things happen inside the

EPC 10 USB

main unit:

I-membrane

is set to a value that will keep the membrane voltage constant, internal stimulation is stopped, the

10 kHz Bessel characteristic for

Filter 1

is selected,

C-Slow Range

is set to ”Off”, and the value of

C-fast

is reduced

by 0.5 pF to avoid oscillations. Rs-compensation will now act as a ”bridge balance”. For the user, these changes
are of little consequence and are mainly designed to make current clamp recording simple and reliable.

Once you have switched to current clamp, you can use

I-membrane

to set a holding current, and you can apply

stimulus pulses via ”External Stim.Input CC”. The scaling of the ”External Stim.Input CC” can be set with the

CC-Gain

control of the

EPC 10 USB

amplifier dialog. With a

CC-Gain

setting of 0.1 pA/mV, the maximum

current for stimulation is

1 nA. With a setting of 1 pA/mV, 10 pA/mV or 100 pA/mV the maximum current is