72 • Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
Auto Button
When the Auto button is pressed to automatically compensate Cp or Whole Cell
capacitance, the MultiClamp Commander applies a series of brief voltage pulses to the
electrode and uses the Membrane Current response to optimize the compensation. The
parameters used in this optimization can be set in the Options/Advanced pane. We
recommend setting the pulse amplitude to be as large as possible without causing
damage to the cell. The amplitude can be positive or negative (default is –50 mV).
The Whole Cell Window Width is the duration of the window (in multiples of Tau, the
fitted time constant of the whole cell transient) over which the algorithm optimizes
whole cell compensation. The best setting depends on the cell type and is best found
by trial and error. As a general rule of thumb, 1 x Tau works best for large cells with a
highly distributed capacitance and 10 x Tau works best for small, compact cells
(default 8 x Tau).
Manual Adjustment of Capacitance Compensation
Although the algorithm used by the Auto button is reasonably robust, and is likely to
work under most circumstances, it may sometimes be necessary to manually adjust the
Cp Fast/Slow or Whole Cell compensation. This is done by using the dual controls,
, or by entering values directly. It is recommended that you practice using these
controls with the PATCH-1U model cell. The best strategy is to first set the
capacitance (pF) value to roughly what is expected (i.e. ~5 pF for electrode
capacitance, ~30-100 pF for whole-cell capacitance) and then to adjust the time
constant (µs) or resistance (M) values, respectively, for optimal compensation. After
these approximate values have been established, iterative adjustment using
becomes easier.
Filtering the Command Stimulus
Under some conditions, such as when very large voltage clamp steps are applied, the
capacitance transients cannot be fully compensated and the amplifier may still saturate.
Under these conditions it may be helpful to reduce the size of the capacitance transient
by slowing down the voltage clamp command step. This can be achieved by filtering
the command stimulus before it is applied to the cell. This filtering can be done within
the MultiClamp 700A. (See Chapter 5,
EXTERNAL COMMAND INPUTS.)
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