PRINCIPLES OF OPERATION
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93
Chapter 8
Zap
The conventional technique for rupturing a membrane patch to go to whole-cell recording
is to apply a pulse of suction. Sometimes this technique damages the cell.
Zap
provides an
alternative method. It applies a pulse of voltage across the patch that ruptures the patch,
presumably by causing dielectric breakdown. A timing circuit lets you find a Zap duration
that is most likely to achieve the desired result without damaging the seal.
Lipid Bilayers
Experimental Techniques
The integrating (PATCH) configuration is uniquely suited for performing
experiments on lipid bilayer membranes. This is because the probe can be
triggered at the onset of a voltage step so that it can quickly charge the membrane.
Also, because the Axopatch 200B is stable while driving purely capacitive loads
(up to 1000 pF), one has the ability of minimizing noise by minimizing access
resistance.
To realize the full potential of the design, the user should externally initiate a reset
at each voltage step applied to the membrane. This is accomplished by applying a
positive going TTL pulse to the FORCED RESET INPUT BNC on the rear panel.
Because of internal timing delays, the positive going edge of the reset pulse can
occur simultaneously with the command step. The reset time is independent of the
length of the reset pulse (it is factory set at 50 µs).
If it becomes necessary to blank the output for longer time periods, the BLANK
ACTIVATE INPUT BNC should be used. When a HI logic level pulse is applied
to this input, the output signal will be blanked for the duration of the pulse. In this
circumstance, both FORCED RESET and BLANK ACTIVATE inputs could be
tied together. For example, if a 1 ms pulse is applied to both BNCs, a normal reset
will be initiated while blanking the output for 1ms.
The following is an explanation of the reset charging process:
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