Guide to Electrophysiological Recording • 53
Chapter 4
Check the Tuning checkbox and observe the Scaled Output: Membrane Potential on a
scope. Move the electrode tip close to where cells are likely to be encountered, and
then increase Pipette Capacitance Neutralization in the MultiClamp Commander to
give the fastest step response. It is advisable to adjust the capacitance neutralization
with the microelectrode as close as possible to the final position, since moving the
electrode can change C
t
and invalidate the setting. It may be wise to slightly under-
compensate, because changes in the solution level could lead to oscillations that may
destroy the cell.
Press the Bridge Balance button. The value (M) found for optimal balance gives the
resistance of the electrode. See Chapter 5,
BRIDGE BALANCE, for further details.
Sometimes the cell is impaled as soon as the microelectrode is pressed against the cell
surface. More often the microelectrode is advanced until there is a slight deflection in
the tip potential. At this point the cell can be impaled by pressing the Buzz button or
the Clear +/Clear - buttons. If these fail, vibrating the microelectrode tip by lightly
tapping on the micromanipulator sometimes works. When the electrode penetrates the
cell there is a sudden change in the Membrane Potential trace, reflecting the
intracellular potential. The voltage response to the Tuning steps will be slower and
much larger, reflecting the membrane time constant and input resistance. After
impaling the cell, it is often helpful to back off the microelectrode slightly and allow
the penetration to stabilize for a few minutes. For some cells it may help to apply a
small DC current to the electrode (enough to produce several mV hyperpolarization)
during the penetration. Selecting the Holding checkbox and slowly increasing the
Holding value can apply this DC current.
Once the penetration has stabilized, you should recheck the Bridge Balance and Pipette
Capacitance Neutralization. Further details on this are given in Chapter 5. It is
sometimes useful to inject a small, brief current pulse at the start of each sweep of data
collection in order to continually check the Bridge Balance setting during the course of
an experiment.
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