Guide to Electrophysiological Recording
•
65
Check the Tuning checkbox and observe the Primary 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 700B
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
Ω
) 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.
Chapter 4
