Reference Section • 65
Chapter 5
It is recommended that Pipette Capacitance Neutralization be set at the same time as
Bridge Balance, because both the electrode capacitance and the electrode resistance
cause errors if left uncompensated. Also, it is easier to correctly balance the bridge
when electrode capacitance is minimized, because the “break” between the rapidly
decaying voltage across the microelectrode and the slowly decaying voltage across the
cell’s membrane resistance is more distinct.
The balancing procedure is the same as in the bath, except that the trace appears more
rounded because of the time constant of the cell membrane. Because the Tuning pulse
width is typically brief compared with the membrane time constant, the voltage
response looks like a series of ramping straight lines. To make it easier to see the fast
voltage step in V
p
on an oscilloscope (Figure 4.5B), it is recommended that the scope
input be AC coupled to remove the resting membrane potential from the signal. The
scope gain can then be turned up without the annoying offset. The M value found by
Bridge Balance is the resistance of the electrode, which may be slightly higher than the
value in the bath because of partial blockage of the tip during penetration.
The residual transient at the start and finish of the current step is due to the finite
response speed of the microelectrode, which is determined in part by the capacitance of
the electrode. The transient can be minimized by correctly setting the Pipette
Capacitance Neutralization control. (See Capacitance Neutralization.) Adjust Pipette
Capacitance Neutralization for the most rapid decay without causing an overshoot.
(See Figure 2.28, Chapter 3.)
To Next Page
Loading...
Need help?
General
