Reference Section • 117
Chapter 5
Series Resistance Compensation
• Theory and practice of compensating the series resistance in V-Clamp mode.
• Adjusted using the
controls in the V-Clamp pane.
• See also Capacitance Compensation, Headstage.
Introduction to R
s
Compensation
Series resistance (R
s
) is defined as the total resistance that is interposed between the
circuitry of the headstage and the membrane of the cell. Contributors to R
s
include:
• The resistance of the solution inside the electrode, dominated by that at the narrow
tip.
• The resistance caused by intracellular organelles that partially clog the electrode
tip.
• The resistance due to glial cells or connective tissue that cover the cell membrane.
• The resistance of the bath solution and the bath electrode (usually minor).
Series resistance causes three major problems in voltage clamp recordings.
1. Steady-state voltage errors. Suppose you are measuring a 1 nA membrane current
under V-Clamp. If R
s
= 10 M, there will be a voltage drop of IRs = 1 nA x
10 M = 10 mV across the series resistance. Since R
s
is interposed between the
headstage and the cell membrane, the actual cell membrane potential will be
10 mV different from the command potential at the headstage. (The direction of
the error will depend on the direction of current flow.) Worse, the error will vary
as the membrane current varies. In extreme situations in the presence of voltage-
gated channels, complete loss of control of membrane potential can occur.
2. Dynamic voltage errors. Following a step change in command potential, the
actual cell membrane potential will respond with an exponential time course with a
time constant given by τ
s
= R
s
C
m
, where C
m
is the cell membrane capacitance.
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