Biomedical Engineering Reference
In-Depth Information
Select V
m
Voltage
Clamp
FIGURE 12.23
Physical set up for the voltage clamp experiment.
are placed outside the seawater bath. Today, these would be connected to an analog-to-
digital converter (ADC) with data stored in a hard disk of a computer. Back in 1952, these
meters were strip chart recorders. The application of a clamp voltage,
V
c
, causes a change in
Na
þ
conductance that results in an inward flow of
Na
þ
ions. This causes the membrane
potential to be more positive than
V
c
. The clamp removes positive ions from inside the cell,
which results in no net change in
V
m
.
The current,
I
m
, is the dependent variable in the volt-
age clamp experiment, and
is the independent variable.
To carry out the voltage clamp experiment, the investigator first selects a clamp voltage
and then records the resultant membrane current,
V
c
I
m
, that is necessary to keep
V
m
at the
clamp voltage. Figure 12.24 shows the resulting
I
m
due to a clamp voltage of
20 mV.
Initially, the step change in
causes a large current to pass through the membrane that
is primarily due to the capacitive current. The clamp voltage also creates a constant leakage
current through the membrane that is equal to
I
l
¼
V
c
E
l
R
l
V
m
ð
12
:
38
Þ
0.002
0.001
0
0
0.002
0.004
0.006
0.008
0.01
−
0.001
−
0.002
Time (s)
FIGURE 12.24
Membrane current
I
m
due to a
20 mV voltage clamp.
