Biomedical Engineering Reference
In-Depth Information
Voltage Clamps
To study the variable voltage-time-resistance channels for
Na
þ
, Hodgkin and
Huxley used a voltage clamp to separate these two dynamic mechanisms so only the
time-dependent features of the channel were examined. Figure 12.21 shows the voltage
clamp experiment by using the equivalent circuit model previously described. The channels
for
K
þ
and
K
þ
and
Na
þ
are represented using variable voltage-time resistances, and the passive
Na
þ
,
K
þ
, and
Cl
are given by a leakage channel with resistance
gates for
(that is, the
Thevenin's equivalent circuit of the passive channels). The function of the voltage clamp
is to suspend the interaction between
R
l
K
þ
channel resistance and the membrane
potential, as shown in Figure 12.22. If the membrane voltage is not clamped, then changes
in
Na
þ
and
Na
þ
and
K
þ
channel resistance modify membrane voltage, which then changes
Na
þ
and
K
þ
channel resistance, and so on, as previously described.
A voltage clamp is created by using two sets of electrodes, as shown in Figure 12.23.In
an experiment, one pair injects current,
I
m
, to keep
V
m
constant, and another pair is used to
Na
þ
and
K
þ
channels,
observe
V
m
. To estimate the conductance in the
I
m
is also measured
during the experiment. Meters for recording
V
m
and
I
m
are shown in Figure 12.21. They
Current Meter
Voltage Meter
Extracellular
I
m
I
Na
R
Na
R
K
I
K
R
Cl
Clamp
Voltage
C
m
I
Na
I
K
V
m
-
-
-
E
Na
E
K
E
Cl
+
+
+
I
m
Intracellular
FIGURE 12.21
Equivalent circuit model of an unmyelinated section of squid giant axon under voltage clamp
conditions. The channels for
K
þ
and
Na
þ
are now represented using variable voltage-time resistances, and the pas-
Na
þ
,
K
þ
, and
Cl
are given by a leakage channel with resistance
sive gates for
pump is illustrated
within the shaded area of the circuit. In the experiment, the membrane is immersed in the seawater bath.
R
l
. The
Na-K
Depolarization
Voltage clamp interrupts this
process at this stage
g
Na
Inward I
Na
FIGURE 12.22
Voltage clamp experiment interrupts the cycle shown in Figure 12.20.
