The Voltage Clamp

We saw in Chapter 6 that the permeability of an excitable cell membrane to sodium and potassium depends on the voltage across the membrane. We also saw that the voltage-induced permeability changes occur at different speeds for the different ionic "gates" on the voltage-sensitive channels. This means that the membrane permeability to sodium, for example, is a function of two variables: voltage and time. Thus, in order to study the permeability in a quantitative way, it is necessary to gain experimental control of one of these two variables. We can then hold that one constant and see how permeability varies as a function of the other variable. The voltage clamp is a recording technique that allows us to accomplish this goal. It holds membrane voltage at a constant value; that is, the membrane potential is "clamped" at a particular

Figure 7-1 A schematic diagram of a voltage-clamp apparatus.

Figure 7-1 A schematic diagram of a voltage-clamp apparatus.

level. We can then measure the membrane current flowing at that constant membrane voltage and use the time-course of changes in membrane current as an index of the time-course of the underlying changes in membrane ionic conductance.

A diagram of the apparatus used to voltage clamp an axon is shown in Figure 7-1. Two long, thin wires are threaded longitudinally down the interior of an isolated segment of axon. One wire is used to measure the membrane potential, just as we have done in a number of previous examples using in-tracellular microelectrodes; this wire is connected to one of the inputs of the voltage-clamp amplifier. The other wire is used to pass current into the axon and is connected to the output of the voltage-clamp amplifier. The other input of the amplifier is connected to an external voltage source, the command voltage, that is under the experimenter's control. The command voltage is so named because its value determines the value of resting membrane potential that will be maintained by the voltage-clamp amplifier.

The amplifier in the voltage-clamp circuit is wired in such a way that it feeds a current into the axon that is proportional to the difference between the command voltage and the measured membrane potential, EC - Em. If that difference is zero (that is, if Em = EC), the amplifier puts out no current, and Em will remain stable. If Em does not equal EC, the amplifier will pass a current into the axon to make the membrane potential move toward the command voltage. For example, if Em is -70 mV and EC is -60 mV, then EC - Em is a positive number. Because the amplifier passes a current that is proportional to that difference, the current will also be positive. That is, the injected current will move positive charges into the axon and depolarize the membrane toward EC. This would continue until the membrane potential equals the command potential of -60 mV. On the other hand, if EC were more negative than Em, EC - Em would be a negative number, and the injected current would be negative. In this case, the current would hyperpolarize the axon until the membrane potential equaled the command voltage.

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