Voltage-clamp allows the investigator to control the transmembrane voltage and subsequently measure current flow through an ion channel after activation. An ion channel can be activated by either a change in transmembrane voltage or a selective ligand, acting as a switching mechanism.
Figure. The cellular membrane as a simplified circuit.
In an experiment using the voltage-clamp method, one controls the membrane voltage in a cell and measures the transmembrane current required to maintain that voltage. The ideal voltage-clamp circuit simply consists of a battery, a switch, a wire, the cell and an ammeter. In an experiment, one controls the membrane voltage and measures the transmembrane current required to maintain that voltage.
When the switch closes, the membrane potential steps instantly to the battery voltage. For simplicity, it is assumed that V
m=0 before the step. There is an impulse of current injecting a charge Q=C
mV cmd; the steady-state current is V cmd/Rm. In this experiment, the voltage is the independent variable. Its value is controlled and equal to the battery value.
The current is the dependent variable; its value is measured by an ammeter. For this reason, the voltage-clamp circuit is sometimes called a "current follower".
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