Current Clamp and Voltage Clamp

In a current-clamp experiment:

one applies a known constant or time-varying current

and

measures the change in membrane potential

caused by the applied current.

 

This type of experiment mimics the current produced by a synaptic input.

 

In a voltage clamp experiment:

one controls the membrane voltage

and

measures the transmembrane current

required to maintain that voltage.

 

Despite the fact that voltage clamp does not mimic a process found in nature, there are three reasons to do such an experiment:

(1) Clamping the voltage eliminates the capacitive current, except for a brief time following a step to a new voltage. The brevity of the capacitive current depends on many factors that are discussed in following chapters.

(2) Except for the brief charging time, the currents that flow are proportional only to the membrane conductance,i.e., to the number of channels.

(3) If channel gating is determined by the transmembrane voltage alone (and is insensitive to other parameters such as the current and the history of the voltage), voltage clamp offers control over the key variable that determines the opening and closing of ion channels.