Shown below is a general purpose voltage clamp circuit used for experiments on
squid axons for many years and simulated in our 1975 series of
papers (
Moore, J. W., F. Ramo'n & R. Joyner. (1975). Axon
voltage-clamp simulations. I. Methods and tests. Biophys. J.
15: 11-24.).
NEURON provides for simulation with the similar realistic control circuit
whose parameters noted in the figure below
show up in the Point Process
Viewer for the "V Clamp".
One feature of this voltage clamp simulator is that it can reproduce our experimental circuits where we standardly ran a quick test the electrophysiological condition of the axon just before voltage clamping. I used a large variable resistance between the output of the control amplifier and the current injecting electrode. In the simulated circuit it is represented by "rstim" but it was also called "access resistance" in my papers. With a large value of rstim, a command voltage pulse is delivered to the axon as if from a current source, allowing an action potential to be generated. When "rstim" was reduced, the controlled source approaches a voltage source and clamped the membrane to the command potential.
Examples of current and voltage plots under the two conditions
