The reason for this becomes easy to understand when you consider what the two different approaches are doing.

In essence the problem is that you are injecting current and measuring voltage through the same electrode, and this leads to a certain error. In voltage clamp you want to do is inject an equal amount of current through your pipette as the one passed through the electrical conductance of the cell membrane, thereby keeping Vm constant at your set value. This is done by a circuit that subtracts the measured Vm from the set value and injects a proportionate amount of current. It is this current that you're measuring in this setting, and assuming that it represents the current through your channel of interest.

The problem with series resistance in this case is that the voltage-drop across this (which has nothing to do with the biologically relevant current you're trying to balance out and therefore measure) depends on the magnitude of the current you're injecting (simple Ohm's law..), which in voltage clamp is not a stable variable, and therefore the error will not be stable either. The larger the current you're trying to measure, the larger the error and this makes traces with large changes in current verry difficult to interpret and sometimes visibly unreliable (oscillations, etc..).

There are ways to reduce this error of course. Using low resistance pipettes, measuring smaller currents, using the series resistance compensation circuit on your new and shiny amplifier, etc. I think many amplifiers today switch between measuring voltage and injecting current in a high frequency manner, but this is also limited by the RC characteristics of your pipette so keeping pipette resistance as low as possible is always a good idea.

Current clamping has less of this problem as you're actually keeping the injected amount of current constant (or at least much more constant than the dynamically changing current values during voltage clamp). Since you can estimate the series resistance fairly well, compensating for the voltage drop across the series resistance of your access to the cell becomes more easily done by something called a bridge-balance circuit. This simply compensates for this linear error and does post-recording cosmetics on your voltage follower trace.

A nice illustration and some much better writing than mine about this can be found in many places but for instance here:

http://www.biologie.ens.fr/~barbour/electronics_for_electrophysiologists.pdf

Cheers!

The reason for this becomes easy to understand when you consider what the two different approaches are doing.

In essence the problem is that you are injecting current and measuring voltage through the same electrode, and this leads to a certain error. In voltage clamp you want to do is inject an equal amount of current through your pipette as the one passed through the electrical conductance of the cell membrane, thereby keeping Vm constant at your set value. This is done by a circuit that subtracts the measured Vm from the set value and injects a proportionate amount of current. It is this current that you're measuring in this setting, and assuming that it represents the current through your channel of interest.

The problem with series resistance in this case is that the voltage-drop across this (which has nothing to do with the biologically relevant current you're trying to balance out and therefore measure) depends on the magnitude of the current you're injecting (simple Ohm's law..), which in voltage clamp is not a stable variable, and therefore the error will not be stable either. The larger the current you're trying to measure, the larger the error and this makes traces with large changes in current verry difficult to interpret and sometimes visibly unreliable (oscillations, etc..).

There are ways to reduce this error of course. Using low resistance pipettes, measuring smaller currents, using the series resistance compensation circuit on your new and shiny amplifier, etc. I think many amplifiers today switch between measuring voltage and injecting current in a high frequency manner, but this is also limited by the RC characteristics of your pipette so keeping pipette resistance as low as possible is always a good idea.

Current clamping has less of this problem as you're actually keeping the injected amount of current constant (or at least much more constant than the dynamically changing current values during voltage clamp). Since you can estimate the series resistance fairly well, compensating for the voltage drop across the series resistance of your access to the cell becomes more easily done by something called a bridge-balance circuit. This simply compensates for this linear error and does post-recording cosmetics on your voltage follower trace.

A nice illustration and some much better writing than mine about this can be found in many places but for instance here:

http://www.biologie.ens.fr/~barbour/electronics_for_electrophysiologists.pdf

Cheers!

As said above, series resistance is only an issue when you are recording voltage and passing current through the same electrode at the same time. This is true in current and voltage clamp. The issue is the same in both configurations: you don't know how much of the recorded voltage drop is across the cell membrane resistance and how much is across the electrode resistance (the series resistance error). The problem tends to be magnified in VC because the clamp is using measured voltage to modulate feedback, but the issue is fundamentally the same. There is actually a detailed discussion of this issue under another RG question a couple of weeks ago. The gold standard for not having to deal with Rs errors is to use two-electrodes, one for current passing and the other for voltage recording. This is not possible in small cells. Next best is to time share the same electrode on a duty cycle (discontinuous single electrode clamp), but this requires a low resistance electrode and relatively iso-potential cell. What most people do, is share the electrode continuously and deal as best they can by using the lowest possible resistance electrode, small amplitude current passing, and electronic compensation (which is far from perfect). In this last case, some caution is required because you probably don't know the exact value of your membrane potential.

Hi simon,

As mentioned above gave you an excellent answers for you question from technically and theratically point of view. Of course, there are several factors affacting series resistance not only under current-clmap but also voltage-clamp. From practical point of view, the most important one is the re-seal (the membrane attampts to close) compared with other factor simply becasue the seal changes all the time during recordings but not others. If this happens, it would delay the onset of currents and definitely affect the the peak of currents and voltage-dependent property, especially fast activating currents, such Na+ currents. In order to minimize the effect of re-seal, you have to use a larger tip of recording electrode (lower resistance); give a less negative pressure during whole-cell process (less membrane would be pull into the tip and less chance of re-seal), and maintain a gentle negative presure during recordings. These may help you to relatively stable the series resistance during the voltage-clamp recordings. In some case, you may see the current drop time-dependently and you think this is run-down, but actually not. To test if the current drop is becasue of the re-seal, you just gently apply a negative presure (without affecting giga-seal), the current would be increased significantly. Good luck.