Mode Control Switches
The most important selection is between the voltage clamp and current clamp modes. This selector switch also has an intermediate position, labelled I=0, which is identical to current clamp except that the normal command inputs do not cause any current to be passed. In voltage clamp, the Optopatch passes a current so as to keep the electrode potential equal to the command potential, and the current is measured. In current clamp, the Optopatch passes a current that is proportional to the command voltage, and the electrode potential is measured. In I=0, the electrode potential is normally measured in the absence of any applied current. This mode is very useful for setting up, or for using the Optopatch as a conventional microelectrode amplifier. However, currents CAN still be passed in this mode if required, by applying a command signal to the (self-contradictory!) I=0 input, which is active only in I=0 mode, and the utility of this will be described later. Please refer to the Plymouth book for further general description of how to use these modes in practice, as the Optopatch is completely conventional in operation in all other respects. The only specific point we should make here is that some patch clamps have separate "fast" and "slow" current clamp modes, in an attempt to provide alternative performance compromises with the conventional-current passing circuit described above, but we need only one current clamp mode.
The other selection is the current-passing range, and the three modes here are called "patch", "small cell" and "big cell". In patch mode, the current sensitivity is high, giving a maximum of 1nA, which corresponds to a 10V output. This situation is equivalent to a 10 gigohm feedback resistor from the signal amplitude point of view. Such currents are large enough to voltage-clamp some whole cells, as well as patches, so we have provided a "small cell" mode, in which the additional facilities needed for whole-cell recordings come into operation, but which can retain the high current sensitivity. "Big cell" mode corresponds to normal cell mode on other patch clamps, and in this mode currents of up to 100nA can be passed, which is equivalent to a resistive headstage with a 100 Megohm feedback resistor. Since the optical current-passing system works well over a very wide current range, we did not feel it necessary to include any current ranges intermediate between these two. However, it is possible to select the higher (100nA) current range instead of 1nA in small cell mode by a switch on the rear panel (actually on the headstage control subpanel). That may appear to be pointless, since one could just select big cell mode instead, but the real difference between the two cell modes is the amount of cell capacitance that can be compensated, as explained next. There are no problems in using the 100nA range when the actual current requirements are considerably lower than this, and the gain stage, described later, has more than enough amplification available to deal with such recording conditions.
In both cell modes, the circuitry for membrane capacitance and access (electrode) resistance compensation - and measurement - comes into operation. The resistance compensation range is from 1 Megohm to (in theory!) 1 gigohm in both cell modes, but the capacitance range increases from 10pF (or 20pF when an offset is selected) in small cell mode to 100pF (200pF) in big cell mode.
Membrane capacitance compensation is active only in voltage clamp mode, but access resistance compensation also occurs in current clamp mode, although it is implemented in a different way. The operation of those controls in voltage clamp mode is quite complicated, so we won't go into that subject here. However, the situation for access resistance compensation is much simpler in current clamp mode, so we can describe it briefly here as well. Referring back to the circuit diagrams that showed the various current clamp circuits, one can see that in all cases the applied current passes through the electrode resistance. Therefore there is a potential difference between the tip of the electrode (which is where we want to be making the measurements) and the headstage input (which is where we are actually making them). However, it is possible to correct for this effect by subtracting an appropriate proportion of the command current input from the voltage output, using what is often, but not entirely correctly, described as a bridge circuit. Again, the Plymouth book describes the use of this type of facility in detail. Note that the access resistance ALSO causes errors in voltage clamp mode, but they are not so easy to correct. This will be discussed in detail later.
