What is discontinuous single electrode voltage-clamp (dSEVC)?
In discontinuous single-electrode voltage-clamp (dSEVC), the tasks of voltage recording and current passing are allocated to the same micropipette. Time-sharing techniques are used to prevent interactions between the two tasks (Fig. 1).
Figure 1. Circuit drawing of the discontinuous single electrode voltage-clamp.
A single micropipette (ME1) penetrates the cell and the voltage recorded (Vp) is buffered by a unity-gain headstage (A1). Assume that Vpis exactly equal to the instantaneous membrane potential (Vm). A sample-and-hold circuit (SH1) samples Vmand holds the recorded value (Vms) for the rest of the cycle.
Vmsis compared with a command voltage (Vcmd) in a differential amplifier (A2). The output of this amplifier becomes the input of a controlled-current source (CCS) if the switch S1 is in the current-passing position. The CCS injects a current into the micropipette that is directly proportional to the voltage at the input of the CCS irrespective of the resistance of the micropipette. The gain of this transconductance circuit is GƮ.
The period of current injection is illustrated at the start of the timing waveform.
Figure 2. Timing waves of the discontinuous single electrode voltage-clamp.
S1 is shown in the current-passing position during which a square pulse of current is injected into the micropipette, causing a rise in Vp. The rate of the rise is limited by the parasitic effects of the capacitance through the wall of the glass micropipette to the solution and the capacitance at the input of the buffer amplifier. The final value of Vpmostly consists of the IR voltage drop across the micropipette due to the passage of current Iothrough the micropipette resistance Rp. Only a tiny fraction of Vpconsists of the membrane potential (Vm) is recorded at the tip.
S1 then switches to the voltage-recording position. When the input of the CCS is 0 volts, its output current is zero and Vppassively decays. During the voltage-recording period, Vpdecays asymptotically towards Vm. Sufficient time must be allowed for Vpto reach within a millivolt or less of Vm. This requires a period of up to nine micropipette time constants (ʈp). At the end of the voltage-recording period, a new sample of Vmis taken and a new cycle begins. The actual voltage used for recording purposes is Vms.
As illustrated in the bottom timing waveform, Vmsmoves in small increments about the average value. The difference between Vms(avg)and Vcmd is the steady-state error (Ɛ) of the clamp that arises because the gain (GƮ) of the CCS is finite. The error becomes progressively smaller as GƮ is increased.
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