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How a VC Experiment Controls Membrane Voltage - Example #2

In the second example, let us set Vc to -40 mV, 30 mV greater than Vm at rest. At the first black arrow at the bottom of the graph, let us activate the circuit that will pass whatever current (figure, orange line) is needed to make Vm = Vc = -40 mV (figure, green line).

To depolarize the cell to -40 mV, positive charge must be brought into the cell to decrease the amount of charge separated across the membrane. The current source brings + charge from the outside to the inside of the cell (how this charge is transferred is illustrated at right). This initial positive current is called the capacitive current (Ic) because it is the current that must be passed to alter the charge (q) on the membrane capacitance to change Vm to Vc.

Remember that dq/dt = I. Therefore, the time integral of current (the area under the I vs. time curve) yields the amount of charge that is transferred across the membrane by the current generator.

After Vm = Vc you would think that the current passed by the current source would go to zero. However, as indicated in the animation, this does not occur; the current passed by the current source remains positive. The reason for this is that when the cell is depolarized, it is no longer in a non-equilibrium steady-state and all the currents do not add up to zero. At -40 mV, the net membrane current flowing through the leakage channels is positive (a net efflux of positive charge flows from the cell). The net positive membrane current tends to repolarize the cell to its original resting potential of -70 mV (see animation). However, the VC circuit does not allow this to occur. When positive current flows through the membrane (Im), the VC circuit brings an equal amount of + charge from outside the cell to the inside to maintain the q on the membrane capacitance at a value that maintains Vm at Vc. Thus, the current flowing through the voltage clamp circuit exactly equals the magnitude and sign of the current flowing through the membrane leakage channels.

At the second black arrow at the bottom of the figure, the circuit is requested to return the membrane potential to its original resting value. To repolarize the membrane, the VC circuit once again passes a capacitive current to return the q on the membrane to its original value. This requires a net transfer of + charge from the inside to the outside of the cell through the current source. By convention, this is a negative current.

These capacitive and leakage currents are always present in a voltage clamp experiment. However, because the voltage clamp technique is typically used to determine the characteristics of the voltage-gated channels, these currents are usually subtracted from the voltage clamp records that are published.
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