The McGill Physiology Virtual Lab

RMP Laboratory

RMP >  Recording Circuit
  The circuit used for measuring the membrane potential is schematized to the right. A recording electrode made of glass (filled with 3 M KCl and with a fine chlorided silver wire inserted inside) is lowered into a muscle cell. The voltage measured is then sent to a high impedance pre-amplifier, and then to the recording system. This page deals with some technical details regarding this recording setup.

Glass micro-electrodes

Electrodes are made of materials which can participate in a reversible reaction with one of the ions in the solution or electrolyte. This permits the conversion of ionic current in solution into electron current in wires.

The most frequently used electrode material in electrophysiology is a silver (Ag) wire coated with a composite of Ag and silver-chloride (AgCl). The following reaction takes place:

Cl- + Ag <--> AgCl + e-

in a solution filled glass micropipette.

The glass micropipette is heated and pulled to a fine tip. It is then filled with the electrolyte solution which provides the necessary fluid bridge between the cell and the electrode.
Very small tip diameters can minimize the electrolyte from entering the cell and changing its normal anion and cation content; however this is done at the expense of noise, diminishing current passing ability and limiting recording bandwidth.

The composition of the electrolyte depends on the type of measurement made. The concentration of the electrolyte is also important: high electrolyte concentrations reduces the electrode resistance, lowers voltage noise and provides a wider recording bandwidth.

Here are some questions to think about:
Why should the microelectrode be filled with KCl as opposed to say NaCl?
What happens to the resistance of the microelectrode if the tip breaks? if the tip is plugged?

A glass micro-pipette: note the fine tip

The microelectrode holder is designed to provide an electrical coupling between the  fluid-filled glass pipette and  the high input impedance pre-amplifier. The wire fits inside the shaft of the electrode above and the glass pipette is secured inside the holder by adjusting the Plexiglas screw.


Ions in solutions and electrodes:
the tip potential

The linear relation between potential difference and current flow, as given by Ohm's law applies to aqueous ionic solutions (cytoplasm...). However certain complications arise due to the following problems:
The Ag/AgCl electrode performs well only in solutions containing Cl- (see the equation above). Since current must flow in a complete circuit: two chlorided silver electrodes are needed. If the electrodes are immersed in two different concentrations of chloride solutions, as they would be when one electrode is in the bathing fluid of the muscle and the other inside the micropipette, there will be a difference in the half-cell potentials (potential difference between the solution and the electrode) at the two electrodes. This tip potential can be subtracted electronically or compensated by adjusting the voltage offset.

The pre-amplifier used in the recording of membrane potential: it has connections to the bath electrode, and the micro-electrode. The output goes to the Powerlab recording system. The position knob adjusts the baseline to zero, when the tip potential is recorded.


High impedance pre-amplifier

The amplifier used is a special DC, high resistance (>10^10 ohms) input unit. The main function of this amplifier is to act as a resistance matching device between the microelectrode (high resistance) and the recording system (low resistance). The reason why this high input impedance amplifier is needed is explained below:
Case 1. Without the high resistance amplifier.
Suppose that the micro-electrode (107
W) is connected directly to the low impedance ( 106W)recording system, and that Em = the membrane potential. During the recording, current will flow through both the high resistance of the recording electrode (Re) and the low resistance of the recording system (Ro). Since these two resistances are in series, the total resistance RT is Re + Ro.

The current flowing in this circuit (from V=IR) will be:
Because the sum of the two resistances is quite low, the current is very high. This is undesirable as it will alter the ionic environment of the cell.
During the flow of this current, a large fraction of the total Em will appear as a voltage drop across the microelectrode:

and only a small fraction of the total voltage drop will occur across the recording system:

Since Vo is our estimate of Em, we naturally want Vo to equal Em as closely as possible. In the case above, Vo will be only a small fraction  (< 1/10) of the actual Em, and this is obviously undesirable.
Case 2. With the high resistance amplifier.
On the other hand, if the microelectrode is connected to a high resistance (Rr = 10^10 ohms) amplifier, the situation is greatly improved.

The current flow will be much reduced:


In addition, most of the voltage drop will now occur across the amplifier, and not across the microelectrode:

Hence, the recorded potential will be almost identical with the actual Em. Therefore using the high resistance amplifier has allowed us to:

1) accurately measure the resting membrane potential
2) significantly reduce the current flowing in the circuit

Please note another advantage of using the high resistance amplifier:

3) The sensitivity of the Em estimate (Vr) to changes in Re (if the electrode is slightly plugged or broken) will be greatly reduced.

To continue to the next section: Procedure, click here