Question

The amplitude of the ventricular action potential as measured with an intracellular microelectrode is some 100 times greater than the QRS wave of the ECG, which represents the ventricular depolarization. Why is this the case? In your discussion focus on the methods used to make these measurements.

Answer 1

Electrodes measure a bioelectric event by converting an ionic potential into an electric potential. Several classes of electrodes exist, namely, 1) Microelectrodes, 2) Surface Electrodes and 3) Needle electrodes.

Microelectrodes are used to measure the biological potential of the cell. As the name suggests, these electrodes have tips that are sufficiently small enough to penetrate a single cell, without damaging it, and also obtain readings from within the cell. Microelectrodes are used for cell attached or whole cell potential recording. In the former case, a metal microelectrode or a glass micropipette touches the cell membrane forming a high-ohmic junction or applies suction to make an uniform connection with the inside of the cell. Microelectrodes have high impedances in mega ohm range because of their smaller size, the typical requirements of a biopotential amplifier. The basic function of a biopotential amplifier is to increase the amplitude of a weak electric signal of biological origin. One of the typical requirements of a biopotential amplifier is that it should have high input impedance, greater than 10 mega ohms. this condition is successfully met by the microelectrode.

In contrast, ECG registers the potential differences in the heart by recording from the body surface, where electrodes are placed in contact with the skin. Such surface electrodes does have a greater surface area and therefore a lower input impedance, relative to microelectrodes. Therefore the amplitude of ventricular action potential when measured by an intracellular microelectrode (small surface area, high input impedance) is almost 100 times more than the corresponding QRS wave of ECG (larger surface area, low input impedance).