Action potentials from the heart's myocytes cause minute voltages to occur on the surface of the thorax, which can be measured with a machine called an electrocardiograph. Normally, transmembrane voltages are measured with a microelectrode placed inside the cell. However, extracellular electrodes can also detect action potentials within a tissue as illustrated in Fig. 1. Imagine the rectangle to represent a block of cardiac muscle containing many cells. One electrode is placed at one end of the muscle, and another electrode at the opposite end. These electrodes are connected to an amplifier that measures the difference in potential between the two electrodes. One electrode is labeled "positive" and another is labeled "negative." The output of the amplifier is connected to a pen recorder, such that an upward deflection occurs whenever the voltage on the electrode marked "positive" is more positive than that on the electrode marked "negative." Similarly, a downward deflection indicates that the voltage on the "positive" electrode is negative with respect to that at the "negative" electrode.
In the resting state (Fig. 1, panel Al), all of the cells in the muscle strip have a transmembrane voltage difference of about 90 mV with the extracellular region positive with respect to the intracellular region. Thus, at rest, the electrode on the left senses the same positive potential as the electrode on the right so that the difference between the two is zero and no deflection is recorded by the pen recorder (panel B, point Al).
Now consider the consequence of stimulating the strip near the left electrode, causing the cells in that region to depolarize, as shown in Fig. 1, panel A2. With depolarization, positive charge enters the cells, leaving the extracellular space negatively charged. At this time, the right electrode becomes more positive than the left electrode, and the pen is deflected in the positive direction, as illustrated in Fig. 1, panel B, point A2. As the depolarization is conducted from cell to cell, the entire strip is eventually depolarized, as shown in panel A3. At this time, there is again no potential difference between the electrodes, and the pen returns to the center, as shown in panel B, point A3. Soon, the cells at the left side will begin to repolarize, returning the extracellular fluid in that region to a positive state, as shown in panel A4. At that time, the electrode on the left finds itself more positive than the electrode on the right so that the pen deflects downward, as shown in panel B, point A4. Finally, when the entire strip repolarizes, all potential differences disappear and the pen again returns to the center position.
The following two points are important to remember:
1. Extracellular electrodes experience a potential difference only when part of the heart is in a depolarized state and part of the heart is polarized.
2. The tissue ahead of a wave of depolarization is positively charged and that behind the wave of depolarization is negatively charged.
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