Electricity and electrophysiology
Direct and Alternating Current
There are two types of electrical current flow that need to be considered when thinking about
electrical signals in biological/physiological systems; direct current and alternating current.
In a direct current (DC) electrical circuit, electrons move through the circuit from the negative
end of the power source (battery) to the positive end. In between the two ends of the battery they
SDVVRYHUD³ORDG´WKHREMHFWWREHSRZHUHG7KHPRYHPHQWRIHOHFtrons in this single direction
powers the load.
In biological systems, a direct current signal is one that deviates from baseline (or zero) in one
direction at a time. Blood pressure is an example of this. Although blood pressure can go up and
down, it does not routinely make continual alternating positive and negative deflections from the
baseline level. Blood pressure may rise, for example, during exercise and decrease, for example,
during sleep but it is not changing up and down on a constant basis.
In an alternating current (AC) electrical circuit, the direction of electron flow is constantly
reversing. In this case, it is simply the movement of electrons over the load that is required to
power it. The electrons do not need to flow constantly in one direction.
In biological systems, an AC signal is one that routinely changes polarity. For example, during
an action potential membrane potential becomes more positive than resting potential and then
more negative. This always happens and it happens very quickly. AC waves can occur at a range
of frequencies from very low to very rapid. The frequency of AC signals is measured in Hertz
(HZ) which is defined as the number of cycles per second.
Any signal (usually electrical in nature) that interferes with the recording of a biological signal
(or non-biological signal) is called noise. One of the most common forms of noise is 60 Hz (or
60 cycle) interference that exists due to the 120 volt/60Hz electricity that flows through electrical
lines. In other words, any electrical appliance that is plugged into the wall (line) power, cause the
occurrence of 60 cycle noise. This noise can radiate through the air (radiated noise) or move
through electrical lines (conducted noise). Electrical noise of various other frequencies can arise
from internal processes in numerous electrical/mechanical devices.
Noise is generally dealt with in three ways: 1) Turn of the source of the noise (i.e., a computer
monitor). This is often not realistic as an important piece of equipment may be the source of the
noise. 2) Grounding the equipment or the preparation that is being recorded from. When
something is grounded, any electrical noise will be preferentially directed through the ground
wire away from your preparation and recording electrode. Ground wires often connect to the
cold water copper pipes in a lab which serve as an excellent ground. 3) Use electrical filters to
remove unwanted noise.