The human body's nervous system uses the ebb and flow of ions to communicate. This ionic transport within and along the nerve fibers can be measured on the surface of the skin using a specific type of electrochemical sensor commonly referred to as the surface recording electrode (sometimes just called the electrode). A good reference on understanding the operation of the electrode can be found in Medical Instrumentation by Webster, 1992.
The purpose of the electrode is to act as a transducer between the ionic transport of the nerve and the electron flow in copper wire. It is the junction between the electrode and the electrolyte that allows such a transduction to take place (see Figure13). The flow of ions in the electrolyte give rise to a flow of electrons (current) in the electrode due to an oxidation or reduction reaction(depending on the direction of current flow) occurring at the interface. What this means, using the example shown in Figure 13, is that anions in the electrolyte will flow to the interface boundary. Cations in the electrolyte will flow away from the interface boundary. To counteract this, electrons in the electrode will flow away from the interface boundary creating a current in the electrode. This process is called oxidation of the metal,C.
Figure 13: The Electrode - Electrolyte Interface
One of the most common types of electrodes is the Ag-AgCl electrode with an electrolyte containing 
. There are two properties that make it a good choice for an electrode. First, it is practically non-polarizable, meaning that current flows freely across the electrode junction. (A polarizable electrode is one in which current does not flow across the electrode junction, thus causing the electrode to behave similarly to a capacitor.) Second, it generates less than 10uV of noise.
In common applications, the Ag-AgCl electrode with electrolyte gel is placed directly on the skin. Figure 14 shows the equivalent circuit for such an arrangement. The entire interface can simply be modeled for most applications as an ideal voltage source with a DC offset and a series impedance. The DC offset is caused by the half cell potentials of the electrode-electrolyte and the electrolyte-skin interface. The impedance can range from 100's of ohms to several Mohms depending on the frequency of the biopotential being measured and preparation of the skin.
Figure 14: The equivalent circuit of an electrode placed on the skin
The surface recording electrode can be used to measure many different biopotentials. For, example, it can be used to measure electrical signals generated from the flexion and extension of the muscles. This signal is refered to as the electromyogram or EMG. This signal varies in frequency from approximately 50Hz to 1000Hz. Its amplitude varies from approximately 10uV to 1mV depending on properties such as the size of the muscle and the amount of exertion.
Another common signal measured by electrodes is the electroencephalogram or EEG. This is the signal caused by neural activity in the brain. It contains frequencies from less than 1 Hz up to 50Hz and amplitudes which are usually less than 10uV. Since the electrode is capable of picking up many other biopotentials in addition to the two mentioned here, its use as an input sensor for HCI is growing rapidly..
