From the 1975 programme:
It is one of the properties of muscle and nerve cells that their activity is accompanied by electrical events which can be detected outside the cells. An extreme manifestation of this effect is found in electric fish where special cells similar to muscle cells are connected in series like a multi-cell battery to produce external voltages sufficient to stun other fish.
This does not mean that muscles are in any sense electrically operated, or that nerves convey signals in the same way as copper wires, but only that chemical changes which take place at cell membranes when they are active also produce electrical signals. These sometimes may start further chemical changes in neighbouring cells thus stimulating them into activity or in the case of a nerve fibre will stimulate the next section of fibre thus causing the activity to travel along it.
Tissue in bulk is quite a good conductor of electricity because it consists mainly of salty water, and skin can be made reasonably conductive by appropriate treatment. In consequence electrodes applied to the skin can pick up signals from tissues deep inside the body. There are problems, however, just due to the fact that the whole of the inside of the body is conducting, because signals from many different sources can be mixed up with one another at the site of the electrodes. It is rather like fixing a microphone on to the outside wall of a large room in which there is a noisy party. Individual conversations, unless they happened to be very close to the microphone, would be impossible to distinguish, but you might be able to say that a lot of talking was going on. The band which produces a large and organised signal would come through quite clearly, and if a number of the party-goers all started to do the same thing like shouting 'fire', or encouraging the teams in a tug-of-war, then their collective signal would become apparent. If one wants to investigate the electrical activity of the body one faces exactly the same problem. An organ like the heart behaves rather like the band, and its signal can be detected almost anywhere on the body; with other tissues, like specific muscles, you try to attach the electrodes as close to the source of the signals as possible, whilst in the case of the brain you may most of the time have to be satisfied with something like the party noise. Nevertheless, the analysis of electrical signals available at the skin surface represents a major source of diagnostic information which can usually be obtained with very little trouble or discomfort.