The human brain contains about 100 000 million nerve cells, each of which is connected to many others—perhaps as many as 50 000 others. The brain is the organ of our thinking and of our memory. It integrates information from the outside world and so allows us to perceive objects and events around us. It organizes our response to these events by movements or other action. It organizes our social behaviour.
Messages are passed between nerve cells by the extraordinarily rapid secretion of tiny packets of specialized chemicals known as neurotransmitters. As a neurotransmitter acts on the next cell in a chain, a brief electric current is generated. These can be recorded by very fine wires placed next to or in a nerve cell, but they are not large enough to be recorded externally over the skin of the head. However, some cells act in rhythmic concert, and these rhythms can be detected as the electroencephalogram (EEG) over the skin of the hands by small electrodes amplified, recorded on tape or disc, and displayed on a moving strip of paper or screen.
Some messages received by a nerve cell are inhibitory—they dampen down the activity of the receiving cell; some are excitatory, enhancing its activity. The receiving nerve cell computes, as it were, these contrasting messages, which determine its own action.