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The EEG is a recording of the brain?s electrical activity, in most cases made from electrodes over the surface of the scalp. It may also be made from electrodes placed directly over the surface of the brain or from needle electrodes inserted into the brain. The recordings are the summation of volume conductor fields produced by millions of interconnecting neurones. The neurone components producing the currents are the dendrites, axons and cell bodies. The architecture of the brain is not uniform but varies with different locations. Thus the EEG can vary depending on the location of the recording electrodes.

Sensory information is transmitted to the brain by frequency modulated trains of action potentials which cause neurone activity in particular regions of the brain depending on the type of sensory information and the site of stimulus in the body. Similarly the decision to initiate a movement, in response to sensory information, arises in various parts of the brain, depending on the type of movement and its location in the body, and gives rise to electrical activity at the corresponding sites.

When analysing the EEG it is convenient to think of the brain as three sections: cerebrum, cerebellum and brain stem. The brain stem is the oldest part in evolutionary terms, and its structure, size and function have changed little in the evolution of the vertebrates.

It is an extension of the spinal chord and has three main functions:
1. Connecting link between the cerebral cortex, cerebellum and spinal chord,
2. Control centre for basic body functions such as respiration, heart and blood
flow regulation,
3. Integration centre for complex reflexes, such as maintenance of body
position and posture.
The cerebellum coordinates voluntary muscle movements and maintains balance. The cerebrum is the dominant part of the central nervous system and has centres for conscious appreciation of sensation, initiation of movement, complex analysis and expressions of emotions and behaviour

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