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THE NORMAL BRAIN AND HOW IT AGES: THE BRAIN’S MESSAGE SYSTEM

As has been mentioned already, nerve fibres connect with parts of other nerve cells. The message travels down the nerve fibre using a process that is often likened to an electrical current flowing down a wire. This is a convenient way of thinking about it, and although it isn’t quite right, for our purposes it is a useful analogy. When it gets to the end of the nerve fibre the message has to ‘switch on’ the next nerve cell or one of its dendrites, and there is a special system to make this possible.

Although it looks as if the connecting parts of two nerve cells are actually in contact with each other when they are viewed through an ordinary microscope, we know from the electron microscope, which can produce an even higher magnification, that there is in fact a gap between the axon that is bringing the message and the part of the cell that is going to receive it. This gap is extremely narrow and the way the first nerve cell activates the second is by releasing a special chemical into this gap. The chemical travels across the gap until it hits a specialized area on the cell which is receiving the message, and the interaction between the chemical and the receiving area, which is called a receptor, switches on the second cell. This neurone then either transmits the message to further cells by a similar process, or reacts to the messages received in some other way.

These chemical messengers are known as neurotransmitters, and if they were to stay in the gap, which is in fact called a synapse, they would continue to stimulate the second nerve cell, which in many cases would result in its death as it would be over-stimulated. The body has, therefore, very cleverly arranged for other chemicals to be present at many of these synapses so that they can break down the neurotransmitter after it has done its job. In many of the illnesses that cause dementia, such as Alzheimer’s disease, there is a gross disturbance of the chemical neurotransmitter system. Developing medicines to try to put this right is one of the approaches to treatment that is still being experimented with, as is described in a later chapter.

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