The most important cells in the brain, the neurones which make up the grey matter, consist of a cell body and the nerve fibres passing from it. The cell body is the powerhouse of the neurone and contains a structure called the nucleus, from which emanate all the instructions for the biochemical processes that occur within the rest of the cell. There is one major nerve fibre called the axon and this can be very long. The end of the axon usually breaks up into many branches, each of which makes contact with the cell body or branches of nerve fibres from other cells. Those that leave the brain either make contact with other nerve cells, in the spinal cord for example, or with other structures such as sensory organs or muscles. There are also numerous shorter nerve fibres attached to each cell body, and these are called dendrites. These make contact with parts of many other neurones. It is thought that some nerve cells die as people grow older, but that the branching of the remaining nerve fibres increases to make up the connections that are lost because cells have died. It seems as if in some types of dementia this compensatory increase in the branching is abnormal or reduced.
As well as neurones there are many other types of cell in the brain. These are usually described collectively as neuroglia. There are several different types of neuroglia, and the function of some of them is uncertain. It used to be considered that they had a principally supportive function but we now know about some of their other functions too. As far as the dementias are concerned, it is possible that one of the more important things neuroglia do is to produce chemicals that other brain cells need, and that this process may become disturbed, leading to a malfunction of neurones which results in their death or prevents them from working properly.
Inside each neurone lies a complicated structure of little fibrils and tubules. This is sometimes referred to as the skeleton of the cell but it is also important in the passage of substances from one end of a nerve fibre to the other. In some types of dementia, including that caused by Alzheimer’s disease, it may well be that a disturbance of these structures and their transport function affects the way the nerve cells work.
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