LIVING WITH ALZHEIMER’S DISEASE: THE NORMAL BRAIN AND HOW IT AGES

Our brain is such an important part of our body, even of our very existence, that it is protected by the bones of the cranial cavity within the skull. Any damage to the head sufficient to break the skull bones has serious consequences for the brain, ranging from loss of consciousness to permanent brain damage, with subsequent impairment of intellect or physical body function.

It is less well known that the brain also has other protective systems. These consist of a series of membranes and a fluid known as the cerebrospinal fluid (CSF). These are arranged in such a way that the brain is enclosed within a tough outer membrane called the dura mater, and within this lies another sack containing the CSF and the brain. Suspending the brain within the bag of fluid protects it not only from the day to day jolts that it would otherwise receive, but also from more serious damage — say a blow on the head that is insufficient to fracture the skull bones. Despite this protection, excessive physical force is still capable of causing damage, although this would be much greater if the fluid wasn’t present. The membranes also stretch tightly between the major subdivisions of the brain, thus limiting the degree to which any part of the brain can be displaced if physical trauma occurs. Our brain is therefore very well-protected from the outside world.

The brain is a pinkish structure, the pink colour coming from the blood that is circulating in the minute blood vessels that nourish the nerve cells. Each brain contains some 10 to 12 billion nerve cells – also called neurones. The brain is so important that it takes up about 20 per cent of the blood that our hearts pump out, and consumes a similar amount of the oxygen that we breathe in through our lungs. Despite requiring such a large amount of blood and oxygen it weighs only a fraction of our total body weight, usually in the region of 2-3 lb (1—1.5 kg). It is often assumed that the larger a person’s brain the more intelligent they are; this is fine for male chauvinists as in general men appear to have larger brains than women! Sadly, however, at least for us men, intelligence or other aspects of mental ability are not related to the size of the brain.

In order to allow us to understand more easily some of the illnesses that cause dementia, this chapter will describe some aspects of the structure of the brain, and how it works. The brain is, however, such an enormously complex organ that it would be confusing, and to a certain extent irrelevant, to explore these subjects at other than a simple level. The description that follows is, therefore, particularly arranged to take into account structures and functions that are important for the understanding of some of the dementing conditions.

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