THE NORMAL BRAIN AND HOW IT AGES: THE STRUCTURE OF THE BRAIN
The brain can be divided in simple terms into four parts. Two of these are the cerebral hemispheres, a left and a right, stretching from just behind the eye to the back of the head. Each hemisphere is divided into four different lobes. The lobe at the front – that which is just behind the eye – is called the frontal lobe, and among its many functions is the control of our behavioural pattern.
Behind the frontal lobe is the parietal lobe and behind that, at the back, is the occipital lobe. The parietal lobe has many functions and is probably best considered as one of the most important parts of the brain for the interpretation and correlation of sensory input — what we can feel when we touch something or are touched. It is also responsible for our interpretation of our body image, that is, our image of our own bodies. Damage to this system may result in an individual neglecting a useful limb or failing to recognize that a limb is paralysed, as occurs after some strokes. The parietal lobe is also responsible for coordinating some functional activities, so that damage to it may result in an inability to execute planned movements, for example doing up buttons. This should be distinguished from a similar difficulty that can result from damage to nerves and muscles. The occipital lobe is important for the interpretation of visual sensation and damage to it will result in disturbance of vision.
The fourth lobe of the brain, the temporal lobe, which lies a little below the parietal lobe and behind that part of the skull that is often referred to as our ‘temple’, is important for many of the dementias, especially Alzheimer’s disease. In lower forms of animal life it is particularly concerned with taste and smell, as it is also in humans. In us, however, it is most important for the organization and processing of memory and has close connections with other areas of the brain. Loss of memory is of course one of the first signs in the development of a dementing illness. The temporal lobe is also important for some aspects of hearing and speech.
As well as connections occurring within the brain between one part and another, each of the hemispheres sends out nerve fibres to the rest of the body, mainly to control what the muscles in the limbs do. They also receive fibres from sensory organs, such as those that are hidden in the skin that tell us about sensations of touch, pain, and temperature. Strangely – and nobody really understands why – each hemisphere makes its connections with the other side of the body; in other words the left hemisphere controls movements in, and receives information from, the right side of the body.
At the back of the brain, sitting below the hemispheres, is a small structure about the size of a man’s thumb. This is called the brain-stem and it joins the hemispheres to the spinal cord. As well as being a structure that has fibres passing through it, to and from the hemispheres and the body, it also has its own specialized functions. These include many of the nerve cells that control the muscles of our face, the muscles that move our eyes and those that are responsible for eating and swallowing, etc. It is also to this area of the brain that sensation from the face is first sent. In addition, the brain-stem contains special centres that are responsible for controlling our breathing, our heartbeat, and other body functions automatically. These are, of course, vital centres that help to keep us alive and as they and many other structures are crammed together in a very small area of brain tissue, damage in the brain-stem can have a very profound effect upon the integrity of the rest of the body. The brain-stem is affected in many of the illnesses that cause dementia, but probably to a lesser extent than the cerebral hemispheres.
Sitting beneath the occipital lobes and behind the brain-stem is the cerebellum, whose importance lies in the control of the reflexes that determine our posture and the state of contraction of the muscles. It is essential for maintaining the body’s equilibrium and for the performance of smoothly coordinated muscular actions. It is involved in some types of dementia, but not very much, if at all, in the commoner illnesses that cause intellectual impairment.
Within the brain are hollow spaces that contain the same fluid, the CSF, that has been mentioned already. The CSF in these hollow spaces, which are called ventricles, escapes through small openings to join the CSF that is contained within the membranes surrounding the brain. A disturbance to this system can result in a particular form of dementia that will be described in a later chapter.
The brain is, roughly speaking, also divided up into white matter and grey matter. The grey matter is not really grey in the living brain but is so described because of the appearance it takes on when it is treated in a particular way after a person has died. It is these grey areas that contain the bodies of the nerve cells, as described below. The largest collection of grey matter lies on the outside of our brains, rather like the rind of an orange, and it is called the cerebral cortex. This structure is affected in many of the dementing illnesses. Buried deep within the brain are other collections of grey matter, some of which are also affected in some of the conditions that cause dementia.
The white matter is only white because it contains a lot of fatty material. This is made up of nerve fibres which pass from the cell bodies to other areas of the brain or the body – the arms and legs for example. Most of these nerve fibres are surrounded by a fatty material which has an important function in helping messages travel down the fibres. It is this fatty material that is white, giving the brain its characteristic appearance. In summary, therefore, the brain can be considered to contain grey matter, which is the cell body of the nerve cells, and white matter which is made up of the fibres either connecting different nerve cells, or parts of them, or travelling to the rest of the body outside the brain.
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