The brain and dementia

2. Parts of the brain

Parts of the brain diagram to show what happens to the brain with dementia.A cross-section diagram of the brain, showing the cerebral hemisphere, brain stem, limbic system and cerebellum

The brain can be divided into different parts: the brain stem and cerebellum, the limbic system, and the cerebral hemispheres (see Figure 1 on right). Each part has different functions.

Brain stem and cerebellum

The brain stem is at the base of the brain. It controls basic bodily functions such as heartbeat and breathing. The nearby cerebellum controls balance and posture. Breathing and staying upright are things that we normally do automatically.

The limbic system

The limbic system is deep inside the brain. It links the brain stem and the cerebral hemispheres. The limbic system includes structures with key roles in memory (the hippocampus) and emotions (the amygdala).

Cerebral hemispheres

The tissue that makes up three-quarters of the brain is called the cerebrum. It is responsible for consciousness, memory, reasoning, language and social skills. A deep groove that runs from the front to the back of the cerebrum divides it into left and right halves: the two cerebral hemispheres.

The left and right cerebral hemispheres have different functions. For example, language is usually dealt with mainly by the left hemisphere. In contrast, awareness of where things are around us is usually dealt with mainly by the right hemisphere.

The surface of the cerebral hemispheres is covered by a thin layer known as the cerebral cortex, sometimes just called the cortex. It contains billions of brain cells called grey matter.

Underneath the cortex are bundles of nerve fibres known as white matter. These transport nerve signals between parts of the cortex and from the cortex to other parts of the brain.

The cortex of each cerebral hemisphere is divided into lobes. There are four lobes in each hemisphere.

The lobes

Cross-section diagram of the brain, showing the temporal lobe, parietal lobe, frontal lobe and occipital lobe

The four lobes are: occipital, temporal, parietal and frontal (see Figure 2 on right). Each lobe does different things, though they also work closely together.

Occipital lobes

The occipital lobes at the back of the brain deal with visual information. When we look at something, light entering the eye is converted into electrical signals to the brain. These signals are analysed first by the visual cortex in the occipital lobes. Damage here can cause blindness. For more information see 'Vision' below.

Parietal lobes

The parietal lobes are in the upper-rear part of the brain. They mainly handle information from our senses about space, perception and size.

The left parietal lobe allows us to tell our left from our right side and where a limb is in front of us. For example, it helps us to bring a fork up to our mouth when we eat. Damage to this lobe is common in Alzheimer's disease and can lead to clumsiness (apraxia), for example when putting on clothes.

The left parietal lobe also plays an important role in reading, writing and processing numbers.

The right parietal lobe helps us recognise objects as three-dimensional. It also helps us to work out where objects - including moving objects - are in relation to each other, and to ourselves. These abilities are used when we pick an object up. Damage to the parietal lobes can cause someone problems with finding their way around places.

Temporal lobes

The temporal lobes are on either side of the brain, near the temples. They deal with memory (including recognition of faces and objects) and language.

Our day-to-day memory of personal experiences (known as episodic memory) is very closely linked to the hippocampus, which is inside the temporal lobe on each side of the brain. The importance of the hippocampus in episodic memory and dementia is explained in the 'Memory' section below.

The outer part of each temporal lobe is where we store general knowledge, which is a different type of memory (known as semantic memory). The left temporal lobe usually deals with facts, the meanings of words and the names of objects. This lobe is central to understanding speech and talking. The right temporal lobe usually deals with visual material. This lobe is central to recognising familiar objects and faces.

Frontal lobes

The frontal lobes are large and complex. They have a wide range of functions. Overall, the frontal lobes are a kind of 'management centre'. They deal with solving problems, setting goals and making decisions, as well as with starting, carrying out and finishing tasks.

This management role is called 'executive function'. We use it when we follow a set of steps, such as making a cup of tea. To do such tasks we have to maintain attention. We also have to briefly hold information (eg how many sugars we have added to a cup of tea) in our working memory. The frontal lobes play an important role in attention and working memory.

Executive function has different aspects and these are dealt with by different parts of the frontal lobes.

The upper parts of the outer surfaces of the frontal lobes are where we organise and plan actions and learn new tasks (see 'Procedural memory'). For example, when learning to drive, these brain areas (together with areas that control movement) help us put together a complex set of actions so they become automatic. These areas also help us switch between tasks or do more than one thing at once. Without them we would get stuck on a task or not be able to concentrate on anything for long.

The upper parts of the middle surfaces of the frontal lobes are important for our interest and motivation. Damage here can cause someone to become apathetic, lethargic and reluctant to do things. It is important to realise that they are not 'just being lazy'.

The area on the underside of the front of the brain controls our social behaviour. For example, it normally prevents us from saying something inappropriate or acting on impulse. There is more about how the frontal lobes control our behaviour in the section 'Emotion and behaviour'.

At the back of the frontal lobes is the motor cortex. This area deals with the planning of movements and the control of certain muscles, such as when we decide to clap our hands, smile or speak.