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In people with Parkinson’s disease there is a degeneration of a specific area of the brain called the substantia nigra. This is a small cluster of cells found within an area called the basal ganglia deep in the centre of the brain. This cluster of cells is usually black in colour, but in people with Parkinson’s disease, this colouring is lost.

This can only be seen when doctors examine the brain tissue after death. Usually, chemicals called neurotransmitters are produced by this area of the brain. They are responsible for helping to send messages between cells within the brain and the rest of the nervous system. In people with Parkinson’s disease there is a substantial reduction in the production of a chemical messenger called dopamine, which is responsible for helping the motor (or movement) coordination centres of the brain (the basal ganglia and the striatum) work efficiently.

A loss of dopamine in people with Parkinson’s disease means that the brain circuits that control movement stop working efficiently. Messages sent by the brain to the muscles do not pass through smoothly, and so ordinary movements like walking, getting up from a chair and putting on clothes become slow and difficult.

The end result is the emergence of the typical signs of the disease: slowness, stiffness and tremor. Doctors tend to refer to slowness of movement as bradykinesia and slowness in starting movements as akinesia, while the stiffness is referred to as rigidity. Normally, the ageing process itself causes the level of dopamine to drop mildly, but in people with Parkinson’s disease around 80 per cent of dopamine has been lost by the time the disease produces physical signs.

In the brain and nervous system there is normally a balance between dopamine and another chemical messenger called acetylcholine. In people with Parkinson’s disease, acetylcholine is relatively more active because of the lack of dopamine, which creates tremor. This is a simplified explanation of what goes wrong in the brain. In reality things are more complicated. Many other chemical messengers are also involved and many pathways within the brain and nervous system stop working properly.