If the cause of an illness is known then it should be possible to devise a therapy to combat that illness. However, if the cause is an anatomical lesion of the brain then it may be possible to devise a rational therapy to treat the symptoms but at first sight it would seem to be impossible to devise a cure because the neuronal loss is irreversible. There are two ways in which the effects of losing neurones might be overcome. One would be to train parallel neuronal circuits to become more efficient: this is the basis of some types of physiotherapy. Alternatively, it may in the future become possible to transplant nerve cells: already this seems to be possible, at least experimentally, with dopaminesecreting cells.

Although the major neuronal deficit in Parkinson's disease and effective therapy have been established for many years, attention has more recently shifted first to another basal ganglia disorder: Huntington's chorea, and to the debilitating illness of Alzheimer's disease (senile dementia). In each of these illnesses the major impact has come from studies of changes in neurotransmitter systems, carried out in the hope that specific changes which might be amenable to drug therapy could be devised on a rational basis. Very recently there has also been a new interest in the cell loss which results from the hypoxia and ischaemia of stroke.

It will be convenient to group together five disorders of the basal ganglia since these are all characterised by dyskinesias (disorders of movement) of various types.