The basal ganglia comprise a group of gray matter structures beneath the cerebral cortex, that surrounds the thalamus and hypothalamus. The basal ganglia play an important role in controlling movement. The motor circuits within the striato-pallidal complex are thought to facilitate desired movement and inhibit unwanted movement through their influence, via the thalamus, mainly on cortical precentral motor regions. Localized damage to parts of the basal ganglia occurs in certain diseases such as Parkinson's disease. Parkinsonism is a common neurological disorder that affects about one person in every 1,000 of the general population and about 2% in the elderly. The diagnosis of Parkinson's disease is based on the presence of two or more of the major symptoms: tremor, rigidity, postural instability, and bradykinesia. The pathological process behind the motor disabilities of Parkinsonism is a progressive degeneration of dopaminergic neurons of the substantia nigra, that results in dopamine depletion in the striatum. Brain dopamine deficiency is sufficient to explain all of the major symptoms of Parkinson's disease.