Parkinson's disease. Trembling hands of a person suffering from Parkinson's disease attempting to thread a needle. Parkinson's disease is caused by the loss of nerve cells from the brain's basal ganglia. Healthy cells in the basal ganglia produce dopamine, a neurotransmitter, that helps to control muscle contraction. However, in Parkinson's disease there is a lack of dopamine, resulting in jerky, involuntary movements. Other symptoms of the disease include a rigid posture, slow movements, and a shuffling walk. Although there is no cure, drug treatments offer some relief but do not halt the progress of Parkinson's disease.