Neurons in Parkinson's disease. Illustration of neurons (nerve cells) in the brain in a case of Parkinson's disease, with a normal neuron at top and an abnormal one at bottom. The abnormal neuron contains Lewy bodies (orange), deposits of protein that are thought to cause the progressive degeneration of the neurons that leads to the symptoms of Parkinson's. These symptoms include, tremor, muscular rigidity, poor balance and depression. Levels of the neurotransmitter dopamine are also reduced (shown in green) in Parkinson's disease. Dopamine replacement therapy (DRT) can be used to treat the symptoms.