Parkinson's disease. Light micrograph of a section through the sabstantia nigra of the brain of a patient with Parkinson's disease showing a neuron (nerve cell, centre) containing Lewy bodies (dark red). Lewy bodies, which are deposits of protein, 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. Although drugs may alleviate the symptoms Parkinson's disease is ultimately fatal.