Substantia nigra. Computer illustration showing a degenerated substantia nigra in Parkinson's disease. The substantia nigra plays an important role in reward, addiction, and movement. Degeneration of this structure is characteristic of Parkinson's disease. In Parkinson's neurons in the substantia nigra contain Lewy bodies, which are deposits of protein, shown here as small red spherical inclusions in the neuron's cytoplasm.