Neurology. Disease named after James Parkinson, 1755-1824. A progressive neurological disorder resulting from the degeneration of neurons in the basal ganglia. Pathologic features include nerve cell loss in the melanin-containing cells in the substantia nigra and a corresponding reduction in dopamine levels. Parkinsonism usually occurs late in life and is characterized by masklike facial expression, tremor of resting muscles, slowing of voluntary movements and weakness of the muscles.