Abstract
Alzheimer's disease and Parkinson's disease are the most common age-related degenerative disorders of the human brain. Both diseases involve multiple neuronal systems and are the consequences of cytoskeletal abnormalities which gradually develop in only a small number of neuronal types. In Alzheimer's disease, susceptible neurons produce neurofibrillary tangles and neuropil threads, while in Parkinson's disease, they develop Lewy bodies and Lewy neurites. The specific lesional pattern of both illnesses accrues slowly over time. Presently available data support the view that fully developed Parkinson's disease with concurring incipient Alzheimer's disease is likely to cause impaired cognition.