Parkinson's disease, motoneuron disease and Alzheimer's disease: origins and interrelationship.

The cause of neurodegenerative disease is considered. For Alzheimer's disease, Parkinson's disease, and motoneuron disease, accumulating evidence over the last 3 years indicates that (a) the usual etiological mechanisms involve a subclinical lesion which kills some neurons and reduces the life expectation of their surviving neighbors, and (b) there may be more than one cause for each of the idiopathic syndromes now termed AD, PD, and MND.