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Plaques without symptoms
Oxford neurologist Masud Husain's patients live a very long time with lots of plaques and tangles, the signature anatomical features of Alzheimer's, and no dementia. A lot more people than expected walk around with every diagnostic feature except the memory loss. These patients are exercisers who stay avidly engaged in learning.
The pattern matches the wider data: people who stay mentally and physically active and socially enriched do much better. The physical changes of Alzheimer's and what those changes do to neurons look somewhat separable.
When motivation takes over
Patients with Parkinson's, who can't generate smooth movement, can still escape a burning building. When motivational systems run high enough, alternate ones seem to take over, the way children with severe ADHD lock into a video game they love for two hours. They can focus, just not on things they aren't interested in.
How can a brain hold plaques without symptoms?
Neurologists call the buffer cognitive reserve, a term associated with Yaakov Stern. Years of education, complex work, and social contact seem to build extra connections that let the brain route around damage. The Nun Study, led by David Snowdon, followed 678 Catholic sisters from 1986. Autopsies showed that some of them had dense Alzheimer's pathology despite sharp minds in their final years.
Which damage tracks memory loss most closely?
Plaques are clumps of a protein called amyloid-beta that build up between neurons. Tangles are twisted strands of a protein called tau that form inside neurons. In autopsy and imaging studies, tau tangles line up with symptoms more closely than amyloid plaques do. The spread of tangles through the brain follows a pattern pathologists describe as Braak stages.
Why can Parkinson's patients move in emergencies?
The pattern is known as paradoxical kinesia. Parkinson's disease destroys dopamine-producing neurons that drive the basal ganglia, which handle automatic, habitual movement. Urgent or emotionally charged situations appear to use other motor pathways that stay intact. Visual cues such as stripes on the floor can also help patients take steps that they otherwise cannot.
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