What happens in the brain when someone becomes demented?

Nerve cells die: The two most common types of dementia are so-called alzheimer's type and multi-infarct type, and in most cases it is some combo of each. The nerves in the brain die in larger numbers and faster than in normals or in those with mild cognitive impairment. In alzheimer's most are inn the hippocampus portion of the brain. In the type involving multiple little strokes, they can be almost anywhere.

Brain damage: Dementia occurs with the loss of brain structure and in particular the neurons - after enough loss of cells and neurons the brain can no longer function normally leading to dementia.