It seems that Alzheimer's, for example, spreads along, for want of a better term, the AD network and Parkinson's along the PD network, etc.

from http://www.sciencedaily.com/releases...0415120955.htm

"The current neuroimaging study, which examined patients with five forms of early age-of-onset dementia -- Alzheimer's disease, behavioral variant frontotemporal dementia, semantic dementia, progressive nonfluent aphasia, and corticobasal syndrome – as well as two groups of healthy controls, showed that each disease targets a different neural network.

"The study suggests that these diseases don't spread across the brain like a wave but instead travel along established neural network pathways," says the lead author of the study, William W. Seeley, MD, assistant professor of neurology at the UCSF Memory and Aging Center."

and

"The findings suggest that network degeneration represents a class-wide neurodegenerative disease phenomenon.

"Something about a network's architecture or biology is either bringing the disease to networked regions or propagating disease between network nodes," says Seeley.