(this article discusses ALS in depth, though there is a short section on Parkinson's.)

" Selective degeneration and death of one or more classes
of neurons is the defining feature of human neurodegenerative
disease. Although traditionally viewed as diseases
mainly affecting the most vulnerable neurons, in most
instances of inherited disease the causative genes are
widely—usually ubiquitously—expressed...We review here the evidence that it is the convergence of damage developed within multiple cell
types, including within neighboring nonneuronal supporting
cells, which is crucial to neuronal dysfunction. Damage
to a specific set of key partner cells as well as to vulnerable
neurons may account for the selective susceptibility of
neuronal subtypes in many human neurodegenerative
diseases,"...

http://jcb.rupress.org/cgi/content/full/jcb.200908164v1

or the pdf version (which displays illustrations/charts, etc much better) though I am unsure if the following is accessible:

http://jcb.rupress.org/cgi/reprint/jcb.200908164v1