🚀 go-pugleaf

There are two separate processes you're talking about. One is the
lifetime of particular cells (In this case nerve cells) and the
other  is the turnover of the actual molecules which make up the
cell. There is quite a range of timescales for both processes.
Most cells in the body (like skin and soft tissues like liver cells)
die quite frequently and are replaced rapidly, even after injury.
However, brain  cells are not replaced, and if one dies, that's it.
Generally, the more highly differentiated the cell type (and nerve
cells and muscle cells are the most complex, arguably, in the body) the
less capable of regeneration or reproduction they are. Hence the
interest in compounds such as nerve growth factor which appear to promote
nerve regeneration, and the transplanting of fetal tissue into adult
brains (since fetal brain cells obviously are still capable of repoduction.

On the other hand, you're right that there is near complete turnover of
the constituent chemicals that make up the cells. The rate of turnover
depends on how tightly bound the stuff is, how freely it diffuses, etc.
Water freely moves and is exchanged rapidly (on the order of days? weeks?)
whereas something like lead can take years. A few molecules don't turnover
at all, in particular the proteins in the lenses of the eye; Hence if
the proteins start to oxidize (causing cataracts) you're in trouble because
the damaged proteins won't get replaced, they're so tightly packed/bound.

In terms of your philosophy paper, its like replcing the pieces of wood in
a wooden boat; the boat stays the same (until its destroyed by attrition) but
its wood is constantly being replaced. 

A Reference: I used Guyton, A textbook of medical physiology,1986, but any
decent neuro/physiol. text should sya something under cell life cycle.