Yes, there are undifferentiated neurons left around in some areas of
brain; if there were some in substantia nigra, then bringing them on-line
might well work even better than L-DOPA.
But no, they're not going to be much help in cases of cortical damage,
even if available. Development occurs in sequences, building on prior
stages (pathfinder cells being the most dramatic examples, some of which
then die like scaffolding being removed). Unless you can recreate the
developmental sequence, a neuron's exploratory processes may simply get
lost (the typical spinal cord regeneration problem).
And no, too, regarding much higher function that depends on memory that is
help in a pattern of cell connections. The reason that neurons don't
divide during life is probably that it would destroy the established
pattern of connections with thousands of other neurons, that constitutes
the memory engram.
William H. Calvin WCalvin at U.Washington.edu
University of Washington NJ-15
Seattle, Washington 98195 FAX:1-206-720-1989