Well, some people think that the brain might be doing something deeply weird with quantum level behaviors to create what we call "consciousness", if that is the case (of course a huge if) then it might not be possible to replicate it in any meaningful sense.

If you look at artificial neural networks and natural ones there are a lot of differences. Artificial neural networks are useful tools for some kind of problems, not physiological models of what happens in our brains.

I can recommend this book if you are interested:

http://www.amazon.co.uk/Philosophy-Artificial-Intelligence-O...