The Triumph Of Deep Learning

The problem is that it is fairly easy to create things that behave like neurons, the brains major component. What is not easy is working out what the whole thing does once you have assembled it. It is assumed that neurons get excited by other neurons and when they get excited enough they "fire" and send their excitement on to other connected neurons. This is very easy to model but how do you determine how the neurons should be connected and what should govern the strengths of connection?