One cubic millimiter of brain tissue contains a hundred thousand (100.000) neurons. Between these 100.000 neurons there is 1 billion (1.000.000.000) connections. In one cubic millimeter that is!!! Your brain contains millions of miles of wiring, tens of times longer than the circumference of our planet.
Sebastian Seung recently published a book called "connectome" where he lays out the hypothesis that "you are your connectome". What is connectome you ask. Your connectome is all your neurons and the connections between all of your neuron. This statement is more attractive than the statement "you are your genes" because unlike genes, your connectome changes in response to everything that happens to you. I have not looked up the numbers, but I am in no doubt that every second of your life is associated with the formation of new synapses, destruction of other synapses and change in strength of yet other synapses.
The only way this hypothesis can really be tested is by mapping the human connectome, which, according to Seung (and I agree), won't happen for a few decades at least...
In the TED lecture professor Seung presents his ideas.
By the way, the brain actually does not contain 100 billion neurons but more like 86 billion neurons. And we do not just use 10% of our brain - we use 100%!
See also: The Human Connectome Project