Well, we have no good idea of where sentience comes from. Some may claim that a soul is responsible, but until someone actually comes up with evidence for a soul, and some mechanism by which it works, that's a bit of a non-answer.
By physically or chemically altering the brain, we can change a person's mental processes. Thus it stands to reason that the physical/electrical/chemical connections in the brain somehow create sentience. But when you look at the level of just a few neurons, it's all just physics and chemistry. We can create things that have the function of a neuron, or a few neurons, and those things probably aren't sentient.
So, in some unexplained way, it's only when you connect billions of neurons (and probably in a specific manner) that you get sentience. The big question is: how much sentience is produced by how many neuronal connections? And are only neurons capable of producing sentience, or are other interconnected systems also capable of sentience? The internet as a whole (with all it's connected computers) has many more connections than a human brain. Is it sentient? I'd guess not, but I have no way of knowing for sure.