[DennisP]

Not always. Lasers, for example, are macroscopic objects, but depend on quantum effects. Same with geiger counters, solar panels, all sorts of modern technology. Listen to the clicks on a geiger counter, and the state of your brain at any given instant is non-deterministic.

Researchers keep finding more ways that life uses quantum physics, including photosynthesis, the avian compass, and (maybe) the sense of smell:

http://www.youtube.com/watch?v=wcXSpXyZVuY

http://www.bbc.co.uk/news/science-environment-12827893

So it wouldn't be all that surprising if they find quantum tricks in the brain, too.

Aside from that, there's radioactivity in your body, including your brain. I saw a speculation once that radioactive decay sparks electrical signals in the brain, which the brain sometimes amplifies enough to change macroscopic behavior. If that's the case, that would prevent our actions from being deterministic.