The impulse.

"At any given moment, the brain has 14 billion neurons firing at a speed of 450 miles per hour. We don’t have control over most of them. When we get a chill... goose bumps. When we get excited... adrenaline. The body naturally follows it’s impulses, which I think is part of what makes it so hard for us to control ours. Of course, sometimes we have impulses we would rather not control, that we later wish we had."

Meredith Grey - Grey´s Anatomy