In Performance Bike magazine recently, they stuck a heart monitor on one of the Phase One (endurance team) riders - I don't think his heart rate went below 163bpm for the entirety of his stint on track. But maybe that's just indicative of the physical exertion and the state of awareness that's required for racing.
I'm kind of used to driving in the middle of the pack, so its only when I find myself upfront that I get nerves. Its counter-productive: you find yourself in a new situation; you say to yourself, "OK, now don't screw this up; so the first thing that happens is you screw it up
Its a version of target fixation. Racing is all about peripheral vision - once you get get stuck on foveal vision, its not going to end well. Where you look is where you'll go, and if you're looking at someone's rear end as you chase them down - odds on you'll hit them in a slow corner. Same thing with new-situation-nerves: concentrate on not making a mistake, and the mistakes will come thick and fast.
The more often you experience the situation, the more time you will be able to give yourself to respond to problems.