The nervous feeling is the adrenaline from the predatory instinct that we have. This gave primitive peoples the strength and endurance to chase down prey and follow the kill through to completion. It also sharpens your perception of the senses and concentration. Quite frankly, if you don't have this at least to some degree, your not really a hunter.
I'm betting you are a hunter.
You've never really lived until you experience that feeling, BTW.