The way that caffeine jolts your brain is this: it blocks certain receptors in the brain and other organs. This causes a set of reactions that results in your body releasing adrenaline. Adrenaline increases your heart rate, blood pressure and blood to the muscles.
However, the reason that these effects exists is because caffeine was developed over time as a poison by plants as a way to kill insects that were trying to eat them! The effects of caffeine on other insects actually paralyzes and kills them. Next time you have a cup of coffee, just think that you're drinking some warm bug poison!