Caffeine and Adenosine
To a nerve cell, caffeine looks like adenosine: Caffeine binds to the adenosine receptor. However, caffeine doesn't slow down the cell's activity like adenosine would. As a result, the cell can no longer identify adenosine because caffeine is taking up all the receptors that adenosine would normally bind to. Instead of slowing down because of the adenosine's effect, the nerve cells speed up.