These yellow structures are adenosine A2A receptors in the shell of the nucleus accumbens of the brain. As we become tired, our bodies produce adenosine (the green small molecule above), which binds to these receptors and they, in turn, send signals to inhibit arousal. In other words, we get sleepy. Enter caffeine! (the orange small molecule above). Caffeine binds to the A2A receptors in place of adenosine and blocks the process, so we wake up.
There have been a lot of good caffeine related stories on the internet this week. Check out Carl Zimmer’s article in the New York Times about how plants evolved to make caffeine. Or watch this video from ASAP Science, doing an excellent job of explaining how caffeine works using fun whiteboard cartoons. Or read this article on BoingBoing in which Dr Kiki Sanford considers the option having an electric current passed through her brain in place of her daily cup of Joe.
But, maybe you better pour yourself another cup of coffee first.