By now you’ve no doubt heard that the brain can create new brain cells. Have you ever wondered how the brain figures out when to do this? According to Hongjun Song, PhD, professor of neurology and director of the Institute for Cell Engineering’s Stem Cell Program at Johns Hopkins Medicine, the hippocampus eavesdrops. That’s right. Cells in the hippocampus (your brain's search engine) listen in on the chemical communication among nearby neurons to find out what is stressing the system and when they need to act. Apparently they pick up communication sent by the neurotransmitter GABA. The communication appears to transmit information about what brain cells experience of the outside world. The Hippocampus then either keeps brain stem cells in reserve, so if you don’t need them, you don’t use them up, or triggers the process of creating new brain cells.