Secret Bonus Neurons Were Found in the Brain

For all the up-sides to our brains—their capacity for reasoning, long-term planning, and remembering movie trivia—until recently, it was thought that they were limited by finitude: that the number of neurons you were born with was all that you were going to get. Once you make those connections to create neural circuits throughout your childhood, you’re pretty much set. So good luck learning Portuguese as an adult, mermão.

But over the last few years, neurogenesis, the generation of new brain cells in your lifetime, has been observed in the hippocampi of the adult human brain. It turns out 700 new neurons are added in each hippocampus per day.

What’s more, the same Swedish team of researchers that observed the new neurons in the hippocampi found yet another cache of new brain cells, in part of the forebrain called the striatum.

According to Clare Wilson at New Scientist , the researchers looked at 30 donated brains, and used the amount of carbon-14 left in the air following Cold War-era nuclear weapons testing to determine the age of the neurons. The lingering radiation from nuclear tests, which is still in the atmosphere, has also been used to determine the age of elephant ivory and bust poachers—a silver lining to metallic-tasting clouds, I guess.

The discovery of the striatum resolves one of the longest-standing mysteries of mammalian neurogenesis. Back in 1962, American biologist Joseph Altman observed the creation of new neurons in rat hippocampi. But in rodents the new neurons migrated elsewhere in the brain, like the brain’s smell centers. In humans, it wasn’t known where the younger neurons were ending up, which reinforced the idea that the matured human brain was too complex, too exceptional, to be adding neurons.

In canaries, new neurons show up in the forebrains during mating season, when the male canaries are learning new songs. Fernando Nottebohm, who discovered this in the canary-brain, theorized that is because the fresh neurons could help store the new song patterns within the neural circuits of the forebrain, the area of the brain that controls complex behaviors.

It’s too soon to say what this cache of new neurons is doing in the human forebrain. Jonas Frisén, who worked on the Swedish research team, told New Scientist that “new neurons may convey some sort of plasticity" in the human brain.

The discovery reveals potential for treating degenerative brain diseases such as Parkinson’s or Huntington’s disease, both of which are caused by the death of neurons in the basal ganglia. “It's very tempting to think that it would be possible to promote the generation of more striatal neurons," says Frisén .

At the very least, and probably only superficially at this point, knowing that the brain can generate more neurons gives hope that something that takes the construction of new neural pathways, say Portuguese fluency, is still within grasp even of the more mature brain. Psychosomatic, maybe, but nevertheless , tri-legal, ta ligado?