New neurons form in adult brains

Once upon a time, it was thought that by the time you were born, you already had all the neurons you were ever going to have. It was only going to be downhill from there, cognitively speaking. Then it was discovered that some new neurons do form even in adult brains, though it’s difficult to differentiate and track them. Now, an international team of researchers has been able to show that hippocampal neurons have a prodigious turnover rate even into old age. And we have nuclear bomb testing to thank for proving that fact.

Between 1955 and 1963, above ground nuclear tests were conducted. These bombs threw large amounts of radioactive carbon (¹⁴C) into the atmosphere, where it was taken up by plants and eventually by the animals that eat those plants, including humans. After a test ban, atmospheric ¹⁴C levels rapidly declined. That means that neurons that were ‘born’ between 1955 and 1963 have more ¹⁴C than neurons that originated before or after the test ban. If all or most of a person’s neurons are formed at birth, then the neurons of someone born before 1955 should contain very little ¹⁴C and the neurons of a person born in 1960 should be very high in ¹⁴C. However, that’s not what the researchers found. Regardless of age at death (I’m sure I don’t need to point out that these tests were conducted on cadavers), each person had a mix of carbon in their neurons, indicating that new neurons were forming and replacing older neurons throughout life.

In fact, the scientists estimated that the turnover rate for hippocampal neurons was 1.75%, meaning that on any given day 700 new neurons are formed. I don’t know about you, but I feel smarter already.

Spalding, K., Bergmann, O., Alkass, K., Bernard, S., Salehpour, M., Huttner, H., Boström, E., Westerlund, I., Vial, C., Buchholz, B., Possnert, G., Mash, D., Druid, H., & Frisén, J. (2013). Dynamics of Hippocampal Neurogenesis in Adult Humans Cell, 153 (6), 1219-1227 DOI: 10.1016/j.cell.2013.05.002.