What Good Are New Brain Cells?

Ever since neuroscientists discovered a decade ago that middle-aged and even old brains keep producing new neurons, they have puzzled over a fundamental question: are these new recruits good for anything, and if so, what? “Intuitively we feel that those new brain cells have to be good for something, but nobody really knows what it is,” said James (Brad) Aimone, a graduate student at the University of California, San Diego.

He and Fred Gage, who led the paradigm-changing discovery of adult neurogenesis (the paradigm being that we’re born with all the neurons we’ll ever have and it’s all downhill from there), have an intriguing suggestion. As they and Janet Wiles of the University of Queensland describe in a new paper in the journal Neuron, neurogenesis at the entryway to the hippocampus, a region of the brain that encodes memories, might help memory in several ways. New neurons of the same age might somehow tag incoming experiences—memories-to-be—that arrive at the same time in such a way that the memories remain forever linked, a process they call pattern integration. The newborn neurons might also tag new hippocampal memories with something like a date stamp, so you know what happened before and after other memories.

“By labeling contemporary events as similar, new neurons allow us to recall events from a certain period,” speculates Gage.

The reason this is speculative is that the suggestions arise from a computational model that Gage, a neurobiologist at the Salk Institute for Biological Studies, and his colleagues used to simulate circuitry in the hippocampus and the region that serves as its entryway, called the dentate gyrus. The scientists still need to study living brains.

But the possibilities are intriguing. Scientists know that newborn neurons make connections to mature brain cells and insinuate themselves into brain circuitry. Then a new crop of neurons is born, eventually joining the existing circuits, too. That sequence means that information reaching the dentate gyrus passes through new neurons of a particular generation, or class. As a result, information about events that occurred around the same time can be tied together by the common experience of passing through that generation of newborn neurons. So if you think back to a vacation, thinking of the hotel you stayed at will retrieve memories of the restaurants you visited and the sightseeing you did.

“Current thinking holds that when we bring up a certain memory, it passes back to the dentate gyrus, which pulls all related bits of information from their offsite storage,” says Gage. “Our hypothesis suggests that cells that were easily excitable bystanders when the memory was formed are engaged as well, providing a hyperlink between all events that happened during their hyperactive youth.”

If neurogenesis is indeed good for memory, the good news is that it's easy to promote. Simple aerobic exercise (mall walking), learning, and environmental enrichment increase the production and survival of new neurons. Chronic stress impairs it.
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