adult neurogenesis

Adult neurogenesis is the making of brand-new neurons — the brain's signaling cells — inside a brain that has already grown up. For a long time scientists assumed you were born with every neuron you would ever have, and that the adult brain could only lose cells, never add them. We now know that is not the whole story: in certain small corners of the mature brain, a reserve of unspecialized parent cells called neural stem cells keeps quietly dividing, and some of their offspring grow into working neurons that wire themselves into the existing circuits.

It does not happen everywhere, and it is not a flood. The clearest site in humans is a sliver of the hippocampus, the seahorse-shaped structure deep in the brain that helps build new memories; many other mammals also make fresh neurons in a region feeding the smell system. Think of it less like rebuilding a whole house and more like a gardener tucking a few new seedlings into one well-tended flower bed. Each newborn cell must survive a risky early stretch, sprout connections, and prove useful, or it is pruned away — only a fraction make it.

Why it matters: these late-arriving neurons are thought to help the hippocampus keep similar experiences from blurring together and to support learning, mood, and flexible memory. Their numbers tend to fall with age and with chronic stress, and rise with exercise and rich, engaging surroundings, which is part of why researchers study them as a possible foothold for keeping the aging brain sharp and for repairing it after injury or disease. Just how much of this carries over to the adult human brain is still actively debated.