Why the Adult Brain Keeps Making New Neurons

Until the late twentieth century, neuroscience took for granted something we now know was wrong: that the adult human brain was born with all the neurons it would ever have, and from then on could only lose them, never generate new ones. In November 1998, researcher Peter Eriksson and his team published a finding that dismantled that idea: the mature human brain keeps generating new neurons, at least in one very specific region that’s especially relevant to learning, the hippocampus.

A discovery that arrived late, for a good reason

Confirming neurogenesis in humans took longer than in animals for a purely methodological reason: identifying a newly formed neuron requires labeling cells at the exact moment they’re dividing, something relatively straightforward to do in lab rats or primates, but far harder to verify ethically in living human brains. Eriksson managed it by analyzing brain tissue from terminal cancer patients who, as part of a separate clinical study, had received a marker that identified dividing cells shortly before they died. It was that unusual design that finally made it possible to confirm in humans what had previously only been shown in other species.

Where new neurons are born

Hippocampal neurogenesis happens in a basal region called the dentate gyrus. There, precursor or stem cells divide and give rise to cells that migrate into the hippocampus, where they differentiate into neurons and form connections with existing ones. The process never fully stops: a few thousand new neurons are produced every day, though a good share of them don’t survive past the first few weeks.

That it’s the hippocampus specifically, one of the regions most involved in learning and memory, that retains this capacity is no coincidence for researchers in the field: it’s still not known precisely how memories are stored, but the overlap between “key area for memory” and “area capable of generating new neurons” opened up a line of research that continues today.

What helps this process, and what slows it down

Research following Eriksson’s identified, in lab animals, two factors with opposite effects on neurogenesis. On one hand, curious, exploratory behavior and enriched environments increase both the formation of new neurons and their odds of surviving. On the other, sustained stress and a lack of stimulation act in exactly the opposite direction: they reduce both. This connects directly to why stress blocks memory: it isn’t just that stress makes it harder to retrieve what you’ve already learned, it can actually slow down the very formation of the neurons that support future learning.

A self-reinforcing loop

One detail makes this finding even more interesting: research by Elizabeth Gould and her team (1999) showed the relationship also runs in reverse, that the act of learning itself increases neurogenesis in the hippocampus. It isn’t just that a stimulating environment favors the appearance of new neurons: the very act of acquiring new information seems to feed back into the process, generating more of the cells that, in turn, sustain the capacity to keep learning. It’s a virtuous circle with a direct practical implication: staying intellectually active, rather than avoiding cognitive effort, may literally be helping the brain preserve its capacity to generate new neurons.

Why it took so long to believe the opposite

That this idea took so long to be confirmed wasn’t a coincidence. For decades, much of neuroscience assumed the adult nervous system was an already-closed structure, incapable of producing new cells beyond initial development. Eriksson’s finding forced a review of that assumption based on concrete data, not just intuition, and opened up a line of research (with implications ranging from cognitive aging to depression, where alterations in hippocampal neurogenesis have been observed) that’s still active thirty years later.

An idea more than a century old, confirmed

Spanish anatomist Santiago Ramón y Cajal, a Nobel laureate in 1906, had already sensed something similar when he compared the brain to a garden full of trees that, “in response to intelligent cultivation, can increase the number of branches, extend their roots over a wider area, and produce more varied and exquisite flowers and fruit.” Cajal was referring mostly to new connections between already-existing neurons, not entirely new neurons, but the underlying idea (that the adult brain retains a capacity for change, that it isn’t a fixed and finished structure) is the same one Eriksson confirmed, from a different angle, nearly a century later. What at the start of the twentieth century was a poetic intuition turned out, with the right tools, to also be a literal biological fact: the adult brain keeps growing, cell by cell, as it keeps learning.

Sources

• Eriksson, P. S., Perfilieva, E., Björk-Eriksson, T., Alborn, A. M., Nordborg, C., Peterson, D. A., & Gage, F. H. (1998). Neurogenesis in the adult human hippocampus. Nature Medicine.
• Gould, E., Beylin, A., Tanapat, P., Reeves, A., & Shors, T. J. (1999). Learning enhances adult neurogenesis in the hippocampal formation. Nature Neuroscience.
• Ramón y Cajal, S. (1894). The Croonian Lecture: La fine structure des centres nerveux. Proceedings of the Royal Society of London.