Neuroplasticity

For a long time, neuroscience textbooks taught a bleak idea: you’re born with all the neurons you’ll ever have, and from there it’s mostly downhill. Modern research has replaced that with a more accurate picture.

The adult brain remains plastic—able to reorganize, reweight connections, and in some regions even generate new neurons—though these capacities can change with age, stress, disease, and lifestyle.

Neuroplasticity is the umbrella term for how the nervous system adapts with experience. It includes:

Synaptic plasticity: strengthening or weakening connections between existing neurons (often discussed via mechanisms like long-term potentiation, LTP).
Structural plasticity: growth or pruning of dendritic spines, changes in axons, and network remodeling over time.
Functional reorganization: brain networks shifting how they allocate processing (common after skill learning and also after injury).
Glial and myelin-related changes: support cells and insulation patterns also adapt, shaping signal speed and efficiency.

Plasticity is what makes practice work: repeated experience nudges circuits toward being more efficient at a task, whether that’s learning vocabulary, playing piano, or recovering function after an injury.

Adult neurogenesis

Adult neurogenesis is a specific form of plasticity: the production of new neurons from neural stem cells/progenitor cells, followed by their maturation and integration into circuits. In mammals, the clearest and most studied site is the dentate gyrus of the hippocampus, where neural stem cells in a “neurogenic niche” can generate new granule neurons.

Does it happen in adult humans?

A well-known flashpoint: one Nature study reported that hippocampal neurogenesis “drops sharply” and becomes undetectable in adults (Sorrells et al. 2018), while other studies using different tissue handling and markers reported persistent neurogenesis across adulthood and aging (Boldrini et al. 2018), including in very old individuals and even in Alzheimer’s disease tissue (Tobin et al. 2019).

Recent reviews emphasize that both the biology and the measurement are complex; differences in fixation, marker choice, and analysis pipelines can change what is detectable (Simard et al. 2025).

So the best summary is: the adult brain does not “turn off” its capacity for neuronal renewal and remodelling, but rates and detectability can vary, and scientists are still refining what’s true across age and health status in humans.

Neural stem cells and the “neurogenic niche”

A key reason adult neurogenesis is possible at all is the presence of neural stem cells—cells capable of self-renewal and differentiation—housed in specialized microenvironments (“niches”) that supply the chemical and structural signals needed for new neurons to survive and integrate.

In the hippocampus, this niche includes local support cells, blood vessels, immune signalling, and molecular pathways that guide each stage of development.

BDNF: a central “fertilizer” for plasticity

BDNF (brain-derived neurotrophic factor) is one of the most important molecules in the plasticity story. It’s a neurotrophin that supports neuron survival and growth, and it helps regulate synapse formation and strengthening—processes closely tied to learning and memory.

BDNF also shows up in discussions of adult neurogenesis: it’s among the factors implicated in regulating the neurogenic process in the adult hippocampus, alongside other intrinsic pathways and external inputs like exercise and stress.

What this means in plain terms

Your adult brain is not a static machine; it’s a living system that continually rewires itself based on what you do, practice, feel, and experience.
In at least some brain regions—most convincingly the hippocampus in mammals—the adult brain can also add new neurons, shaped by biology and environment.
“More neurogenesis” isn’t automatically “more intelligence,” but neurogenesis and broader plasticity are tightly linked to how the brain supports learning, memory, and stress resilience.

References

Sorrells SF et al. (2018). Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults. Nature. “https://www.nature.com/articles/nature25975
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Boldrini M et al. (2018). Human Hippocampal Neurogenesis Persists throughout Aging. Cell Stem Cell. “https://www.cell.com/cell-stem-cell/fulltext/S1934-5909%2818%2930121-8
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Tobin MK et al. (2019). Human Hippocampal Neurogenesis Persists in Aged Adults and Alzheimer’s Disease Patients. Cell Stem Cell. “https://pubmed.ncbi.nlm.nih.gov/31130513/
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Simard S et al. (2025). Adult Hippocampal Neurogenesis in the Human Brain: Updates, Challenges, and Perspectives. Neuroscientist. “https://pubmed.ncbi.nlm.nih.gov/38757781/
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Brickman AM et al. (2014). Enhancing dentate gyrus function with dietary flavanols improves cognition in older adults. Nature Neuroscience. “https://pmc.ncbi.nlm.nih.gov/articles/PMC4940121/
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Brickman AM et al. (2023). Dietary flavanols restore hippocampal-dependent memory in older adults with lower diet quality and lower habitual flavanol consumption. PNAS. “https://www.pnas.org/doi/10.1073/pnas.2216932120
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