Adult neurogenesis is the process of generating new neurons in the brain after it has fully developed. For decades, scientists believed the brain stopped producing neurons after childhood. That view has been overturned. The adult brain continues to create new nerve cells in at least two specific regions, and these newborn neurons play measurable roles in memory, learning, and mood regulation.
Where New Neurons Are Born
Two areas of the adult brain consistently produce new neurons. The first is the hippocampus, a structure buried deep in the temporal lobe that is essential for forming memories. Specifically, new neurons arise in a thin strip of tissue called the subgranular zone within the hippocampus’s dentate gyrus. Once born, these cells migrate a short distance into the surrounding tissue and wire themselves into existing brain circuits.
The second region is the subventricular zone, which lines the brain’s fluid-filled ventricles. Stem cells here produce neurons that travel to the olfactory bulb, the brain’s smell-processing center. This migration is well documented in rodents and primates, though its extent in humans is still debated. Researchers have also found hints of neuron production in other brain areas, but these two zones remain the most established and best studied.
How a New Neuron Develops
Creating a functional neuron from a stem cell is not instant. The process unfolds in four broad phases: proliferation, early survival, maturation, and late survival. During proliferation, a neural stem cell divides and produces intermediate progenitor cells. These progenitors initially resemble support cells called glia before shifting to a neuronal identity.
The cells then exit the division cycle and enter a long maturation phase. They extend dendrites (the branching input structures) toward neighboring tissue and send an axon to connect with a distant target region of the hippocampus called CA3. A signaling molecule called GABA, normally known for calming neural activity, actually drives the maturation of these young cells and guides their integration into circuits. A growth factor produced by nearby mature neurons promotes this process by boosting GABA release from surrounding cells, helping the newcomers differentiate and survive.
In mice, a new hippocampal neuron takes roughly four to six weeks after its initial cell division to reach full functional maturity. In macaque monkeys, this timeline stretches beyond six months. During those weeks or months, the young neurons pass through a critical window of heightened flexibility, responding more readily to stimulation than their older neighbors. Eventually, they become electrically and structurally indistinguishable from neurons that have been there since birth.
What New Neurons Actually Do
The hippocampus handles a specific cognitive challenge: telling similar experiences apart. You parked your car in the same garage yesterday and today, but in different spots. Retrieving the right memory without confusing it with the wrong one requires a process called pattern separation, the ability to take overlapping inputs and file them as distinct memories.
New neurons are central to this ability. During their critical window of heightened plasticity (around four to eight weeks old in mice), young neurons are more excitable and more responsive to new information than mature ones. This expanded capacity for plasticity helps the dentate gyrus encode fine distinctions between similar contexts. When researchers blocked neurogenesis through irradiation or genetic manipulation, animals lost the ability to discriminate between similar environments and defaulted to generalizing. When neurogenesis was stimulated through exercise or genetic techniques, discrimination improved.
New neurons in the olfactory bulb serve a parallel function, helping distinguish between similar odors. In both regions, adult-born neurons appear to act as modulators that enhance the brain’s ability to process complex, overlapping information.
What Suppresses Neurogenesis
Chronic stress is one of the most potent inhibitors of new neuron production. When you are under sustained stress, the adrenal glands release cortisol (in humans) or corticosterone (in rodents). These stress hormones bind to receptors on neural progenitor cells and suppress their proliferation. Studies in primates showed that prolonged social stress, such as being subordinate in a social hierarchy, significantly reduced cell division in the dentate gyrus.
The mechanism is fairly direct. Stress hormones enter the nucleus of progenitor cells and alter gene expression in two ways: by activating certain genes and by blocking other transcription factors that would otherwise promote cell growth and survival. Landmark experiments in the early 1990s showed that removing the adrenal glands in rats (eliminating the source of stress hormones) boosted the production of new cells, while administering adrenal hormones suppressed it. This finding helped establish the link between chronic stress, reduced neurogenesis, and the cognitive and emotional symptoms seen in depression.
The Connection to Depression and Antidepressants
One of the more provocative findings in this field is the relationship between neurogenesis and how antidepressants work. SSRIs, the most commonly prescribed class of antidepressants, typically take three to six weeks to produce noticeable mood improvements. This therapeutic delay has long puzzled researchers, since the drugs alter brain chemistry within hours.
The neurogenesis hypothesis offers a partial explanation. SSRIs increase the proliferation and survival of new neurons in the hippocampus, but only after 14 or more days of treatment. Shorter treatment regimens do not produce the same effect. Since a new neuron needs up to seven weeks to become fully functional after its initial cell division, the timeline for neurogenesis roughly mirrors the clinical delay patients experience before feeling better. SSRIs also appear to “rejuvenate” existing mature neurons, shifting their electrical properties to resemble those of younger, more plastic cells. This combination of generating new neurons and refreshing old ones may expand the hippocampus’s capacity to process emotional and contextual information in healthier ways.
This does not mean depression is simply a deficit of new neurons. The relationship is more nuanced. But the overlap between factors that reduce neurogenesis (chronic stress, elevated cortisol) and factors that contribute to depression has made this one of the more actively studied connections in neuroscience.
How Neurogenesis Changes With Age
The rate of new neuron production drops substantially over a lifetime. In macaque monkeys, neurogenesis decreases by approximately 70% between young adulthood and middle age, with an additional significant decline between middle and old age. Both the size of the subventricular zone and the number of migrating young neurons shrink in older animals compared to younger adults. While detailed quantitative data in humans remains limited, the pattern appears similar across species.
This decline is one reason age-related memory difficulties tend to involve exactly the kinds of tasks new neurons support, like distinguishing between similar experiences or encoding new contextual details. The brain does not stop producing neurons entirely in old age, but the volume drops enough to meaningfully reduce the hippocampus’s regenerative capacity.
Exercise and Other Lifestyle Factors
Aerobic exercise is the most consistently supported lifestyle factor for boosting neurogenesis. In mouse studies, moderate treadmill running roughly doubled the number of surviving new cells in the dentate gyrus compared to sedentary controls. Moderate exercise also increased proliferation by about 48%. Interestingly, intensity matters: forced intense exercise did not produce the same benefits as moderate exercise, with cell counts in the high-intensity group falling back to levels statistically indistinguishable from sedentary animals.
Environmental enrichment, meaning living in a complex, stimulating environment with opportunities for exploration, also increases neurogenesis. Researchers have proposed that higher neurogenesis following enrichment serves an adaptive purpose: it enhances pattern separation, helping the brain faithfully encode the details of a complex environment and recognize it more quickly in the future. This suggests that novelty, exploration, and moderate physical activity work together to maintain the brain’s regenerative potential across the lifespan.