Prior to the mid-20th century, adult neurogenesis - the generation of new nerve cells - was deemed to be impossible and unnecessary. Why would the developed brain require new neurons? Despite John Altman's significant discovery in 1965 of adult neurogenesis in rats, this question has remained without a concrete answer even to this day.
The process of neurogenesis is thought to occur in the hippocampus of the temporal lobe. This begins with the fabrication of dormant neural stem cells: slow-growing and multipotent cells which later divide into transit amplifying cells and transient intermediate progenitors (TACs and TIPs). These cells are much more rapidly-dividing and go on to create neuroblasts, cells which will soon differentiate into new neurons. This entire process lasts around 20 days and involves many activation hormones - the functions of which are still not entirely known - and stimuli specific to each individual type of neuron.
It is not just internal factors which promote neuronal growth; extrinsic stimuli have been proven to play a significant role in this process. These include: learning and working the memory, physical exercise, the surrounding environment and severe brain injuries. Lifestyle has an impact on the process of neurogenesis, but how does nerve growth affect our lives?
With the help of magnetic resonance imaging technology, a potential correlation has been witnessed between the instance of depression in patients and a decreased hippocampal size. This is a leading topic of discussion as it remains unclear whether the lack of neurogenesis is a risk factor for depression or if this reduction in capacity is just a symptom of mental decline. Studies carried out in rats point towards the latter, as while glucocorticoids (steroid hormones) have been proven to inhibit neurogenesis, when the inverse was tested with the blockage of neurogenesis, no change in mental stress was witnessed in the rodents. What this essentially means is that a lack of neurogenesis is simply a corollary of poor mental health and stress.
If depression leads to stunted regeneration, could stimulating neurogenesis have the opposite effect of preventing or even reversing mental health decline? Although this is not yet a theory which can definitively be proven, there is little harm in boosting physical activity and environment quality as a way to supplement well-being in patients and as a preventative measure in healthy people. Perhaps in the future the correlation will be more definitive and we may be able to improve the medications on offer to tackle the root cause of these conditions; for now neurogenesis remains an elusive - yet promising - topic of interest.
sources:
Science Direct 3/11/23
National Library of Medicine 3/11/23
Neuroscientifically Challenged 3/11/23
Behave, Robert Sapolsky 3/11/23