Ron Murphy asked:
Neurons don't divide, but new neurons are created from stem cells? How do these new neurons come to be used in the brain? I presume they don't migrate, but instead extend their dendrites and axons. But aren't the neuron bodies still building up in specific sites of generation?
Scientists discovered, much to their surprise about 15 years ago, that, contrary to prevailing wisdom, nerve cells in the central nervous system, in other words the brain and spinal cord, are actively dividing into adult life.
We previously thought that once you go beyond the early years of life, probably one or two, no more nerve cells are born in the brain. Some studies, however, done on cancer patients who were taking drugs that labelled up dividing cells, led people to realise that new nerve cells were being produced into adulthood in certain sites in the nervous system.
One of these sites is in the hippocampus. What happens is there are cells in a specific region of the hippocampus - a site on both sides of the brain concerned with learning and memory - and located in a site called the subcortical plate - containing a population of stem cells, which bud off and they produce one new neurone and one new stem cell.
Those new neurones then migrate. They use glia, which are supporting cells in the brain almost like tight ropes, and they walk along these tight ropes, crawling along them to a site where they want to end up.
After that, their fate is one of two things. One, they can survive, the other is that they die off. A proportion die, and a proportion do survive, and we think they're very important for learning and memory as well as maintaining a healthy psyche.
This is because, if you look at how anti-depressants work, people thought they were just working via an increase in the levels of certain nerve transmitter chemicals. It turns out that's not the whole story because what they've now discovered is that these nerve cells can be made to survive in much greater proportions if you take anti-depressants.
So they think that one of the reasons why people who are on anti-depressants get better from their depression, and it takes several weeks for the depressive symptoms to go away despite the fact that you can register a change in the nerve transmitter chemicals almost immediately, is because it’s taking a while for new nerve cells to accumulate in the brain. They then work a bit like a neurological sticking plaster. They plaster over the damage and they help to wire themselves in, integrate and restore more normal neurochemistry in the brain.
So, I hope that answers the question.
Basically, you do give birth to new neurones, they do migrate to certain restricted targets in the nervous systems of adults and that goes on right until the day you die...