Biologists have shown in rats that chronic stress makes stem cells in the brain produce more myelin-producing cells and fewer neurons, possibly affecting the speed of connections between cells as well as memory and learning. This could explain why stress leads to mental illness, such as PTSD, anxiety and mood disorders, later in life.<img src="http://feeds.feedburner.com/~r/sciencedaily/~4/XeMS3Kq0yHY" height="1" width="1"/>
