February 11, 2014
Science Daily/University of California - Berkeley
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.

Doctors know that people with stress-related illnesses, such as post-traumatic stress disorder (PTSD), have abnormalities in the brain, including differences in the amount of gray matter versus white matter. Gray matter consists mostly of cells -- neurons, which store and process information, and support cells called glia -- while white matter is composed of axons, which create a network of fibers that interconnect neurons. White matter gets its name from the white, fatty myelin sheath that surrounds the axons and speeds the flow of electrical signals from cell to cell.

How chronic stress creates these long-lasting changes in brain structure is a mystery that researchers are only now beginning to unravel.

In a series of experiments, Daniela Kaufer, UC Berkeley associate professor of integrative biology, and her colleagues, including graduate students Sundari Chetty and Aaron Freidman, discovered that chronic stress generates more myelin-producing cells and fewer neurons than normal. This results in an excess of myelin -- and thus, white matter -- in some areas of the brain, which disrupts the delicate balance and timing of communication within the brain.

"We studied only one part of the brain, the hippocampus, but our findings could provide insight into how white matter is changing in conditions such as schizophrenia, autism, depression, suicide, ADHD and PTSD," she said.
The hippocampus regulates memory and emotions, and plays a role in various emotional disorders.

Kaufer and her colleagues published their findings in the Feb. 11 issue of the journal Molecular Psychiatry.

Does stress affect brain connectivity?
Kaufer's findings suggest a mechanism that may explain some changes in brain connectivity in people with PTSD, for example. One can imagine, she said, that PTSD patients could develop a stronger connectivity between the hippocampus and the amygdala -- the seat of the brain's fight or flight response -- and lower than normal connectivity between the hippocampus and prefrontal cortex, which moderates our responses.

"You can imagine that if your amygdala and hippocampus are better connected, that could mean that your fear responses are much quicker, which is something you see in stress survivors," she said. "On the other hand, if your connections are not so good to the prefrontal cortex, your ability to shut down responses is impaired. So, when you are in a stressful situation, the inhibitory pathways from the prefrontal cortex telling you not to get stressed don't work as well as the amygdala shouting to the hippocampus, 'This is terrible!' You have a much bigger response than you should."
http://www.sciencedaily.com/releases/2014/02/140211153559.htm