Experts already know that people suffering from chronic stress are prone to experiencing mental health problems - such as anxiety and mood disorders - later in life. Now, a new study from researchers at the University of California, Berkeley, explains why.
Previous research has shown that people with post-traumatic stress disorder (PTSD) and other stress-related conditions have abnormalities in the brain.
The human brain is made up of "gray matter" and "white matter" and scientists have noticed that the proportions of white versus gray matter is different in people with stress illnesses, compared with other people. But so far, scientists have not been able to explain why these differences in the brain occur.
White matter gets its name from the white, fatty "sheath" of myelin coating it - the electrically insulating layer that forms around nerves and accelerates the transmission of electrical signals between cells. The new study focused on cells in the brain that produce myelin - the electrically insulating layer that forms around nerves.
Too much white matter
The researchers found that an excess of white matter is found in some areas of the brain in people who experience chronic stress. It seems that the experience of chronic stress causes more myelin-producing cells to be generated, with fewer neurons than normal.
An excess of white matter is found in some areas of the brain in people who experience chronic stress.
The consequence of this is that the excess of myelin causes the "delicate balance" of the brain to be disrupted, with communication between brain cells slipping out of their normal timing.
To see how this happens, the researchers conducted tests on the hippocampus region in the brains of adult rats. But they found that the neural stem cells in the rats' hippocampi behaved in unexpected ways.
Previously, it was thought that these stem cells would only mature into neurons, or a type of cell called an astrocyte. However, when the rats were experiencing chronic stress, these stem cells matured into another form of cell - an oligodendrocyte. Oligodendrocytes are the cells that produce myelin.
As well as producing the myelin in white matter, oligodendrocytes also aid the formation of synapses - the structures that allow nerve cells to connect and exchange information with each other.
Could stress cause changes in brain connectivity?
The researchers wonder if people with PTSD undergo changes in brain connectivity. For instance, if their hippocampus (which regulates memory and emotions) might develop a stronger connection to their amygdala (where the "fight or flight" response is processed).
This theory could also mean that PTSD patients might have weaker connectivity between the hippocampus and the 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," says study author Prof. Daniela Kaufer.
"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."
Prof. Kaufer and her team are conducting further investigations to test this hypothesis.
Because chronic stress is known to also affect memory and learning ability, Kaufer and her team think that this may be accounted for by the rat stem cells maturing into myelin-producing cells rather than the neurons that process and transmit the electrical information necessary for learning and memory skills.
In 2012, Medical News Today reported on a study that found it may be possible to identify people who could be susceptible to PTSD by analyzing scans of their brain.