Chronic stress kills brain cells and shrinks neurons. The area of the brain where most of this damage is being done is in the hippocampus. A part of the brain involved in memory and learning.
There is a significant link between a lifetime of stressful experience, exposure to the specific hormones that are generated by stress and shrinkage in areas of the brain that are responsible for memory and learning. The main hormone that is responsible for a lot of the problems are called glucocorticoids and they cause damage when brain cells are exposed to them over an extended time.
Glucocorticoids cause brain cells to shrink, and dendrite branches which brain cells use to communicate, shrivel up and die. Long-term exposure to this hormone kills neurons or makes them more vulnerable to being destroyed when there is a traumatic brain injury or a severe shock to the brain such as a stroke.
Stress and the hippocampus
The hippocampus contains a high number of adrenal steroid receptors and this area of the brain is especially plastic and is responsible for certain types of memory and learning. The amygdala is also a very important stress target, and is an area of the brain that is routinely misunderstood and the subject of many myths and half-truths which we discuss later. The amygdala controls the physiological and behavioural responses that are linked to strong emotions, including fear, but not exclusively fear. The prefrontal cortex (PFC), on the other hand, plays a central role in managing working memory and the executive functions. All three regions, the hippocampus, the amygdala and the prefrontal cortex, are targets of stress hormones.
The hippocampus is the part of your brain that handles explicit memories, like your telephone number or the name of an acquaintance. It holds a large number of glucocorticoid receptors and so we know that it is structured to participate in the overall stress response. Depression can increase glucocorticoid levels in the blood, Yvette Sheline of the Washington University made a comparison of the hippocampi of people who were recovering from long-term and severe depression with a control group. The control group did not have a history of depression but were matched by age, gender, height, and education. They found that people with a history of depression had a hippocampus that was 15% smaller than the control group.
Tamara Gurvits and Roger Pitman from Harvard University discovered a 26% decrease in the hippocampal volume of combat veterans suffering from PTSD when compared to combat veterans without PTSD, thereby isolating the stress as the causal factor in the reduction of hippocampal volume. Douglas Bremner at Yale University found a 12% reduction in the volume of the left hippocampus in adults who had suffered from post traumatic stress disorder in their childhood.
Although these studies do not prove that stress caused their brains to shrink, it is possible that people suffering from depression also are starting out with reduced hippocampal volume. So we are not certain of a causal relationship, but we are seeing a very strong correlation between these two things.
A study that was conducted with people suffering from Cushing’s syndrome that helped to provide a little more clarity on this matter. Cushing’s syndrome is caused by a tumour that stimulates the adrenal glands to produce huge amounts of glucocorticoids. Monica Starkman from the University of Michigan scanned the brains of people suffering from this syndrome and discovered that the hippocampus had experienced atrophy in the exact same areas where the concentration of glucocorticoid receptors was high. In nearby regions of the brain that had fewer glucocorticoid receptors, there was no evidence of atrophy. Studies like this point us towards stress being a causal factor in maladaptive changes to brain structure.
The good news is that the brain damage related to stress is not necessarily permanent. Some studies have shown that short-term exposure to glucocorticoids cause neurons to shrink, but they were able to rebound to normal levels when the hormone levels normalised. However, long-term exposure can cause irreversible damage. We see this with Cushing’s syndrome because areas of the brains that have been badly damaged begin to recover, after the tutor is removed, because flow of glucocorticoids is halted.
The amount of stress you subjectively experience is not as important as the amount of stress hormones, such as glucocorticoids, to which your brain is exposed. This toxic brain state is at the root of all the negative effects of stress. We must focus on stress response, of which subjective experience is just one component.
Stress and memory
Your memories are the foundation of your identity, providing you with a stable sense of what is real and what you need in order to function in your day-to-day life. Although memory is vitally and fundamentally important, the way it functions in our brain is still somewhat a mystery to neuroscientists. Many attempts have been made to develop an accurate computer model of human memory and these failed attempts have demonstrated to us the complexity and counterintuitive nature of memory. Biological memory is very different to the memory on your computer’s hard drive or on your iCloud because it is embodied.
Biological memories are subject to slow deterioration and disappearance; they can also be counterfeit. Memory is taken for granted, it is only when it fails that we notice something is there (or not there). I’m not just talking about severe breakdowns in memory seen in Alzheimer’s disease, I’m also talking about the times during the year when seemingly small things are forgotten leading to crises.