How the past 12 months have changed your brain

Welcome back to Inside Your Head 🧠, a weekly newsletter exploring why your brain makes you think, feel, and act the way you do, written by Dana Smith, Elemental’s senior writer and a former brain scientist. Forwarded by a friend? Subscribe here so you won’t miss the next one.

It’s been a year since the U.S. went into a state of national shutdown; a year of stress, fear, trauma, loneliness, and loss. The mental health toll of the pandemic is clear: According to the Household Pulse Survey, a weekly mental health screening of roughly 73,000 Americans conducted by the National Center for Health Statistics and the U.S. Census Bureau, as many as 43% of Americans reported symptoms of depression and anxiety during the past year. This is an increase from 26% in 2017, which was already not great.

But do these psychiatric symptoms correspond to real, physiological changes among our 86 billion neurons? I spoke with several psychologists and neuroscientists about this question, and they confirmed that yes, the past 12 months have changed our brains, and the culprit is extreme chronic stress.

Your brain, changed by chronic stress

• Stress and the body’s response to it is not inherently bad. In fact, an acute stress response is a good thing that helps us to survive. Chronic stress, however, is a different beast, one that can lead to changes in the brain that open the door for psychiatric disorders, particularly depression, anxiety, PTSD, and substance use disorders.
• Under chronic stress, the adrenal glands are no longer releasing a pulse of cortisol into the bloodstream; it’s a continuous stream. Some of that cortisol passes through the blood-brain barrier and activates glucocorticoid receptors on brain cells. Long-term activation of those receptors can cause profound and lasting changes to the structure and function of brain cells.
• One change that occurs is mediated by the brain’s immune system — a family of cells, called microglia, that are located throughout the brain. Under normal circumstances, one role of microglia is to chop up and clear out damaged or unused synapses — the connections between neurons that enable brain cells to communicate. In small doses, this is a normal part of healthy brain maintenance. During periods of severe stress, however, many more microglia are produced, and they become activated by the cortisol circulating in the brain. The excess, over-activated microglia can then start to prune out synapses that are still necessary and functional.
• “As a consequence, you’ll lose the complexity of your neurons, and if it’s pronounced enough, it can cause a number of negative outcomes,” says James Herman, PhD, a professor of psychiatry and behavioral neuroscience at the University of Cincinnati. “It can impair memory, it can impair cognitive processing, and it can even impair the ability of the brain to control stress responses. So in many ways, these types of cellular responses can actually even make the stress worse, almost like in a positive feedback loop.”
• Three brain regions that appear to be particularly affected by chronically high cortisol levels and the subsequent culling of synapses are the amygdala, which triggers fear and anxiety responses; the hippocampus, an area important for learning and memory, as well as mood and emotion regulation; and the prefrontal cortex, the brain’s executive center involved in things like future planning and impulse control.
• The synapse pruning that occurs during times of chronic stress can disrupt the balance between these brain regions. In anxiety disorders, and post-traumatic stress disorder in particular, the prefrontal cortex and the amygdala become less connected, and there is greater than normal activity in the amygdala and less in the prefrontal cortex. As a result, there’s nothing holding the amygdala back from constantly sounding the alarm, so you get more stressed out over more things.
• With depression, the prefrontal cortex and the hippocampus are most affected, and both see a loss in volume and function. This happens because the activation of the glucocorticoid receptors in the area prevents new growth in the region. As a result, people with depression have significantly smaller hippocampi than non-depressed people, which is thought to contribute to greater difficulty regulating their mood.
• “What happens with stress is that you lose synaptic structure in the prefrontal cortex, and it’s thought that actually removes the brakes on the amygdala, and it allows the amygdala to overdrive,” Herman says. “The amygdala is being told to grow, whereas the hippocampus and the prefrontal cortex have been told to retract. It’s very interesting biology how you kind of shift the emphasis of the brain toward a mode that is more associated with fear and negative affect.”

Thanks for reading! Today’s newsletter is an excerpt from the first in a series of stories I’m working on called “Your Poor Pandemic Brain” to mark the one-year anniversary of the U.S. going into shutdown and what it’s done to our mental health. Read the full version here, and watch out for another installment in the coming weeks. In the meantime, email or tweet me how you’re coping with the stress of the past year.