More than just “flight” or “fight”: how stress affects the brain and body

You are walking through a dysfunctional area late at night when you suddenly see a dark silhouette ahead of you. In an emotional sense, it scares you. Yet, even before you realized fear, you were going through a chemical reaction in your body called stress.

Basically, everything begins in the amygdala – this paired part of the brain, located in the temporal lobes of the hemispheres, continuously monitors outside stimuli, isolating from them those that pose a serious threat to life and health. The amygdala double checks the evidence (dark silhouette) against memories of similar situations in the past (recently your friend was robbed on the street) and general information (it is not safe in the dark in this area) – the threat has been identified. The alarming signal reaches the diencephalon, where the hypothalamus triggers an ancient fight-or-flight mechanism. A hormone-induced state of alert affects the entire body. When adrenaline is released, the heart rate and respiration increase, the muscles receive more blood and become tense. In order to provide energy to the body, cortisol releases glucose into the bloodstream. To conserve resources, the immune system, inflammation, and digestion are slowed.

When we are stressed, we become stronger and more resilient, increasing our chances of surviving dangerous situations. However, this mechanism consumes a lot of energy. Therefore, our bodies aren’t designed for prolonged stress. In an ideal world, it would return to its original state as soon as you realize that you’ve mistaken the shadow of a cat for a robber. Stress can last a long time, sometimes increasing, sometimes decreasing, but never completely disappearing. Our stressors are problems at work, quarrels in the family, financial difficulties and existential fears, which are harder to cope with than a momentary fright.

Chronic stress produces the same physiological response as acute stress. Mostly, we don’t notice it because it is a little weaker: breathing is not as rapid, but shallow; muscles are compressed, but less (usually the lower jaw strains, and the shoulders rise slightly). The body, however, also expends a significant amount of resources during chronic stress, and all important organ systems are affected. Perhaps this is why some scientists believe stress is the main reason people do not live 150 years, but half as long.

• The nervous system and the brain. The things that don’t kill us don’t make us stronger. Moreover, the more stress we face in life, the more sensitive our brain becomes to them and the harder it is to handle them. According to scientists, mice subjected to chronic stress have an increased response to danger in the amygdala. Perhaps this is why chronic stress often leads to depression and anxiety, since these disorders are also characterized by overactive amygdalas. Additionally, stress kills nerve cells. Quite literally. In fact, chronic stress accelerates the death of hippocampal neurons, which leads to cognitive impairment, affective disorders and depression.
• The cardiovascular system. When under stress, the pulse quickens and the blood vessels dilate. Stress and pressure on the heart are increasing, which increases the risk of heart disease. A study on mice found that heat shock proteins are more abundant in the cells of the coronary arteries, aorta and myocardium under stress. They are related to the development of hypertension, coronary heart disease, and atherosclerosis.
• The endocrine system. The glucocorticoids, including cortisol, which are synthesized during times of stress, regulate the immune response and reduce inflammation – this is how the body prevents an overreaction in the event of an injury. Chronic stress disrupts the communication between the hormone response to stress and the immune system, which can result in impaired immunity, chronic fatigue, metabolic dysfunction, and depression.
• Digestive system. Perhaps the gut is not a “second brain” as neuroscientist Emeran Meyer describes it. Nevertheless, 100 million neurons in the gut have a direct – superfast connection with the brain. A disruption of peristalsis regulation by the nervous system can lead to functional disorders, like irritable bowel syndrome. Stress, along with intestinal infections, contributes to intestinal problems. Stress can also affect the stomach, within which there are many nerve endings – disrupting its normal function, altering absorption rates and causing increased sensitivity to irritants such as spicy food, alcohol, or drugs.

Stress interferes with making good decisions

Neuronal connections in the brain are rearranged as a result of chronic stress. Learning, spatial orientation, and working memory are affected. The decision tree itself is changing as well. In one experiment, mice were trained to press levers to obtain treats from two feeders. After they were separated into two groups, one group was chronically stressed for three weeks. The rules were changed afterward: you no longer needed to push the lever to get treats from one trough. However, the stressed mice continued to press the levers. Their brain scans showed that the activity of the prefrontal cortex (responsible for analysis and decision-making) had decreased, while the sensory-motor zone, on the other hand, had increased. Stressed mice appear to have chosen their usual behavioral strategy without regard to the actual conditions or the consequences of their decision.

Chronic stress affects men and women differently

Women are more likely to suffer from depression, post-traumatic stress disorder, anxiety, and mood disorders. This may be due to heightened stress responses. Women are more sensitive to stressors, and they also feel the effects of stressors for a longer period of time. The trend is not limited to humans. A study found that after three days of cohabitation with an aggressive neighbor, male mice were wary of unfamiliar relatives for two weeks, and female mice for more than 10 weeks. What makes the female body more vulnerable to stress? There can be many reasons for this. It has been shown, for example, that neurons in the brain of women use more receptors to communicate with the neurotransmitter corticoliberin, which triggers the stress response.

Strange but true: oxytocin, a hormone associated with pleasant emotions (warmth, affection, sexual arousal), increases women’s anxiety after stress. The female hormone estrogen can also affect stress sensitivity: mice with high estrogen levels tend to groom more anxiously – brushing helps to distract them from stress and calm them down.

Pessimism is caused by stress

The conclusion was reached by a team of psychologists and neuroscientists from the UK, Germany, and the US. In an experiment, 91 adults participated in an economic game. Using images of TVs and smartphones on a virtual conveyor belt, they had to determine which plant they were viewing – one for assembling phones, one for assembling TVs. The participants each invested in the production of either televisions or telephones. Subjects who correctly guessed the plant (and it happened to be the one they invested in) received a monetary reward, while those who incorrectly predicted it lost money. Half of the participants were told before the experiment that they would have to speak in front of the panel – this made them feel more stressed.

Statistical analysis of the results showed that, under the influence of stress, participants assumed they were at the wrong plant. The authors note that in stressful situations, any argument for an undesirable outcome had more weight for the participants than in calm conditions. In a physiological sense, it makes sense: by being pessimistic, the body is more likely to prepare for the worst. If you need to make the most objective decision about your future, it is better to exclude the influence of stress factors.

Stress interferes with planning

An interesting study was conducted by Stanford University researchers. The participants in the experiment donned virtual reality helmets and explored more than a dozen cities along a predetermined route. Afterwards, they were “transported” to one of these virtual cities and told to reach a certain point. A half of the participants were warned that they would be electrocuted from time to time, which resulted in a stress response. Scientists studied how subjects in varying states of stress and rest navigated. Participants who were not threatened with electric shocks were more likely to find a shorter, more convenient route. In the stress group, subjects were more likely to choose the learned route, ignoring the opportunity to shorten the distance.

An MRI study revealed that stress inhibited the activity of the hippocampus, which is responsible for memory. As a result, the subjects did not use their knowledge of the area, but acted mechanically. How does it relate to the future plans? A special kind of memory is required. Memory for the future is the ability to remember what you have to do in the future. The study authors claim that this function also suffers from stress.