Stress responseThe biology of stress in your body

For most people today, the threat of encountering a wild bear in everyday life is quite small. Yet the likelihood of experiencing a stress reaction, as if avoiding or defending yourself from a bear, remains high.

In our bodies, this stress response can manifest itself the moment we are called to give a presentation at work, or perhaps during a nerve-wracking first meeting: sweaty palms, discomfort in the gut, a feeling of pins and needles on the skin. This is the reality of living as social, modern humans with biological systems that have evolved over millions of years.

“Human social interaction is the most stressful thing we do,” says Cliff Summers, a behavioral neuroscientist at the University of South Dakota.

Despite the bodily discomfort, this acute stress response it often means that your nervous system and brain are functioning properly, performing useful and vital tasks. Although biologists and neuroscientists are still unraveling the complex interplay of hormones, neurotransmitters and physiological effects that determine our behavior and health, resulting in stress responses.

Acute stress response: stress hormones to stress responses

Stress gets a bad rap as something of a villain to the human experience. But it is often a useful and productive force for change and survival.

For a start, one lens in the research field defines stress as “a state of homeostasis that is induced.” (This idea is based on the work of Hans Selyewho became the father of stress theory in the mid-1900s) In fact, this version of stress is an element of everyday life for most security-seeking creatures.

So what exactly happens in the body in the face of stress, anxiety or nervousness when it comes to an acute stress reaction?

Read more: What is anxiety and how can anxiety defeat us?

Stress hormones

Hormonally speaking, a stressful situation—whether it involves an unexpected bear or an angry boss in your office—raises two key chemicals in the human body: adrenaline and cortisol, according to Summers.

Together, these two stress hormones (and some neurotransmitters) coordinate to optimize the flow of blood sugar to the brain and in turn execute the fight-or-flight mechanics in the physical body. Let’s start with the effects of cortisol.

When cortisol is pumped into the bloodstream, the hormone blocks the uptake of sugar in most cells in the body. This leaves an influx of sugar in the bloodstream available for the brain to absorb, ultimately enhancing cognitive function to deal with a stressful encounter.

While this is playing out, the body’s stress response also triggers the release of adrenaline (also called epinephrine) from the adrenal glands. The hormone adrenaline floods the bloodstream and quickly spreads to all parts of the body, such as the eyes, heart, blood vessels and respiratory tract.

The main function of epinephrine or adrenaline is to release and assist in the conversion of stores of sugar and nutrients stored in cells in muscles and other organs.

Stress reactions

The resulting surge of energy is something we can see manifested in athletes, such as the runner participating in a marathon, the proverbial mother displaying a feat of strength to save her child, or the unfortunate hiker encountering a bear.

“You actually have more energy to run because the adrenaline and cortisol are working to release that energy into your system,” says Summers.

Also, this epinephrine can cause some of the localized reactions, such as sweaty hands or stomach aches, that many of us identify as feelings of nervousness or stress. In part, this is because adrenal receptors are found in our glands and intestines. In addition, when the hormone adrenaline attaches to receptors in the intestines, for example, it stops the occurrence of typical contractions, which causes discomfort.

Read more: Can you predict a panic attack?

The danger of constant stress

In both animals and humans, the frequency, severity and pace of this stress response can lead to very different long-term effects on the body and mind, according to Summers research on mice.

“We [tend to] I think stress is a bad thing,” says Summers. “As it turns out, the experiments I’ve done show pretty strongly that a strong stress response is a good thing, only if it lasts for a short time.”

Ideally, the stress response should turn on quickly and turn off quickly, switching between the sympathetic and parasympathetic nervous systems, respectively. If the stressed fight-or-flight state remains engaged for extended periods of time, it can lead to multiple and severe health consequences.

“High levels of stress will make you sick and kill you,” says Summers.

Read more: How your brain’s ‘fingerprints’ can diagnose disease

A study of the stress spectrum

One subtle aspect of the study is that highly stressful events may suppress cognitive function, rather than moderately stressful situations that optimize brain function with elevated cortisol levels.

These findings suggest that the balance of brain hormones and body response falls on a complex spectrum that Summers and other researchers believe I am still studying. Emerging research also illuminates two distinct groups: stress-resistant individuals and stress-sensitive individuals.

As one example, Summers says she watches this game in her classroom during exams. While some students seem to perform better because of the stress of taking tests, many others do worse under pressure, whether or not they have studied the material.

Other researchers have studied actual levels of the stress hormone in young medical school students preparing for an exam, and they measured cortisol levels up to nine times higher than during a calm period.

Summers says the focus of much of today’s research in the field is the study of “What makes some people sensitive to stress? And what makes other people calmer?’

Answers to questions related to the stress response are likely to point to key stress-fighting tools and practices that could improve the overall well-being of many humans and animals.

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