Whereas many of us suffer from stress and are on sick leave because of the consequences of stress for health, most experience stress without becoming sick. Thus, stress does not necessarily lead to illness or other problems, and you don’t need to worry about it most of the time.
On the contrary, stress is usually something good! It helps us to give the energy and focus to deal with problems. Think of tensions in relationships or problems at the workplace, for example, that stress you out for a while but also enable you to solve these tensions and problems.
At its core, stress is a protection mechanism. It protects somebody from physical or emotional harm, and could even help to survive difficult situations. This clear benefit to life reflects the biological success of stress across essentially all creatures living on our planet. Stress and stress reactions occur from bacteria to humans, and from plants to animals. Some anti-stress molecules found in bacteria are also found in humans!
Stress is therefore nothing special, in the sense that essentially everybody will experience stress throughout life, and probably many times. Also, stress is nothing special as it occurs in all species populating planet Earth. Stress is good, and what is good tends to be preserved during evolution.
This article will show you how different stress mechanisms have evolved over time, from the synthesis of heat shock proteins in unicellular organisms to activation of certain brain and hormonal systems in fish, amphibia, reptiles, birds and mammals (including humans). It aims to help you realize that stress is a normal part of life, and usually something good. Problems only arise when stress becomes chronic. Chronic stress can cause disease, and should be avoided and be dealt with as soon as it presents itself.
Heat shock proteins
Heat shock proteins form a special class of proteins in the body. They are synthesized when an organism is under stress.
They have a very particular function in the cells of the body. Heat shock proteins get rid of damaged proteins following stress. Damaged proteins can become toxic to cells, so heat shock proteins serve an important function to reduce stress at the cellular level.
As their name already implies, heat shock proteins have been discovered in laboratory experiments, in which cells were exposed to a sudden increase in temperature. As all functions of cells depend on temperature, this heat shock disrupts the equilibrium of many processes in the cell. Disruption of equilibrium is one of the core concepts of stress. One could thus say that these cells cultured under temperature swings are under stress. Heat shock proteins are the first line of defense to protect cellular health and to help to reestablish or form a new equilibrium to terminate the stress.
Heat shock proteins are found in all living things: from simple bacteria and microorganisms that are basically composed of one cell only, to complex organisms consisting of billions of cells like ourselves. In all of these, heat shock proteins serve a similar stress-protective function.
Stress in vertebrate animals and humans
As plants and animals become more complex during evolution, they cannot not solely rely on stress reactions at the cellular level by heat shock proteins. They will need more advanced systems to detect stressors (things that cause stress), and to coordinate stress responses that will be elicited throughout the body.
In vertebrates (these are the animals with a spinal column: fish, amphibia, reptiles, birds and mammals including humans), two additional stress systems have developed. They use the same brain structures and the same messenger molecules in all of these animals. Therefore, the biology of stress has been studied in animals, most notably fish, mice and rats in the laboratory, and chimpanzees and macaques in the wild.
It turns out that all vertebrates have a rapid stress system that is operational within seconds after an encounter with a stressor, and a slow stress system that is effective within minutes.
The fast stress response system is a neural system. It consists of a particular brain region, the hypothalamus, the brainstem and the central part of the adrenal gland, known as the medulla. In short, the stress hormone corticotropin-releasing factor, or CRF, is produced in the hypothalamus, and released in the brainstem. The brainstem sends a fast neural signal to the adrenal medulla. This results in a rapid secretion of the stress hormone adrenaline (US: epinephrine) into the blood. Adrenaline accelerates heart rate, increases blood pressure, augments breathing and prepares the muscles for action. The body is now ready for a “fight-or-flight” reaction to combat or escape from the stressor.
The slow, hormonal, stress system also starts with CRF in the hypothalamus in the brain, but this time it is released in the pituitary gland (or hypophysis) at the base of the brain. The pituitary gland secretes the stress hormone adrenocorticotropic hormone, or ACTH, which travels through the blood until it reaches the cortical part of the adrenal gland. Here, ACTH stimulates the production and release of the stress hormone cortisol. Cortisol supports the actions of adrenaline, and plays furthermore a role in the reallocation of energy in favor of organs that need to be activated to deal with the stressor.
The two major stress systems in the human brain and body. Both start in the hypothalamus (blue) with the stress protein CRF. In the neural pathway (the sympathetic nervous system, green), CRF is released in the locus coeruleus. The locus coeruleus produces noradrenaline (NA, US: norepinephrine), which is released in the spinal cord. The stress signal reaches the adrenal medulla via the splanchnic nerve, which releases acetylcholine (Ach) to promote the synthesis and release of adrenaline (epinephrine). In the hormonal pathway (the Hypothalamus-Pituitary-Adrenal axis, red), CRF is released in the pituitary gland, which responds by secreting the stress hormone ACTH. ACTH travels through the blood to reach the adrenal cortex, where it promotes the synthesis and release of cortisol. In the image, the face is to the left, and the back of the head is to the right.
Implications for stress in humans
You may wonder whether the high similarity between the stress systems of fish and humans could explain why so many people have stress. The idea behind this argument is that we are equipped with a fish brain to deal with stress, and that this may not be sufficient to face the high demands in our society that cause stress. If you think this, you are not alone. Some psychologists would agree with you. They would argue that there is not much we can do to reduce stress. We just don’t have the brains for it.
However, this argument does not apply for three reasons:
- Whereas many employees will go on sick leave because of stress, most employees do not. This suggests that the majority of employees do manage stress rather well. Our human brain would therefore be capable of dealing with stress.
- The stress systems are merely output systems of the brain. This is to say that the stress systems are only activated after the brain has sensed problems that need to be dealth with. What is sensed and experienced as stress differs a lot between fish and humans. Fish will experience mostly physical stress, such as oxygen shortage and temperature changes. At times, they may be attacked by predators or by conspecifics that are stronger and bigger, and therefore higher in rank in the group (psychosocial stress). In humans, most of the stress we experience is of psychosocial nature, caused by problems in our social interactions with others.
- Human brains are much more developed than fish brains, especially the cortex. This is the big part of the brain that you would see when you look directly under the skull. Simply put, our brains have enabled us to build societies that are more complex than those of any other animal species on Earth. We can therefore handle the demands that a complex society imposes, and this includes dealing with stress. Information about stress is first processed by other parts of the brain, before the brain’s stress systems are being called to action. Thus, whereas the stress systems in fish and humans are quite similar, when and how they are activated differs enormously. This makes the coupling between stressors, the processing of stressful information by the brain, and the activation of the stress systems fit the life of fish and that of humans.
Variability in stress reactions between people
From this article, you may have gotten the impression that the reactions to stress by the two stress systems are rather uniform among animals and people. The fast neural and the slow hormonal systems are activated to start adaptation to a stressor in all vertebrates, including humans.
However, people (and animals) differ in their sensitivity and resilience to stress. Some people may be easily stressed, even by little problems that arise every day, whereas others do not seem to be bothered by major stressful events.
Such differences are rooted in the different personalities of each of us. Personalities are to a large extent determined by genetics (some may have genes that make one stress-sensitive or stress-resilient), development (children, adults and the elderly may experience stress differently) and previous exposures to stressful situations (memory and experience).
This is, by the way, similar in animals. Biologists have observed that animals each have their own unique personality. Animals of the same species therefore display differences in their stress sensitivity and resilience.
Finally, personality may determine whether stress preferentially activates the fast stress system to produce adrenaline (epinephrine) or the slow stress system to produce cortisol. As a rough rule of thumb, persons who immediately deal with stress actively are the ones who use the fast stress system the most. Consequently, they are also the ones who have the highest risk of developing cardiovascular diseases by the actions of elevated adrenaline levels in the blood, such as cardiac arrest.
The success of stress
We hope that after reading this article you will realize that stress is a very common condition. The bodily reactions to stress serve to help you combat the stressor (the thing that caused your stress), and are something good. The fact that the stress systems are such good defenders of health is quite remarkable, and explains why the stress systems have not changed much in vertebrates. Never change a winning team!
Most of the time, you don’t have to worry about stress. It is often rather the opposite: stress makes you worry about something that provokes your stress, and this helps to focus on dealing with the stressor. Only when stress becomes long-lasting is your health in danger, and in this case, you should worry about stress. However, with the information we provide here at Stressinsight, you will learn how to deal with chronic stress effectively and even how to prevent it in many cases. You will come to understand that you do not have to reduce the activity of the stress systems, but rather that of processes in the brain that are “upstream” of the stress systems.
