Stress and the sympathetic nervous system
Understanding stress
Erwin van den Burg
Understanding stress
10 min

How the sympathetic nervous system influences your body during stress

10 min

The sympathetic nervous system enables you to deal with stress, and prepares the body for the "fight-or-flight" response. But when it is active too long it may have adverse effects on the body. How can you tell that your sympathetic nervous system is switched on?

When you encounter a stressful situation, the sympathetic nervous system prepares the body for “fight-and flight” reactions. These reactions have evolved as a survival mechanism, enabling humans and other mammals to respond quickly to life threatening situations. During prolonged or chronic stress, activation of the sympathetic nervous system may have adverse effects on your body.

Sympathetic nervous system and the fight or flight response

A stressful situation, whether something psychological such as persistent worry about losing a job or approaching deadlines, can trigger a series of stress reactions to deal with these stressful situations. The sympathetic nervous system is the body’s way to rapidly launch stress responses. It will make the heart pound faster and accelerate breathing. Muscles will tense and sweating will occur.

The combination of these reactions is known as the “fight or flight” response. It enables a human being (or any other vertebrate animal) to actively combat the stressor (the condition that causes stress) or to run away from it.  This is because the faster heartbeat and breathing rate will provide more oxygen to the muscles, which are quickly prepared for immediate action. Sweaty hands are thought to be useful to climb into trees or on rocks, for example, as sweat enhances grip to the branches or stones. Useful for our ancestors living on the savannah, but not very practical in our modern society most of the times. Sweating will also lower body temperature, which will rise as a consequence of stress.

In humans and animals alike, the sympathetic nervous system makes fast action during stress possibleThe flight reaction of this mouse is activated by the sympathetic nervous system and may help to escape from the hungry owl.

The fight or flight response is an evolutionary old mechanism, and can already be found in fish (except for the sweating of course). The sympathetic nervous system as the orchestrator of a series of fast stress responses has proven a successful physiological strategy to deal with stressors throughout all classes of vertebrate animals (fish, amphibians, reptiles, birds and mammals).

However, there is a downside to the activation of the sympathetic nervous system during stress. The body and the sympathetic nervous system can overreact to stressful situations that are not life threatening (such as getting stuck in a traffic jam or when watching a football match). Remember that the fight or flight response has evolved as a survival mechanism! Also, when the stress persists because it is too severe so that it cannot be dealt with properly, the stress reactions will take a toll on the body.

Over the years, scientists have found that repetitive stress can induce severe health problems. Chronic and repetitive stress contribute to high blood pressure, the formation of artery-clogging deposits, and cause profound changes in the brain that may promote anxiety, depression and addiction. Furthermore, chronic stress may lead to obesity, as stressed people tend to eat more palatable food (chocolate, hamburgers, etc.) and because they sleep and exercise less.

Signaling danger

The stress reactions start in the amygdala. This is an almond-shaped structure located low in the brain. The amygdala is the alert center of the brain. It receives information from the eyes, ears and other senses, and can interpret whether something you see, hear, smell or taste is dangerous.

The amygdala activates the sympathetic nervous system during danger and stressThe amygdala (red) is the alert center of the brain. The amygdala activates the stress reactions in the hypothalamus, which is located immediately next to it (yellow).

Thus, when someone sees an oncoming car, the eyes rapidly transmit the visual information to the amygdala. The amygdala will interpret the oncoming car as dangerous, and will send immediately an alarm signal to the hypothalamus. This all happens in a fraction of a second, so fast that the visual system has not even finished processing the image of the oncoming car! People will jump out of harm's way without having to think about it!

The hypothalamus is located at the base of the brain and is an important regulator of many functions in the body. It communicates to the body in two ways, one of them being the autonomic nervous system. The autonomous nervous system controls such involuntary functions as blood pressure, heartbeat, breathing, and the constriction or dilatation of small blood vessels and airways in the lungs.

The autonomous nervous system is composed of two branches. The first is the sympathetic nervous system. It triggers the fight and flight reactions and provides the body with a burst of energy so that it can respond to the oncoming danger. The second branch is the parasympathetic nervous system. It counteracts the actions of the sympathetic nervous system, calming the body down after the danger has passed.

After the amygdala has sent the danger signal, the hypothalamus activates the sympathetic nervous system. It does so by means of secreting a small protein, which is known as CRF, in the brainstem. The brainstem contains a group of neurons that is known as the locus coeruleus. Here, CRF activates a particular group of cells. These cells respond by the production of noradrenaline (or norepinephrine).

The sympathetic nervous system leads to the release of adrenaline into the bloodThe amygdala activates the hypothalamus, which will respond by secreting CRF in the locus coeruleus. The locus coeruleus will then synthesize noradrenaline (NA, also known as norepineprhine), which will be released in the spinal cord. Next, the spinal cord will produce yet another messenger molecule acetylcholine (Ach), which is released from the splanchnic nerve in the medulla of the adrenal gland. Finally, the adrenal medulla secretes adrenaline (US: epinephrine) into the blood.

Noradrenaline (norepinephrine) is transported to a large number of brain regions and ensures that a human or animal is alert and vigilant. This gives focus, and allows us to concentrate on the stressor.

One of the brain areas where noradrenaline ends up is another group of cells in the brain stem where the large intestinal nerve begins. Noradrenaline activates this nerve, which then sends electric impulses to the adrenal gland. The adrenal gland lies on top of the kidneys. The large intestinal nerve releases the messenger molecule acetylcholine in the adrenal gland. Cells in the marrow of the adrenal gland react to this with the release of the stress hormone adrenaline, also known as epinephrine, perhaps the best-known stress hormone.

Adrenaline causes the frequency of the heartbeat and blood pressure to increase, the muscles to tighten, energy to be released from the liver and muscles, and breathing to accelerate. In short, all reactions necessary for immediate action, and giving rise to many of the symptoms of stress.

Symptoms of the activated sympathetic nervous system

There are several symptoms that can make you aware that the sympathetic nervous system is activated, and signal that you are in a condition of stress. These symptoms include:

·       Pounding of the heart

·       Sweating

·       Stomach aches

·       Tense muscles

·       Insomnia

·       Feeling alert

·       Irritability

·       Increased breathing

·       Having dilated pupils

·       Sharpening of sight, hearing, smelling and other senses

Most of these are related to the fight or flight response, and focus attention to the threat that has caused stress. All these symptoms reflect increased oxygen intake, which is sent to the brain, the heart and the muscles. Combined with increased release of blood sugar (glucose) and fats from storage sites in the body (mainly the liver), you will be ready to take immediate action.

Variations in autonomic nervous system activation between people

Although activation of the sympathetic nervous system during stress, and activation of the parasympathetic nervous system to end the stress reactions, are rather universal mechanisms, they can differ in intensity from person to person.

Recently, researchers studying stress responses in children have found differences between activation of the autonomic nervous system during stress.

About two thirds of the children tested responded to stress with a small or moderate activation of the sympathetic system. Such activation is sufficient to deal with the stressor, and at the same time is not very strong so that adverse effects of stress are not likely to occur.

Other children showed increased activity of the parasympathetic system, thus inhibiting mostly stress reactions that would work against dealing with stress. These children would not be able to elicit proper fight or flight responses.

Still another group of children appeared to be aroused, or stressed, already before a stressor presents itself. This suggests that the anticipation of the occurrence of a potential stress in the near future is sufficient to activate stress reactions. Stress reactions to the stressor itself are therefore low.

Smaller groups of children either displayed activation of both the sympathetic and parasympathetic nervous systems (these children use more than one stress system to deal with stress), whereas others hardly respond to a stressor at all.

Researchers have further found that children who did not respond to a stressor come from low socioeconomic families. Also, they found that children who used more than one stress system were in majority raised in families in which stress and conflicts occur on a regular basis.

It is currently not known how these children would perform during stress when reaching adulthood. Nevertheless, it seems that early life stress and socio-economic status influence which stress systems are being used and which are not. This not only applies to the autonomic nervous system, but also to another, slower stress system. This system is based on stress hormones coming from the pituitary and adrenal glands, leading to the secretion of cortisol into the blood. This is the topic of another article.

Measuring the stress systems you use

Sympathetic and parasympathetic nervous system activity can easily be measured.

Measuring the so-called pre-ejection period of the heart can assess sympathetic nervous system activity. Basically, this indicates the time that the heart needs to empty its ventricle, squirting the blood into the blood vessels. The pre-ejection period can be easily calculated from an electrocardiogram (ECG) and an impedance cardiogram (ICG). The ECG measures the electrical activity of the heart, which changes during the filling of the heart with blood and the ejection of the blood into the blood vessels. The ICG measures electrical activity in the entire chest. Both diagrams give specific waves, which can be used as points of reference to calculate the pre-ejection period. It is the distance between the beginning of the Q wave in the ECG (reflecting activation of the ventricular septum, which separates the two ventricles of the heart) and point B on the ICG (reflecting the opening of the aortic valve, which allows the blood to leave the heart). The pre-ejection period is regulated by the sympathetic nervous system. The shorter the period, the more active the sympathetic nervous system is.

PEP, stress and the sympathetic nervous systemThe PEP, the pre-ejection period, can be derived from an electrocardiogram (ECG) and an impedance cardiogram (ICG). It is calculated as the difference in time between the beginning of point Q on the ECG and point B on the ICG. The shorter this period, the more the sympathetic nervous system is active.

Parasympathetic activation can be assessed from ECGs coupled to breathing frequency. When inhaling, the heart beat frequency is higher than when exhaling. This variability in heart rhythm is known as respiratory sinus arrhythmia (RSA) and is regulated to a large extent by the parasympathetic system. Thus, when heart frequency variability is high, the activity of the parasympathetic nervous system is high.

Respiration can be used to determine parasympathetic activityRespiratory sinus arrhythmia (RSA). The heart rate increases during inhalation and decreases during exhalation, resulting in the longest heart period during exhalation (L-IBI) and the shortest heart period during inhalation (S-IBI). From Nederend I, Jongbloed M, De Geus E, Blom N and Ten Harkel A, 2016, Postnatal Cardiac Autonomic Nervous Control in Pediatric Congenital Heart Disease Journal of Cardiovascular Development & Disease 3, 16.

Measuring the activity of the sympathetic and parasympathetic activities might help to understand how you act during times of stress. Unfortunately, these measurements are not often done in clinical practice. However, you could assess some symptoms of sympathetic activation yourself, not only by looking at the symptoms of sympathetic activation (such as heart rate, see above), but also by thinking how you behave during stress. Are you taking action during stress? Or do you rather stay put and wait until the stress passes? As a rule of thumb, people who take action usually rely more on the sympathetic nervous system than people who are more passive.

Please note that active coping with stress is not necessarily better than passive coping. It all depends on the situation which coping style would be better. In situations where things can be changed to overcome stress, an active approach may be the way to go. In situations where change is not likely to be achieved, a passive strategy may be more beneficial. This is because active coping costs a lot of energy, which could be spared when active coping would not be efficient to deal with stress.

In this article, you have learned that the sympathetic nervous system manifests itself essentially instantaneously after the stress has begun. Being aware of the signs that the sympathetic nervous system has been switched on helps you realize that you are under stress. Very useful of course, but there is still another stress system that becomes active to help you deal with stressors: the hypothalamus-pituitary-adrenal system. You can read about this slower system in this article.