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10 Jul 2024

Sympathetic Nervous System (SNS): What It Is & Function?

5 mins to read
What is sympathetic nervous system, also known as the fight-or-flight response? Learn why it is important & how to keep it at its best.

The sympathetic nervous system, known as the fight-or-flight response, helps prepare our body to respond to stress or imminent danger. Without it, we wouldn't be able to, for example, jump out of the path of an oncoming car or act quickly in an emergency to care for a sick child.

This stress response has evolved as a survival mechanism for both animals and humans, helping us deal with life-threatening situations. The response involves the release of hormones that trigger a bunch of physiological responses, which is essential to helping us get through stressful situations.

However, it has become clear that chronic episodes where the stress responses are activated can impair the body in many ways and contribute to long-term health problems.

What is the sympathetic nervous system?

The sympathetic nervous system is one part of the autonomic system. The autonomic system controls the automatic processes of our body that happen without us realising, such as our heartbeat, breathing, digestion, sweating, and more.

When we are faced with stress, our eyes and ears send a message to the brain, which activates the sympathetic nervous system and triggers the fight-or-flight response. It works in opposite ways to the parasympathetic nervous system, the other part of the autonomic system.

The parasympathetic nervous system stops the intense responses and instead promotes rest and calm after the danger has passed.

What happens when the sympathetic nervous system is activated?

Once the brain receives the message of danger, it sends signals through the autonomic nerves to the adrenal glands, which release the adrenaline hormone. This results in an increase in a series of physical responses to give you the strength, reflexes, energy and endurance to help your body get through the danger.

These include:

  • Increase in heart rate to improve delivery of oxygen to other parts of the body
  • Dilation of pupils to help sharpen vision
  • Slowing down of the digestive system so that energy can be diverted to other parts of the body that need it
  • More oxygen being sent to the brain, which increases alertness
  • Lungs opening wider so more oxygen can be taken in

As Harvard University describes, this all happens efficiently even before the brain's visual centres can process it. This is why, for instance, someone may be able to put their foot on the brakes as soon as they see an oncoming car without even thinking about it.

After this initial adrenaline surge, if there are signals of ongoing danger, the brain activates the second part of the stress response, prompting the adrenal glands to release cortisol. This helps the body stay on high alert. Eventually, when the threat subsides and cortisol levels reduce, the parasympathetic nervous system comes in to reduce the stress response.

Importance of the sympathetic nervous system

While the system is often associated with emergencies or when there is immediate danger, it also plays a vital role in our daily lives.

Firstly, it helps regulate the body's involuntary functions, such as breathing, digestion, heart rate, and blood pressure. For example, sweating is an example of how it's activated during exercise to help us cool down. 

In addition, according to an article in the Neuroimmunomodulation Journal, the system works together with the immune system, such as when there is an infection, to help clear the body of the intruder, whether it's a bacteria or virus. Secondly, it helps us cope with the psychological stressors in our lives, such as the demands of home and work.

Chronic stress and activation of the sympathetic nervous system

Although the sympathetic nervous system plays its role during stressful situations, whether in our everyday lives or in emergencies, if the stress is chronic, the accompanying continuous surges in adrenaline can negatively impair the body's processes, leading to health problems.

For example, it can damage blood vessels and arteries or increase blood pressure. In addition, persistently high cortisol levels can lead to inflammation and negatively impair many of your body's processes, including the immune system. High cortisol levels can also increase appetite and storage of unused nutrients as fat.

Stress reduction tips

Though we cannot predict sudden emergencies and threats, we can manage daily stress to minimise the frequency with which our sympathetic nervous system is triggered. This can be achieved through the following activities that can help activate the parasympathetic nervous system, helping you relax and thereby lowering stress.

Relaxation practices

Engaging in what helps you relax, such as yoga, meditation, gentle walking, or a gentle activity or hobby such as gardening, drawing, or a craft, can help you slow down and take your mind off stress.


Exercise promotes deeper breathing and helps relieve muscle tension. It also lowers the levels of stress hormones in the body, such as adrenaline and cortisol. Additionally, it helps produce endorphins, the happy hormones, and according to Harvard University, the body's natural painkillers.

Social support

Socialising with others can take your mind off stress, and talking with the right people about stress may help alleviate emotional stress.



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