What Is the Sympathetic Nervous System?

Fight, Flight, Freeze, Fawn Responses

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The sympathetic nervous system (SNS) is responsible for the body's stress response and is activated when you perceive danger. The brain sends messages to the rest of the body to prepare for and respond to danger, initiating a fight, flight, freeze, or fawn stress response. If the SNS is chronically activated, it can impact your health.

Learn about the sympathetic nervous system function and your body's response to perceived danger.

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Function of the Sympathetic Nervous System

The primary function of the sympathetic nervous system is to keep your body safe from danger. When you perceive a threat, your sympathetic nervous system helps prepare your body and mind for impending danger. This is known as a stress response.

Types of Stress Responses

Scientists initially categorized the stress response as a fight or flight response. More recently, two more reactions have been identified: Freeze or fawn. Sympathetic nervous system stress responses include:

  • Fight response: This response involves aggression directed toward the danger or perceived danger. For example, someone threatened by another person may become physically defensive and attack or punch that person.
  • Flight response: This response may also be called the flee response. It involves avoiding danger or perceived danger by running, driving, or moving in a different direction. For example, a person may run out of a burning building and keep running until they are far away from it.
  • Freeze response: This response involves holding as still as possible while the danger or perceived danger is present. For example, opossums and other animals are known for "playing dead" when threatened.
  • Fawn response: This response involves acting to prevent or decrease conflict. For example, someone being robbed may willingly hand over valuable or cherished belongings to remain physically safe.

Sympathetic Nervous System Anatomy

The two main parts of the sympathetic nervous system are the brain and the spinal cord.

  • The brain is responsible for processing the threat.
  • The spinal cord communicates the threat to other parts of the body.

Once the brain is alerted to a threat, it sends messages via neurons (chemical messengers) to clusters of nerve cells called ganglia, which are responsible for sensory and motor responses. Then, the ganglia send those neurons to the body parts that respond to danger, such as muscles needed to run away or fight.

Sympathetic vs. Parasympathetic Nervous System (PNS)

The sympathetic and parasympathetic nervous systems (PNS) comprise the autonomic nervous system. They work together to shift the mind and body between alertness and calmness. The PNS response is sometimes called "rest and digest" because when the PNS is activated, the body responds by increasing digestion, sleep, the tendency toward social interactions, and repairing injuries.

How the Sympathetic Nervous System Works

When the brain senses danger, the amygdala, responsible for interpreting external stimuli, sends a message to the hypothalamus, which is responsible for maintaining our body's baseline state. Then the brain releases hormones that cause the body to react.

For example, adrenaline (epinephrine) helps get oxygen to the muscles by opening the airways and telling the blood vessels to send more blood to the heart and lungs.

What Happens When the Sympathetic Nervous System Is Activated?

The sympathetic nervous system causes physical changes all over the body. When you experience a fight, flight, freeze, or fawn reaction, your brain sends messages to the rest of your body to prepare for danger. Your body responds by increasing blood sugar levels and releasing fat for energy to protect against danger.

Some other parts of your body that respond when the sympathetic nervous system is activated include:

  • Eyes: The pupils of the eyes get bigger to let in more light so the threat can be seen more easily.
  • Lungs: The lungs open up to provide more oxygen to muscles, and the brain, which helps muscles work better and the brain becomes more alert.
  • Heart: The heart beats faster to pump more blood to organs needing more blood flow.
  • Digestive system: The digestive system slows so more energy can be used to respond to the threat or danger.

Sometimes the sympathetic nervous system becomes activated when there is no real danger or remains activated after the threat is gone. Long-term activation of the sympathetic nervous system has been linked to health concerns such as:

It can also lead to physical effects such as fatigue.


The sympathetic nervous system helps to protect you from danger. When you perceive danger, your brain and spinal cord release chemicals and send messages to other body parts to respond. Physical changes take place to make the body and mind more able to respond to the threat. The parasympathetic nervous system creates an opposite response after the threat is gone, which allows the body and mind to rest and recover.

16 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Ashley Olivine, Ph.D., MPH
Dr. Ashley Olivine is a health psychologist and public health professional with over a decade of experience serving clients in the clinical setting and private practice. She has also researched a wide variety psychology and public health topics such as the management of health risk factors, chronic illness, maternal and child wellbeing, and child development.