Your sympathetic nervous system is part of your autonomic nervous system. It could be called your “automatic” nervous system, as it is responsible for many functions that you don’t have to think about to control. This can include control of your heart rate, blood pressure, digestion, urination and sweating, among other functions.

What’s the difference between the sympathetic and parasympathetic nervous systems?

Your sympathetic and parasympathetic nervous systems have opposite roles. While the SNS carries signals that put your body’s systems on alert, the parasympathetic carries signals that relax those systems.

The two systems work together to keep your body in balance. Your SNS takes the lead for as long as is necessary to get you through a period of danger. Then, your parasympathetic nervous system steps in and returns things to normal.

Sympathetic-parasympathetic-nervous-systems What is the sympathetic nervous system (SNS)?

Your sympathetic nervous system is best known for its role in responding to dangerous or stressful situations. In these situations, your SNS activates to speed up your heart rate, deliver more blood to areas of your body that need more oxygen or other responses to help your get out of danger.

Function of the Sympathetic Nervous System

What does the SNS do? Its purpose?

Your SNS controls your “fight-or-flight” response. Danger or stress activates your sympathetic nervous system, which can cause several things to happen in your body. In response to danger or stress, your sympathetic nervous system may affect your:

  • Eyes: Enlarge your pupils to let more light in and improve your vision.
  • Heart: Increase your heart rate to improve the delivery of oxygen to other parts of your body.
  • Lungs: Relax your airway muscles to improve oxygen delivery to your lungs.
  • Digestive tract: Slow down your digestion so its energy is diverted to other areas of your body.
  • Liver: Activate energy stores in your liver to an energy that can be used quickly.

These effects help you in situations where you might need to think or act quickly. They improve your eyesight, reflexes, endurance and strength. Your SNS also activates at times when your body’s under trauma, like when you’re exercising or are sick.

Your sympathetic nervous system activity also affects your immune system and your body’s repair processes. These effects can help your body start repairs on an injury quickly if you get hurt.


Your sympathetic nervous system uses chemicals called neurotransmitters to communicate. Specifically, these chemicals are norepinephrine, epinephrine and acetylcholine.


Where is your sympathetic nervous system located?

Most of the signals that your sympathetic nervous system sends start in your spinal cord. The signals leave your spinal cord and activate structures called ganglia. Your sympathetic ganglia then send the necessary signals far and wide to different parts of your body. This could include your heart, lungs, arteries, sweat glands and digestive system.

What is it made of?

The components of your sympathetic nervous system are similar to those found in other parts of your nervous system. The main type of cell is a neuron, which can generate and receive signals.

Conditions and Disorders

What are the common conditions and disorders that affect the SNS?

There are many conditions and causes of SNS problems. Common examples include:

  • Type 2 diabetes. Uncontrolled Type 2 diabetes can damage your autonomic nervous system, including your SNS. An example of this is orthostatic hypotension, where your blood pressure drops when you stand up. Diabetes-related neuropathy can damage the nerves that normally trigger a blood pressure increase reflex when you stand.
  • Anxiety disorders and chronic stress. Anxiety and chronic stress can strain your SNS. Over time, that can increase your risk of obesity and other metabolic problems.
  • Cancer. Pheochromocytomas are a type of cancer that affects your adrenal glands, which are at the top of your kidneys. Your adrenal glands produce neurotransmitters, like adrenaline and norepinephrine. This kind of cancer makes these glands release too much adrenaline and norepinephrine, which keeps your sympathetic nervous system far more active than needed.
  • Genetic conditions. Genetic conditions like amyloidosis can affect your sympathetic nervous system.
  • Horner’s syndrome. This condition affects a small part of the sympathetic nervous connections in your face. This can cause a small pupil, a lack of facial sweating and eyelid drooping.
  • Infections. Nerve damage can happen because of viruses and certain bacteria.
  • Multiple system atrophy.This severe condition is similar to Parkinson’s disease, damaging autonomic nerves over time.
  • Sexual dysfunction. Your sympathetic and parasympathetic nervous systems play a role in sexual function. Individuals with a penis with sympathetic nervous system damage may experience priapism, an erection lasting at least four hours. Without treatment, priapism can lead to permanent erectile dysfunction.
  • Trauma. Injuries can cause nerve damage, which may be long-term or even permanent. This is especially the case when you have injuries to your spinal cord that damage or cut off SNS connections farther down.

What are some common signs and symptoms of sympathetic nervous system problems?

Several potential symptoms can happen with the SNS, including:

  • Heart rhythm issues (including arrhythmias).
  • Constipation.
  • Dizziness or passing out when standing up.
  • Eyelid droop (ptosis).
  • Fast heart rate (tachycardia), even when resting.
  • Sexual dysfunction, including priapism.
  • Sweating too much (hyperhidrosis) or not sweating enough (anhidrosis).
  • Trouble digesting food (including gastroparesis).
  • Trouble swallowing (dysphagia).